OF NEWTON'S NATURAL PHILOSOPHY
BY PROFESSOR MENDELÉEFF
A LECTURE DELIVERED AT THE ROYAL INSTITUTION OF GREAT BRITAIN
ON FRIDAY, MAY 31, 1889
Nature, inert to the eyes of the ancients, has been revealed to us as
full of life and activity. The conviction that motion pervaded all
things, which was first realised with respect to the stellar universe,
has now extended to the unseen world of atoms. No sooner had the human
understanding denied to the earth a fixed position and launched it along
its path in space, than it was sought to fix immovably the sun and the
stars. But astronomy has demonstrated that the sun moves with unswerving
regularity through the star-set universe at the rate of about 50
kilometres per second. Among the so-called fixed stars are now discerned
manifold changes and various orders of movement. Light, heat,
electricity--like sound--have been proved to be modes of motion; to the
realisation of this fact modern science is indebted for powers which have
been used with such brilliant success, and which have been expounded so
clearly at this lecture table by Faraday and by his successors. As, in
the imagination of Dante, the invisible air became peopled with spiritual
beings, so before the eyes of earnest investigators, and especially
before those of Clerk Maxwell, the invisible mass of gases became peopled
with particles: their rapid movements, their collisions, and impacts
became so manifest that it seemed almost possible to count the impacts
and determine many of the peculiarities or laws of their collisions. The
fact of the existence of these invisible motions may at once be made
apparent by demonstrating the difference in the rate of diffusion through
porous bodies of the light and rapidly moving atoms of hydrogen and the
heavier and more sluggish particles of air. Within the masses of liquid
and of solid bodies we have been forced to acknowledge the existence of
persistent though limited motion of their ultimate particles, for
otherwise it would be impossible to explain, for example, the celebrated
experiments of Graham on diffusion through liquid and colloidal
substances. If there were, in our times, no belief in the molecular
motion in solid bodies, could the famous Spring have hoped to attain any
result by mixing carefully-dried powders of potash, saltpetre and sodium
acetate, in order to produce, by pressure, a chemical reaction between
these substances through the interchange of their metals, and have
derived, for the conviction of the incredulous, a mixture of two
hygroscopic though solid salts--sodium nitrate and potassium acetate?
In these invisible and apparently chaotic movements, reaching from the
stars to the minutest atoms, there reigns, however, a harmonious order
which is commonly mistaken for complete rest, but which is really a
consequence of the conservation of that dynamic equilibrium which was
first discerned by the genius of Newton, and which has been traced by his
successors in the detailed analysis of the particular consequences of the
great generalisation, namely, relative immovability in the midst of
universal and active movement.
But the unseen world of chemical changes is closely analogous to the
visible world of the heavenly bodies, since our atoms form distinct
portions of an invisible world, as planets, satellites, and comets form
distinct portions of the astronomer's universe; our atoms may therefore
be compared to the solar systems, or to the systems of double or of
single stars: for example, ammonia (NH_{3}) may be represented in the
simplest manner by supposing the sun, nitrogen, surrounded by its planets
of hydrogen; and common salt (NaCl) may be looked on as a double star
formed of sodium and chlorine. Besides, now that the indestructibility of
the elements has been acknowledged, chemical changes cannot otherwise be
explained than as changes of motion, and the production by chemical
reactions of galvanic currents, of light, of heat, of pressure, or of
steam power, demonstrates visibly that the processes of chemical reaction
are inevitably connected with enormous though unseen displacements,
originating in the movements of atoms in molecules. Astronomers and
natural philosophers, in studying the visible motions of the heavenly
bodies and of matter on the earth, have understood and have estimated the
value of this store of energy. But the chemist has had to pursue a
contrary course. Observing in the physical and mechanical phenomena which
accompany chemical reactions the quantity of energy manifested by the
atoms and molecules, he is constrained to acknowledge that within the
molecules there exist atoms in motion, endowed with an energy which, like
matter itself, is neither being created nor capable of being destroyed.
Therefore, in chemistry, we must seek dynamic equilibrium not only
between the molecules, but also in their midst among their component
atoms. Many conditions of such equilibrium have been determined, but much
remains to be done, and it is not uncommon, even in these days, to find
that some chemists forget that there is the possibility of motion in the
interior of molecules, and therefore represent them as being in a
condition of death-like inactivity.
Chemical combinations take place with so much ease and rapidity, possess
so many special characteristics, and are so numerous, that their
simplicity and order were for a long time hidden from investigators.
Sympathy, relationship, all the caprices or all the fancifulness of human
intercourse, seemed to have found complete analogies in chemical
combinations, but with this difference, that the characteristics of the
material substances--such as silver, for example, or of any other
body--remain unchanged in every subdivision from the largest masses to
the smallest particles, and consequently these characteristics must be
properties of the particles. But the world of heavenly luminaries
appeared equally fanciful at man's first acquaintance with it, so much
so, that the astrologers imagined a connection between the
individualities of men and the conjunctions of planets. Thanks to the
genius of Lavoisier and of Dalton, man has been able, in the unseen world
of chemical combinations, to recognise laws of the same simple order as
those which Copernicus and Kepler proved to exist in the planetary
universe. Man discovered, and continues every hour to discover, _what_
remains unchanged in chemical evolution, and _how_ changes take place in
combinations of the unchangeable. He has learned to predict, not only
what possible combinations may take place, but also the very existence of
atoms of unknown elementary substances, and has besides succeeded in
making innumerable practical applications of his knowledge to the great
advantage of his race, and has accomplished this notwithstanding that
notions of sympathy and affinity still preserve a strong vitality in
science. At present we cannot apply Newton's principles to chemistry,
because the soil is only being now prepared. The invisible world of
chemical atoms is still waiting for the creator of chemical mechanics.
For him our age is collecting a mass of materials, the inductions of
well-digested facts, and many-sided inferences similar to those which
existed for Astronomy and Mechanics in the days of Newton. It is well
also to remember that Newton devoted much time to chemical experiments,
and while considering questions of celestial mechanics, persistently kept
in view the mutual action of those infinitely small worlds which are
concerned in chemical evolutions. For this reason, and also to maintain
the unity of laws, it seems to me that we must, in the first instance,
seek to harmonise the various phases of contemporary chemical theories
with the immortal principles of the Newtonian natural philosophy, and so
hasten the advent of true chemical mechanics. Let the above
considerations serve as my justification for the attempt which I propose
to make to act as a champion of the universality of the Newtonian
principles, which I believe are competent to embrace every phenomenon in
the universe, from the rotation of the fixed stars to the interchanges of
chemical atoms.
In the first place I consider it indispensable to bear in mind that, up
to quite recent times, only a one-sided affinity has been recognised in
chemical reactions. Thus, for example, from the circumstance that red-hot
iron decomposes water with the evolution of hydrogen, it was concluded
that oxygen had a greater affinity for iron than for hydrogen. But
hydrogen, in presence of red-hot iron scale, appropriates its oxygen and
forms water, whence an exactly opposite conclusion may be formed.
During the last ten years a gradual, scarcely perceptible, but most
important change has taken place in the views, and consequently in the
researches, of chemists. They have sought everywhere, and have always
found, systems of conservation or dynamic equilibrium substantially
similar to those which natural philosophers have long since discovered in
the visible world, and in virtue of which the position of the heavenly
bodies in the universe is determined. There where one-sided affinities
only were at first detected, not only secondary or lateral ones have been
found, but even those which are diametrically opposite; yet among these,
dynamical equilibrium establishes itself not by excluding one or other of
the forces, but regulating them all. So the chemist finds in the flame of
the blast furnace, in the formation of every salt, and, with especial
clearness, in double salts and in the crystallisation of solutions, not a
fight ending in the victory of one side, as used to be supposed, but the
conjunction of forces; the peace of dynamic equilibrium resulting from
the action of many forces and affinities. Carbonaceous matters, for
example, burn at the expense of the oxygen of the air, yielding a
quantity of heat, and forming products of combustion, in which it was
thought that the affinities of the oxygen with the combustible elements
were satisfied. But it appeared that the heat of combustion was competent
to decompose these products, to dissociate the oxygen from the
combustible elements, and therefore to explain combustion fully it is
necessary to take into account the equilibrium between opposite
reactions, between those which evolve and those which absorb heat.
In the same way, in the case of the solution of common salt in water, it
is necessary to take into account, on the one hand, the formation of
compound particles generated by the combination of salt with water, and,
on the other, the disintegration or scattering of the new particles
formed, as well as of these originally contained. At present we find two
currents of thought, apparently antagonistic to each other, dominating
the study of solutions: according to the one, solution seems a mere act
of building up or association; according to the other, it is only
dissociation or disintegration. The truth lies, evidently, between these
views; it lies, as I have endeavoured to prove by my investigations into
aqueous solutions, in the dynamic equilibrium of particles tending to
combine and also to fall asunder. The large majority of chemical
reactions which appeared to act victoriously along one line have been
proved capable of acting as victoriously even along an exactly opposite
line. Elements which utterly decline to combine directly may often be
formed into comparatively stable compounds by indirect means, as, for
example, in the case of chlorine and carbon; and consequently the
sympathies and antipathies which it was thought to transfer from human
relations to those of atoms should be laid aside until the mechanism of
chemical relations is explained. Let us remember, however, that chlorine,
which does not form with carbon the chloride of carbon, is strongly
absorbed, or, as it were, dissolved, by carbon, which leads us to suspect
incipient chemical action even in an external and purely surface contact,
and involuntarily gives rise to conceptions of that unity of the forces
of nature which has been so energetically insisted on by Sir William
Grove and formulated in his famous paradox. Grove noticed that platinum,
when fused in the oxyhydrogen flame, during which operation water is
formed, when allowed to drop into water decomposes the latter and
produces the explosive oxyhydrogen mixture. The explanation of this
paradox, as of many others which arose during the period of chemical
renaissance, has led, in our time, to the promulgation by Henri
Sainte-Claire Deville of the conception of dissociation and of
equilibrium, and has recalled the teaching of Berthollet, which,
notwithstanding its brilliant confirmation by Heinrich Rose and Dr.
Gladstone, had not, up to that period, been included in received chemical
views.
Chemical equilibrium in general, and dissociation in particular, are now
being so fully worked out in detail, and supplied in such various ways,
that I do not allude to them to develop, but only use them as examples by
which to indicate the correctness of a tendency to regard chemical
combinations from points of view differing from those expressed by the
term hitherto appropriated to define chemical forces, namely, 'affinity.'
Chemical equilibria, dissociation, the speed of chemical reactions,
thermochemistry, spectroscopy, and, more than all, the determination of
the influence of masses and the search for a connection between the
properties and weights of atoms and molecules--in one word, the vast mass
of the most important chemical researches of the present day--clearly
indicate the near approach of the time when chemical doctrines will
submit fully and completely to the doctrine which was first announced in
the _Principia_ of Newton.
In order that the application of these principles may bear fruit it is
evidently insufficient to assume that statical equilibrium reigns alone
in chemical systems or chemical molecules: it is necessary to grasp the
conditions of possible states of dynamical equilibria, and to apply to
them kinetic principles. Numerous considerations compel us to renounce
the idea of statical equilibrium in molecules, and the recent yet
strongly-supported appeals to dynamic principles constitute, in my
opinion, the foundation of the modern teaching relating to atomicity, or
the valency of the elements, which usually forms the basis of
investigations into organic or carbon compounds.
This teaching has led to brilliant explanations of very many chemical
relations and to cases of isomerism, or the difference in the properties
of substances having the same composition. It has been so fruitful in its
many applications and in the foreshadowing of remote consequences,
especially respecting carbon compounds, that it is impossible to deny its
claims to be ranked as a great achievement of chemical science. Its
practical application to the synthesis of many substances of the most
complicated composition entering into the structure of organised bodies,
and to the creation of an unlimited number of carbon compounds, among
which the colours derived from coal tar stand prominently forward,
surpass the synthetical powers of Nature itself. Yet this teaching, as
applied to the structure of carbon compounds, is not on the face of it
directly applicable to the investigation of other elements, because in
examining the first it is possible to assume that the atoms of carbon
have always a definite and equal number of affinities, whilst in the
combinations of other elements this is evidently inadmissible. Thus, for
example, an atom of carbon yields only one compound with four atoms of
hydrogen and one with four atoms of chlorine in the molecule, whilst the
atoms of chlorine and hydrogen unite only in the proportions of one to
one. Simplicity is here evident, and forms a point of departure from
which it is easy to move forward with firm and secure tread. Other
elements are of a different nature. Phosphorus unites with three and with
five atoms of chlorine, and consequently the simplicity and sharpness of
the application of structural conceptions are lost. Sulphur unites only
with two atoms of hydrogen, but with oxygen it enters into higher orders
of combination. The periodic relationship which exists among all the
properties of the elements--such, for example, as their ability to enter
into various combinations--and their atomic weights, indicate that this
variation in atomicity is subject to one perfectly exact and general law,
and it is only carbon and its near analogues which constitute cases of
permanently preserved atomicity. It is impossible to recognise as
constant and fundamental properties of atoms, powers which, in substance,
have proved to be variable. But by abandoning the idea of permanence, and
of the constant saturation of affinities--that is to say, by
acknowledging the possibility of free affinities--many retain a
comprehension of the atomicity of the elements 'under given conditions;'
and on this frail foundation they build up structures composed of
chemical molecules, evidently only because the conception of manifold
affinities gives, at once, a simple statical method of estimating the
composition of the most complicated molecules.
I shall enter neither into details, nor into the various consequences
following from these views, nor into the disputes which have sprung up
respecting them (and relating especially to the number of isomerides
possible on the assumption of free affinities), because the foundation or
origin of theories of this nature suffers from the radical defect of
being in opposition to dynamics. The molecule, as even Laurent expressed
himself, is represented as an architectural structure, the style of which
is determined by the fundamental arrangement of a few atoms, whilst the
decorative details, which are capable of being varied by the same forces,
are formed by the elements entering into the combination. It is on this
account that the term 'structural' is so appropriate to the contemporary
views of the above order, and that the 'structuralists' seek to justify
the tetrahedric, plane, or prismatic disposition of the atoms of carbon
in benzene. It is evident that the consideration relates to the statical
position of atoms and molecules and not to their kinetic relations. The
atoms of the structural type are like the lifeless pieces on a chess
board: they are endowed but with the voices of living beings, and are not
those living beings themselves; acting, indeed, according to laws, yet
each possessed of a store of energy which, in the present state of our
knowledge, must be taken into account.
In the days of Haüy, crystals were considered in the same statical and
structural light, but modern crystallographers, having become more
thoroughly acquainted with their physical properties and their actual
formation, have abandoned the earlier views, and have made their
doctrines dependent on dynamics.
The immediate object of this lecture is to show that, starting with
Newton's third law of motion, it is possible to preserve to chemistry all
the advantages arising from structural teaching, without being obliged to
build up molecules in solid and motionless figures, or to ascribe to
atoms definite limited valencies, directions of cohesion, or affinities.
The wide extent of the subject obliges me to treat only a small portion
of it, namely of _substitutions_, without specially considering
combinations and decompositions, and even then limiting myself to the
simplest examples, which, however, will throw open prospects embracing
all the natural complexity of chemical relations. For this reason, if it
should prove possible to form groups similar, for example, to H_{4} or
CH_{6} as the remnants of molecules CH_{4} or C_{2}H_{7} we shall not
pause to consider them, because, as far as we know, they fall asunder
into two parts, H_{2} + H_{2} or CH_{4} + H_{2}, as soon as they are even
temporarily formed, and are incapable of separate existence, and
therefore can take no part in the elementary act of substitution. With
respect to the simplest molecules which we shall select--that is to say,
those of which the parts have no separate existence, and therefore cannot
appear in substitutions--we shall consider them according to the periodic
law, arranging them in direct dependence on the atomic weight of the
elements.
Thus, for example, the molecules of the simplest hydrogen compounds--
HF H_{2}O H_{3}N H_{4}C
hydrofluoric acid water ammonia methane
correspond with elements the atomic weights of which decrease
consecutively
F = 19, O = 16, N = 14, C = 12.
Neither the arithmetical order (1, 2, 3, 4 atoms of hydrogen) nor the
total information we possess respecting the elements will permit us to
interpolate into this typical series one more additional element; and
therefore we have here, for hydrogen compounds, a natural base on which
are built up those simple chemical combinations which we take as typical.
But even they are competent to unite with each other, as we see, for
instance, in the property which hydrofluoric acid has of forming a
hydrate--that is, of combining with water; and a similar attribute of
ammonia, resulting in the formation of a caustic alkali, NH_{3},H_{2}O,
or NH_{4}OH.
Having made these indispensable preliminary observations, I may now
attack the problem itself and attempt to explain the so-called structure
or rather construction, of molecules--that is to say, their constitution
and transformations--without having recourse to the teaching of
'structuralists,' but on Newton's dynamical principles.
Of Newton's three laws of motion, only the third can be applied directly
to chemical molecules when regarded as systems of atoms among which it
must be supposed that there exist common influences or forces, and
resulting compounded relative motions. Chemical reactions of every kind
are undoubtedly accomplished by changes in these internal movements,
respecting the nature of which nothing is known at present, but the
existence of which the mass of evidence collected in modern times forces
us to acknowledge as forming part of the common motion of the universe,
and as a fact further established by the circumstance that chemical
reactions are always characterised by changes of volume or the relations
between the atoms or the molecules. Newton's third law, which is
applicable to every system, declares that, 'action is also associated
with reaction, and is equal to it.' The brevity of conciseness of this
axiom was, however, qualified by Newton in a more expanded statement,
'the action of bodies one upon another are always equal, and in opposite
directions.' This simple fact constitutes the point of departure for
explaining dynamic equilibrium--that is to say, systems of conservancy.
It is capable of satisfying even the dualists, and of explaining, without
additional assumptions, the preservation of those chemical types which
Dumas, Laurent, and Gerhardt created unit types, and those views of
atomic combinations which the structuralists express by atomicity or the
valency of the elements, and, in connection with them, the various
numbers of affinities. In reality, if a system of atoms or a molecule be
given, then in it, according to the third law of Newton, each portion of
atoms acts on the remaining portion in the same manner, and with the same
force as the second set of atoms acts on the first. We infer directly
from this consideration that both sets of atoms, forming a molecule, are
not only equivalent with regard to themselves, as they must be according
to Dalton's law, but also that they may, if united, replace each other.
Let there be a molecule containing atoms A B C, it is clear that,
according to Newton's law, the action of A on B C must be equal to the
action of B C on A, and if the first action is directed on B C, then the
second must be directed on A, and consequently then, where A can exist in
dynamic equilibrium, B C may take its place and act in a like manner. In
the same way the action of C is equal to the action of A B. In one word
every two sets of atoms forming a molecule are equivalent to each other,
and may take each other's place in other molecules, or, having the power
of balancing each other, the atoms or their complements are endowed with
the power of replacing each other. Let us call this consequence of an
evident axiom 'the principle of substitution,' and let us apply it to
those typical forms of hydrogen compounds which we have already
discussed, and which, on account of their simplicity, and regularity,
have served as starting-points of chemical argument long before the
appearance of the doctrine of structure.
In the type of hydrofluoric acid, HF, or in systems of double stars, are
included a multitude of the simplest molecules. It will be sufficient for
our purpose to recall a few: for example, the molecules of chlorine,
Cl_{2}, and of hydrogen, H_{2}, and hydrochloric acid, HCl, which is
familiar to all in aqueous solution as spirits of salt, and which has
many points of resemblance with HF, HBr, HI. In these cases division into
two parts can only be made in one way, and therefore the principle of
substitution renders it probable that exchanges between the chlorine and
the hydrogen can take place, if they are competent to unite with each
other. There was a time when no chemist would even admit the idea of any
such action; it was then thought that the power of combination indicated
a polar difference of the molecules in combination, and this thought set
aside all idea of the substitution of one component element by another.
Thanks to the observations and experiments of Dumas and Laurent fifty
years ago, such fallacies were dispelled, and in this manner the
principle of substitution was exhibited. Chlorine and bromine acting on
many hydrogen compounds, occupy immediately the place of their hydrogen,
and the displaced hydrogen, with another atom of chlorine or bromine,
forms hydrochloric acid or bromide of hydrogen. This takes place in all
typical hydrogen compounds. Thus chlorine acts on this principle on
gaseous hydrogen--reaction, under the influence of light, resulting in
the formation of hydrochloric acid. Chlorine acting on the alkalis,
constituted similarly to water, and even on water itself--only, however,
under the influence of light and only partially because of the
instability of HClO--forms by this principle bleaching salts, which are
the same as the alkalis, but with their hydrogen replaced by chlorine. In
ammonia and in methane, chlorine can also replace the hydrogen. From
ammonia is formed in this manner the so-called chloride of nitrogen,
NCl_{3}, which decomposes very readily with violent explosion on account
of the evolved gases, and falls asunder as chlorine and nitrogen. Out of
marsh gas, or methane, CH_{4}, may be obtained consecutively, by this
method, every possible substitution, of which chloroform, CHCl_{3}, is
the best known, and carbon tetrachloride, CCl_{4}, the most instructive.
But by virtue of the fact that chlorine and bromine act, in the manner
shown, on the simplest typical hydrogen compounds, their action on the
more complicated ones may be assumed to be the same. This can be easily
demonstrated. The hydrogen of benzene, C_{6}H_{6}, reacts feebly under
the influence of light on liquid bromine, but Gustavson has shown that
the addition of the smallest quantity of metallic aluminium causes
energetic action and the evolution of large volumes of hydrogen bromide.
If we pass on to the second typical hydrogen compound--that is to say,
water--its molecule, HOH, may be split up in two ways: either into an
atom of hydrogen and a semi-molecule of hydrogen peroxide, HO, or into
oxygen, O, and two atoms of hydrogen, H; and therefore, according to the
principle of substitution, it is evident that one atom of hydrogen can
exchange with hydrogen oxide, HO, and two atoms of hydrogen, H, with one
atom of oxygen, O.
Both these forms of substitution will constitute methods of
oxidation--that is to say, of the entrance of oxygen into the compound--a
reaction which is so common in nature as well as in the arts, taking
place at the expense of the oxygen of the air or by the aid of various
oxidising substances or bodies which part easily with their oxygen. There
is no occasion to reckon up the unlimited number of cases of such
oxidising reactions. It is sufficient to state that in the first of these
oxygen is directly transferred, and the position, the chemical function,
which hydrogen originally occupied, is, after the substitution, occupied
by the hydroxyl. Thus ammonia, NH_{3}, yields hydroxylamine, NH_{2}(OH),
a substance which retains many of the properties of ammonia.
Methane and a number of other hydrocarbons yield, by substitution of the
hydrogen by its oxide, methyl alcohol, CH_{3}(OH), and other alcohols.
The substitution of one atom of oxygen for two atoms of hydrogen is
equally common with hydrogen compounds. By this means alcoholic liquids
containing ethyl alcohol, or spirits of wine, C_{2}H_{5}(OH), are
oxidised until they become vinegar, or acetic acid, C_{2}H_{3}O(OH). In
the same way caustic ammonia, or the combination of ammonia with water,
NH_{3},H_{2}O, or NH_{4}(OH), which contains a great deal of hydrogen, by
oxidation exchanges four atoms of hydrogen for two atoms of oxygen, and
becomes converted into nitric acid, NO_{2}(OH). This process of
conversion of ammonium salts into saltpetre goes on in the fields every
summer, and with especial rapidity in tropical countries. The method by
which this is accomplished, though complex, though involving the agency
of all-permeating micro-organisms, is, in substance, the same as that by
which alcohol is converted into acetic acid, or glycol,
C_{2}H_{4}(OH)_{2}, into oxalic acid, if we view the process of oxidation
in the light of the Newtonian principles.
But while speaking of the application of the principle of substitution
to water, we need not multiply instances, but must turn our attention to
two special circumstances which are closely connected with the very
mechanism of substitutions.
In the first place, the replacement of two atoms of hydrogen by one atom
of oxygen may take place in two ways, because the hydrogen molecule is
composed of two atoms, and therefore, under the influence of oxygen, the
molecule forming water may separate before the oxygen has time to take
its place. It is for this reason that we find, during the conversion of
alcohol into acetic acid, that there is an interval during which is
formed aldehyde, C_{2}H_{4}O, which, as its very name implies, is
'alcohol dehydrogenatum,' or alcohol deprived of hydrogen. Hence aldehyde
combined with hydrogen yields alcohol; and united to oxygen, acetic acid.
For the same reason there should be, and there actually are, intermediate
products between ammonia and nitric acid, NO_{2}(HO), containing either
less hydrogen than ammonia, less oxygen than nitric acid, or less water
than caustic ammonia. Accordingly we find, among the products of the
deoxidation of nitric acid and the oxidation of ammonia, not only
hydroxylamine, but also nitrous oxide, nitrous and nitric anhydrides.
Thus, the production of nitrous acid results from the removal of two
atoms of hydrogen from caustic ammonia and the substitution of the oxygen
for the hydrogen, NO(OH); or by the substitution, in ammonia, of three
atoms of hydrogen by hydroxyl, N(OH)_{3}, and by the removal of water:
N(OH)_{3} - H_{2}O = NO(OH). The peculiarities and properties of nitrous
acid--as, for instance, its action on ammonia and its conversion, by
oxidation, into nitric acid--are thus clearly revealed.
On the other hand, in speaking of the principle of substitution as
applied to water, it is necessary to observe that hydrogen and hydroxyl,
H and OH, are not only competent to unite, but also to form combinations
with themselves, and thus become H_{2} and H_{2}O_{2}; and such are
hydrogen and the peroxide thereof. In general, if a molecule A B exists,
then molecules A A and B B can exist also. A direct reaction of this kind
does not, however, take place in water, therefore undoubtedly, at the
moment of formation, hydrogen reacts on hydrogen peroxide, as we can show
at once by experiment; and further because hydrogen peroxide, H_{2}O_{2},
exhibits a structure containing a molecule of hydrogen, H_{2}, and one of
oxygen, O_{2}, either of which is capable of separate existence. The
fact, however, may now be taken as thoroughly established, that, at the
moment of combustion of hydrogen or of the hydrogen compounds, hydrogen
peroxide is always formed, and not only so, but in all probability its
formation invariably precedes the formation of water. This was to be
expected as a consequence of the law of Avogadro and Gerhardt, which
leads us to expect this sequence in the case of equal interactions of
volumes of vapours and gases; and in hydrogen peroxide we actually have
such equal volumes of the elementary gases.
The instability of hydrogen peroxide--that is to say, the ease with
which it decomposes into water and oxygen, even at the mere contact of
porous substances--accounts for the circumstance that it does not form a
permanent product of combustion, and is not produced during the
decomposition of water. I may mention this additional consideration that,
with respect to hydrogen peroxide, we may look for its effecting still
further substitutions of hydrogen by means of which we may expect to
obtain still more highly oxidised water compounds, such as H_{2}O_{3} and
H_{2}O_{4}. These Schönbein and Bunsen have long been seeking, and
Berthelot is investigating them at present. It is probable, however, that
the reaction will stop at the last compound, because we find that, in a
number of cases, the addition of four atoms of oxygen seems to form a
limit. Thus, OsO_{4}, KClO_{4}, KMnO_{4}, K_{2}SO_{4}, Na_{3}PO_{4}, and
such like, represent the highest grades of oxidation.[1]
[1] Because more than four atoms of hydrogen never unite with one atom
of the elements, and because the hydrogen compounds (_e.g._ HCl,
H_{2}S, H_{3}P, H_{4}Si) always form their highest oxides with four
atoms of oxygen, and as the highest forms of oxides (OsO_{4},
RuO_{4}) also contain four of oxygen, and eight groups of the
periodic system, corresponding to the highest basic oxides R_{2}O,
RO, R_{2}O_{3}, RO_{2}, R_{2}O_{5}, RO_{3}, R_{2}O_{7}, and RO_{4},
imply the above relationship, and because of the nearest analogues
among the elements--such as Mg, Zn, Cd, and Hg; or Cr, Mo, W, and
U; or Si, Ge, Sn, and Pt; or F, Cl, Br, and I, and so forth--not
more than four are known, it seems to me that in these
relationships there lies a deep interest and meaning with regard to
chemical mechanics. But because, to my imagination, the idea of
unity of design in Nature, either acting in complex celestial
systems or among chemical molecules, is very attractive, especially
because the atomic teaching at once acquires its true meaning, I
will recall the following facts relating to the solar system. There
are eight major planets, of which the four inner ones are not only
separated from the four outer by asteroids, but differ from them in
many respects, as, for example, in the smallness of their diameters
and their greater density. Saturn with his ring has eight
satellites, Jupiter and Uranus have each four. It is evident that
in the solar systems also we meet with these higher numbers four
and eight which appear in the combination of chemical molecules.
As for the last forty years, from the times of Berzelius, Dumas, Liebig,
Gerhardt, Williamson, Frankland, Kolbe, Kekulé, and Butleroff, most
theoretical generalisations have centred round organic or carbon
compounds, we will, for the sake of brevity, leave out the discussion of
ammonia derivatives, notwithstanding their simplicity with respect to the
doctrine of substitutions; we will dwell more especially on its
application to carbon compounds, starting from methane, CH_{4}, as the
simplest of the hydrocarbons, containing in its molecule one atom of
carbon. According to the principles enumerated we may derive from CH_{4}
every combination of the form CH_{3}X, CH_{2}X_{2}, CHX_{3}, and CX_{4},
in which X is an element, or radicle, equivalent to hydrogen--that is to
say, competent to take its place or to combine with it. Such are the
chlorine substitutes already mentioned, such is wood-spirit, CH_{3}(OH),
in which X is represented by the residue of water, and such are numerous
other carbon derivatives. If we continue, with the aid of hydroxyl,
further substitutions of the hydrogen of methane we shall obtain
successively CH_{2}(OH)_{2}, CH(OH)_{3}, and C(OH)_{4}. But if, in
proceeding thus, we bear in mind that CH_{2}(OH)_{2} contains two
hydroxyls in the same form as hydrogen peroxide, H_{2}O_{2} or (OH)_{2},
contains them--and moreover not only in one molecule, but together,
attached to one and the same atom of carbon--so here we must look for the
same decomposition as that which we find in hydrogen peroxide, and
accompanied also by the formation of water as an independently existing
molecule; therefore CH_{2}(OH)_{2} should yield, as it actually does,
immediately water and the oxide of methylene, CH_{2}O, which is methane
with oxygen substituted for two atoms of hydrogen. Exactly in the same
manner out of CH(OH)_{3} are formed water and formic acid, CHO(OH), and
out of C(OH)_{4} is produced water and carbonic acid, or directly
carbonic anhydride, CO_{2}, which will therefore be nothing else than
methane with the double replacement of pairs of hydrogen by oxygen. As
nothing leads to the supposition that the four atoms of hydrogen in
methane differ one from the other, so it does not matter by what means we
obtain any one of the combinations indicated--they will be identical;
that is to say, there will be no case of actual isomerism, although there
may easily be such cases of isomerism as have been distinguished by the
term metamerism.
Formic acid, for example, has two atoms of hydrogen, one attached to the
carbon left from the methane, and the other attached to the oxygen which
has entered in the form of hydroxyl, and if one of them be replaced by
some substance X it is evident that we shall obtain substances of the
same composition, but of different construction, or of different orders
of movement among the molecules, and therefore endowed with other
properties and reactions. If X be methyl, CH_{4}--that is to say, a group
capable of replacing hydrogen because it is actually contained with
hydrogen in methane itself--then by substituting this group for the
original hydrogen we obtain acetic acid, CCH_{3}O(OH), out of formic, and
by substitution of the hydrogen in its oxide or hydroxyl we obtain methyl
formate, CHO(OCH_{3}). These substances differ so much from each other
physically and chemically that at first sight it is hardly possible to
admit that they contain the same atoms in identically the same
proportions. Acetic acid, for example, boils at a higher temperature than
water, and has a higher specific gravity than it, whilst its metameride,
methyl formate, is lighter than water, and boils at 30°--that is to say,
it evaporates very easily.
Let us now turn to carbon compounds containing two atoms of carbon to the
molecule, as in acetic acid, and proceed to evolve them from methane by
the principle of substitution. This principle declares at once that
methane can only be split up in the four following ways:--
1. Into a group CH_{3} equivalent with H. Let us call changes of this
nature methylation.
2. Into a group CH_{2} and H_{2}. We will call this order of
substitutions methylenation.
3. Into CH and H_{3}, which commutations we will call acetylenation.
4. Into C and H_{4}, which may be called carbonation.
It is evident that hydrocarbon compounds containing two atoms of carbon
can only proceed from methane, CH_{4}, which contains four atoms of
hydrogen by the first three methods of substitution; carbonation would
yield free carbon if it could take place directly, and if the molecule of
free carbon--which is in reality very complex, that is to say strongly
polyatomic, as I have long since been proving by various means--could
contain only C_{2} like the molecules O_{2}, H_{2}, N_{2}, and so on.
By methylation we should evidently obtain from marsh gas, ethane,
CH_{3}CH_{3} = C_{2}H_{6}.
By methylenation--that is, by substituting group CH_{2} for
H_{2}--methane forms ethylene, CH_{2}CH_{2} = C_{2}H_{4}.
By acetylenation--that is, by substituting three atoms of hydrogen,
H_{3}, in methane--by the remnant CH, we get acetylene, CHCH =
C_{2}H_{2}.
If we have applied the principles of Newton correctly, there should not
be any other hydrocarbons containing two atoms of carbon in the molecule.
All these combinations have long been known, and in each of them we can
not only produce those substitutions of which an example has been given
in the case of methane, but also all the phases of other substitutions,
as we shall find from a few more instances, by the aid of which I trust
that I shall be able to show the great complexity of those derivatives
which, on the principle of substitution, can be obtained from each
hydrocarbon. Let us content ourselves with the case of ethane,
CH_{3}CH_{3}, and the substitution of the hydrogen by hydroxyl. The
following are the possible changes:--
1. CH_{3}CH_{2}(OH): this is nothing more than spirit of wine, or ethyl
alcohol, C_{2}H_{5}(OH) or C_{2}H_{6}O.
2. CH_{2}(OH)CH_{2}(OH): this is the glycol of Würtz, which has shed so
much light on the history of alcohol. Its isomeride may be
CH_{3}CH(OH)_{2}, but as we have seen in the case of CH(OH)_{2}, it
decomposes, giving off water, and forming aldehyde, CH_{3}CHO, a
substance capable of yielding alcohol by uniting with hydrogen, and of
yielding acetic acid by uniting with oxygen.
If glycol, CH_{2}(OH)CH_{2}(OH), loses its water, it may be seen at once
that it will not now yield aldehyde, CH_{3}CHO, but its isomeride,
CH_{2}CH_{2}/O, the oxide of ethylene. I have here indicated in a special
manner the oxygen which has taken the place of two atoms of the hydrogen
of ethane taken from different atoms of the carbon.
3. CH_{3}C(OH)_{3} decomposed as CH(OH)_{3}, forming water and acetic
acid, CH_{3}CO(OH). It is evident that this acid is nothing else than
formic acid, CHO(OH), with its hydrogen replaced by methyl. Without
examining further the vast number of possible derivatives, I will direct
your attention to the circumstance that in dissolving acetic acid in
water we obtain the maximum contraction and the greatest viscosity when
to the molecule CH_{3}CO(OH) is added a molecule of water, which is the
proportion which would form the hydrate CH_{3}C(OH)_{3}. It is probable
that the doubling of the molecule of acetic acid at temperatures
approaching its boiling-point has some connection with this power of
uniting with one molecule of water.
4. CH_{2}(OH)C(OH)_{3} is evidently an alcoholic acid, and indeed this
compound, after losing water, answers to glycolic acid, CH_{2}(OH)CO(OH).
Without investigating all the possible isomerides, we will note only that
the hydrate CH(OH)_{2}CH(OH)_{2} has the same composition as
CH_{2}(OH)C(OH)_{3}, and although corresponding to glycol, and being a
symmetrical substance, it becomes, on parting with its water, the
aldehyde of oxalic acid, or the glyoxal of Debus, CHOCHO.
5. CH(OH)_{2}C(OH_{3}), from the tendency of all the preceding,
corresponds with glyoxylic acid, an aldehyde acid, CHOCO(OH), because the
group CO(OH), or carboxyl, enters into the compositions of organic acids,
and the group CHO defines the aldehyde function.
6. C(OH)_{3}C(OH)_{3} through the loss of 2H_{2}O yields the bibasic
oxalic acid CO(OH)CO(OH), which generally crystallises with 2H_{2}O,
following thus the normal type of hydration characteristic of ethane.[2]
[2] One more isomeride, CH_{2}CH(OH), is possible--that is, secondary
vinyl alcohol, which is related to ethylene, CH_{2}CH_{2}, but
derived by the principle of substitution from CH_{4}. Other
isomerides, of the composition C_{2}H_{4}O, such, for example, as
CCH_{3}(OH), are impossible, because it would correspond with the
hydrocarbon CHCH_{3} = C_{2}H_{4}, which is isomeric with ethylene,
and it cannot be derived from methane. If such an isomeride existed
it would be derived from CH_{2}, but such products are, up to the
present, unknown. In such cases the insufficiency of the points of
departure of the statical structural teaching is shown. It first
admits constant atomicity and then rejects it, the facts serving to
establish either one or the other view; and therefore it seems to
me that we must come to the conclusion that the structural method
of reasoning, having done a service to science, has outlived the
age, and must be regenerated, as in their time was the teaching of
the electro-chemists, the radicalists, and the adherents of the
doctrine of types. As we cannot now lean on the views above stated,
it is time to abandon the structural theory. They will all be
united in chemical mechanics, and the principle of substitution
must be looked on only as a preparation for the coming epoch in
chemistry, where such cases as the isomerism of fumaric and maleic
acids, when explained dynamically, as proposed by Le Bel and Van't
Hoff, may yield points of departure.
Thus, by applying the principle of substitution, we can, in the simplest
manner, derive not only every kind of hydrocarbon compound, such as the
alcohols, the aldehyde-alcohols, aldehydes, alcohol-acids, and the acids,
but also combinations analogous to hydrated crystals which usually are
disregarded.
But even those unsaturated substances, of which ethylene, CH_{2}CH_{2},
and acetylene, CHCH, are types, may be evolved with equal simplicity.
With respect to the phenomena of isomerism, there are many possibilities
among the hydrocarbon compounds containing two atoms of carbon, and
without going into details it will be sufficient to indicate that the
following formulæ, though not identical, will be isomeric substantially
among themselves:--CH_{3}CHX_{2} and CH_{2}XCH_{2}X, although both
contain C_{2}H_{4}X_{2}; or CH_{2}CX_{2} and CHXCHX, although both
contain C_{2}H_{2}X_{2}, if by X we indicate chlorine or generally an
element capable of replacing one atom of hydrogen, or capable of uniting
with it. To isomerism of this kind belongs the case of aldehyde and the
oxide of ethylene, to which we have already referred, because both have
the composition C_{2}H_{4}O.
What I have said appears to me sufficient to show that the principle of
substitution adequately explains the composition, the isomerism, and all
the diversity of combination of the hydrocarbons, and I shall limit the
further development of these views to preparing a complete list of every
possible hydrocarbon compound containing three atoms of carbon in the
molecule. There are eight in all, of which only five are known at
present.[3]
[3] Conceding variable atomicity, the structuralists must expect an
incomparably larger number of isomerides, and they cannot now
decline to acknowledge the change of atomicity, were it only for
the examples HgCl and HgCl_{2}, CO and CO_{2}, PCl_{3} and PCl_{5}.
Among those possible for C_{3}H_{6} there should be two isomerides,
propylene and trimethylene, and they are both already known. For
C_{3}H_{4} there should be three isomerides: allylene and allene are
known, but the third has not yet been discovered; and for C_{3}H_{2}
there should be two isomerides, though neither of them is known as yet.
Their composition and structure are easily deduced from ethane, ethylene,
and acetylene, by methylation, by methylenation, by acetylenation and by
carbonation.
1. C_{3}H_{8} = CH_{3}CH_{2}CH_{3} out of CH_{3}CH_{3} by methylation.
This hydrocarbon is named propane.
2. C_{3}H_{6} = CH_{3}CHCH_{2} out of CH_{3}CH_{3} by methylenation. This
substance is propylene.
3. C_{3}H_{6} = CH_{2}CH_{2}CH_{2} out of CH_{3}CH_{3} by methylenation.
This substance is trimethylene.
4. C_{3}H_{4} = CH_{3}CCH out of CH_{3}CH_{3} by acetylenation or from
CHCH by methylation. This hydrocarbon is named allylene.
5. C_{3}H_{4} = CHCH/CH_{2} out of CH_{3}CH_{3} by acetylenation, or from
CH_{2}CH_{2} by methylenation, because CH_{2}CH/CH = CHCH/CH_{2}. This
body is as yet unknown.
6. C_{3}H_{4} = CH_{2}CCH_{2} out of CH_{2}CH_{2} by methylenation. This
hydrocarbon is named allene, or iso-allylene.
7. C_{3}H_{2} = CHCH/C out of CH_{3}CH_{3} by symmetrical carbonation, or
out of CH_{2}CH_{2} by acetylenation. This compound is unknown.
8. C_{3}H_{2} = CC/CH_{2} out of CH_{3}CH_{3} by carbonation, or out of
CHCH by methylenation. This compound is unknown.
If we bear in mind that for each hydrocarbon serving as a type in the
above tables there are a number of corresponding derivatives, and that
every compound obtained may, by further methylation, methylenation,
acetylenation, and carbonation, produce new hydrocarbons, and these may
be followed by a numerous suite of derivatives and an immense number of
isomeric substances, it is possible to understand the limitless number of
carbon compounds, although they all have the one substance, methane, for
their origin. The number of substances is so enormous that it is no
longer a question of enlarging the possibilities of discovery, but rather
of finding some means of testing them analogous to the well-known two
which for a long time have served as gauges for all carbon compounds.
I refer to the law of even numbers and to that of limits, the first
enunciated by Gerhardt some forty years ago, with respect to
hydrocarbons, namely, that their molecules always contain an even number
of atoms of hydrogen. But by the method which I have used of deriving all
the hydrocarbons from methane, CH_{4}, this law may be deduced as a
direct consequence of the principle of substitutions. Accordingly, in
methylation, CH_{3} takes the place of H, and therefore CH_{2} is added.
In methylenation the number of atoms of hydrogen remains unchanged, and
at each acetylenation it is reduced by two, and in carbonation by four,
atoms--that is to say, an even number of atoms of hydrogen is always
added or removed. And because the fundamental hydrocarbon, methane,
CH_{4}, contains an even number of atoms of hydrogen, all its derivative
hydrocarbons will also contain even numbers of hydrogen, and this
constitutes the law of even numbers.
The principle of substitutions explains with equal simplicity the
conception of the limiting compositions of hydrocarbons C_{_n_}H_{2_n_ +
2}, which I derived, in 1861,[4] in an empirical manner from accumulated
materials available at that time, and on the basis of the limits to
combinations worked out by Dr. Frankland for other elements.
[4] 'Essai d'une théorie sur les limites des combinaisons organiques,'
par D. Mendeléeff, 2/11 août 1861, _Bulletin de l'Académie i. d.
Sc. de St. Pétersbourg_, t. v
Of all the various substitutions the highest proportion of hydrogen is
yielded by methylation, because in that operation alone does the quantity
of hydrogen increase; hence, taking methane as a point of departure, if
we imagine methylation effected (_n_ - 1) times we obtain hydrocarbon
compounds containing the highest quantities of hydrogen. It is evident
that they will contain CH_{4} + (_n_ - 1)CH_{2}, or C_{_n_}H_{2_n_} +
{2}, because methylation leads to the addition of CH_{2} to the compound.
It will thus be seen that by the principle of substitution--that is to
say, by the third law of Newton--we are able to deduce, in the simplest
manner, not only the individual composition, the isomerism, and relations
of substances, but also the general laws which govern their most complex
combinations without having recourse either to statical constructions, to
the definition of atomicities, to the exclusion of free affinities, or to
the recognition of those single, double or treble bonds which are so
indispensable to structuralists in the explanation of the composition and
construction of hydrocarbon compounds. And yet, by the application of the
dynamical principles of Newton, we can attain to that chief and
fundamental object, the comprehension of isomerism in hydrocarbon
compounds, and the forecasting of the existence of combinations as yet
unknown, by which the edifice raised by structural teaching is
strengthened and supported. Besides--and I count this for a circumstance
of special importance--the process which I advocate will make no
difference in those special cases which have been already so well worked
out, such as, for example, the isomerism of the hydrocarbons and
alcohols, even to the extent of not interfering with the nomenclature
which has been adopted, and the structural system will retain all the
glory of having worked up, in a thoroughly scientific manner, the store
of information which Gerhardt had accumulated about the middle of the
fifties, and the still higher glory of establishing the rational
synthesis of organic substances. Nothing will be lost to the structural
doctrine except its statical origin; and as soon as it will embrace the
dynamic principles of Newton, and suffer itself to be guided by them, I
believe that we shall attain for chemistry that unity of principle which
is now wanting. Many an adept will be attracted to that brilliant and
fascinating enterprise, the penetration into the unseen world of the
kinetic relations of atoms, to the study of which the last twenty-five
years have contributed so much labour and such high inventive faculties.
D'Alembert found in mechanics that if inertia be taken to represent
force, dynamic equations may be applied to statical questions, which are
thereby rendered more simple and more easily understood.
The structural doctrine in chemistry has unconsciously followed the same
course, and therefore its terms are easily adopted; they may retain their
present forms provided that a truly dynamical--that is to say,
Newtonian--meaning be ascribed to them.
Before finishing my task and demonstrating the possibility of adapting
structural doctrines to the dynamics of Newton, I consider it
indispensable to touch on one question which naturally arises, and which
I have heard discussed more than once. If bromine, the atom of which is
eighty times heavier than that of hydrogen, takes the place of hydrogen,
it would seem that the whole system of dynamic equilibrium must be
destroyed.
Without entering into the minute analysis of this question, I think it
will be sufficient to examine it by the light of two well-known
phenomena, one of which will be found in the department of chemistry and
the other in that of celestial mechanics, and both will serve to
demonstrate the existence of that unity in the plan of creation which is
a consequence of the Newtonian doctrines. Experiments demonstrate that
when a heavy element is substituted for a light one in a chemical
compound--for example, for magnesium, in the oxide of that metal, an atom
of mercury, which is 8-1/3 times heavier--the chief chemical
characteristics or properties are generally, though not always,
preserved.
The substitution of silver for hydrogen, than which it is 108 times
heavier, does not affect all the properties of the substance, though it
does some. Therefore chemical substitutions of this kind--the
substitution of light for heavy atoms--need not necessarily entail
changes in the original equilibrium; and this point is still further
elucidated by the consideration that the periodic law indicates the
degree of influence of an increment of weight in the atom as affecting
the possible equilibria, and also what degree of increase in the weight
of the atoms reproduces some, though not all, of the properties of the
substance.
This tendency to repetition--these periods--may be likened to those
annual or diurnal periods with which we are so familiar on the earth.
Days and years follow each other, but, as they do so, many things change;
and in like manner chemical evolutions, changes in the masses of the
elements, permit of much remaining undisturbed, though many properties
undergo alteration. The system is maintained according to the laws of
conservation in nature, but the motions are altered in consequence of the
change of parts.
Next, let us take an astronomical case--such, for example, as the earth
and the moon--and let us imagine that the mass of the latter is
constantly increasing. The question is, what will then occur? The path of
the moon in space is a wave-line similar to that which geometricians have
named epicycloidal, or the locus of a point in a circle rolling round
another circle. But in consequence of the influence of the moon it is
evident that the path of the earth itself cannot be a geometric ellipse,
even supposing the sun to be immovably fixed; it must be an epicycloidal
curve, though not very far removed from the true ellipse--that is to say,
it will be impressed with but faint undulations. It is only the common
centre of gravity of the earth and the moon which describes a true
ellipse round the sun. If the moon were to increase, the relative
undulations of the earth's path would increase in amplitude, those of the
moon would also change, and when the mass of the moon had increased to an
equality with that of the earth, the path would consist of epicycloidal
curves crossing each other, and having opposite phases. But a similar
relation exists between the sun and the earth, because the former is also
moving in space. We may apply these views to the world of atoms, and
suppose that in their movements, when heavy ones take the place of those
that are lighter, similar changes take place, provided that the system or
the molecule is preserved throughout the change.
It seems probable that in the heavenly systems, during incalculable
astronomical periods, changes have taken place and are still going on
similar to those which pass rapidly before our eyes during the chemical
reaction of molecules, and the progress of molecular mechanics may--we
hope will--in course of time permit us to explain those changes in the
stellar world which have more than once been noticed by astronomers, and
which are now so carefully studied. A coming Newton will discover the
laws of these changes. Those laws, when applied to chemistry, may exhibit
peculiarities, but these will certainly be mere variations on the grand
harmonious theme which reigns in nature. The discovery of the laws which
produce this harmony in chemical evolution will only be possible, it
seems to me, under the banner of Newtonian dynamics, which has so long
waved over the domains of mechanics, astronomy, and physics. In calling
chemists to take their stand under its peaceful and catholic shadow I
imagine that I am aiding in establishing that scientific union which the
managers of the Royal Institution wish to effect, who have shown their
desire to do so by the flattering invitation which has given me--a
Russian--the opportunity of laying before the countrymen of Newton an
attempt to apply to chemistry one of his immortal principles.
APPENDIX II
THE PERIODIC LAW OF THE CHEMICAL ELEMENTS
BY PROFESSOR MENDELÉEFF
FARADAY LECTURE DELIVERED BEFORE THE FELLOWS OF THE
CHEMICAL SOCIETY IN THE THEATRE OF THE ROYAL INSTITUTION,
ON TUESDAY, JUNE 4, 1889
The high honour bestowed by the Chemical Society in inviting me to pay a
tribute to the world-famed name of Faraday by delivering this lecture has
induced me to take for its subject the Periodic Law of the Elements--this
being a generalisation in chemistry which has of late attracted much
attention.
While science is pursuing a steady onward movement, it is convenient
from time to time to cast a glance back on the route already traversed,
and especially to consider the new conceptions which aim at discovering
the general meaning of the stock of facts accumulated from day to day in
our laboratories. Owing to the possession of laboratories, modern science
now bears a new character, quite unknown, not only to antiquity, but even
to the preceding century. Bacon's and Descartes' idea of submitting the
mechanism of science simultaneously to experiment and reasoning has been
fully realised in the case of chemistry, it having become not only
possible but always customary to experiment. Under the all-penetrating
control of experiment, a new theory, even if crude, is quickly
strengthened, provided it be founded on a sufficient basis; the
asperities are removed, it is amended by degrees, and soon loses the
phantom light of a shadowy form or of one founded on mere prejudice; it
is able to lead to logical conclusions, and to submit to experimental
proof. Willingly or not, in science we all must submit not to what seems
to us attractive from one point of view or from another, but to what
represents an agreement between theory and experiment; in other words, to
demonstrated generalisation and to the approved experiment. Is it long
since many refused to accept the generalisations involved in the law of
Avogadro and Ampère, so widely extended by Gerhardt? We still may hear
the voices of its opponents; they enjoy perfect freedom, but vainly will
their voices rise so long as they do not use the language of demonstrated
facts The striking observations with the spectroscope which have
permitted us to analyse the chemical constitution of distant worlds,
seemed, at first, applicable to the task of determining the nature of the
atoms themselves; but the working out of the idea in the laboratory soon
demonstrated that the characters of spectra are determined, not directly
by the atoms, but by the molecules into which the atoms are packed; and
so it became evident that more verified facts must be collected before it
will be possible to formulate new generalisations capable of taking their
place beside those ordinary ones based upon the conception of simple
substances and atoms. But as the shade of the leaves and roots of living
plants, together with the relics of a decayed vegetation, favour the
growth of the seedling and serve to promote its luxurious development, in
like manner sound generalisations--together with the relics of those
which have proved to be untenable--promote scientific productivity, and
ensure the luxurious growth of science under the influence of rays
emanating from the centres of scientific energy. Such centres are
scientific associations and societies. Before one of the oldest and most
powerful of these I am about to take the liberty of passing in review the
twenty years' life of a generalisation which is known under the name of
the Periodic Law. It was in March 1869 that I ventured to lay before the
then youthful Russian Chemical Society the ideas upon the same subject
which I had expressed in my just written 'Principles of Chemistry.'
Without entering into details, I will give the conclusions I then arrived
at in the very words I used:--
'1. The elements, if arranged according to their atomic weights, exhibit
an evident _periodicity_ of properties.
'2. Elements which are similar as regards their chemical properties have
atomic weights which are either of nearly the same value (_e.g._
platinum, iridium, osmium) or which increase regularly (_e.g._ potassium,
rubidium, cæsium).
'3. The arrangement of the elements, or of groups of elements, in the
order of their atomic weights, corresponds to their so-called _valencies_
as well as, to some extent, to their distinctive chemical properties--as
is apparent, among other series, in that of lithium, beryllium, barium,
carbon, nitrogen, oxygen, and iron.
'4. The elements which are the most widely diffused have _small_ atomic
weights.
'5. The _magnitude_ of the atomic weight determines the character of the
element, just as the magnitude of the molecule determines the character
of a compound.
'6. We must expect the discovery of many yet _unknown_ elements--for
example, elements analogous to aluminium and silicon, whose atomic weight
would be between 65 and 75.
'7. The atomic weight of an element may sometimes be amended by a
knowledge of those of the contiguous elements. Thus, the atomic weight of
tellurium must lie between 123 and 126, and cannot be 128.
'8. Certain characteristic properties of the elements can be foretold
from their atomic weights.
'The aim of this communication will be fully attained if I succeed in
drawing the attention of investigators to those relations which exist
between the atomic weights of dissimilar elements, which, so far as I
know, have hitherto been almost completely neglected. I believe that the
solution of some of the most important problems of our science lies in
researches of this kind.'
To-day, twenty years after the above conclusions were formulated, they
may still be considered as expressing the essence of the now well-known
periodic law.
Reverting to the epoch terminating with the sixties, it is proper to
indicate three series of data without the knowledge of which the periodic
law could not have been discovered, and which rendered its appearance
natural and intelligible.
In the first place, it was at that time that the numerical value of
atomic weights became definitely known. Ten years earlier such knowledge
did not exist, as may be gathered from the fact that in 1860 chemists
from all parts of the world met at Karlsruhe in order to come to some
agreement, if not with respect to views relating to atoms, at any rate as
regards their definite representation. Many of those present probably
remember how vain were the hopes of coming to an understanding, and how
much ground was gained at that Congress by the followers of the unitary
theory so brilliantly represented by Cannizzaro. I vividly remember the
impression produced by his speeches, which admitted of no compromise, and
seemed to advocate truth itself, based on the conceptions of Avogadro,
Gerhardt, and Regnault, which at that time were far from being generally
recognised. And though no understanding could be arrived at, yet the
objects of the meeting were attained, for the ideas of Cannizzaro proved,
after a few years, to be the only ones which could stand criticism, and
which represented an atom as--'the smallest portion of an element which
enters into a molecule of its compound.' Only such real atomic
weights--not conventional ones--could afford a basis for generalisation.
It is sufficient, by way of example, to indicate the following cases in
which the relation is seen at once and is perfectly clear:--
K = 39 Rb = 85 Cs = 133
Ca = 40 Sr = 87 Ba = 137
whereas with the equivalents then in use--
K = 39 Rb = 85 Cs = 133
Ca = 20 Sr = 43·5 Ba = 68·5
the consecutiveness of change in atomic weight, which with the true
values is so evident, completely disappears.
Secondly, it had become evident during the period 1860-70, and even
during the preceding decade, that the relations between the atomic
weights of analogous elements were governed by some general and simple
laws. Cooke, Cremers, Gladstone, Gmelin, Lenssen, Pettenkofer, and
especially Dumas, had already established many facts bearing on that
view. Thus Dumas compared the following groups of analogous elements with
organic radicles:--
Diff. Diff. Diff. Diff.
Mg = 12} P = 31} O = 8}
}8 }44 }8
Li = 7 } Ca = 20} As= 75} S = 16}
}16 }3 × 8 }44 }3 × 8
Na = 23} Sr = 44} Sb = 119} Se = 40}
}16 }3 × 8 }2 × 44 }3 × 8
K = 39 } Ba = 68} Bi = 207} Te = 64}
and pointed out some really striking relationships, such as the
following:--
F = 19.
Cl = 35·5 = 19 + 16·5.
Br = 80 = 19 + 2 × 16·5 + 28.
I = 127 = 2 x 19 + 2 × 16·5 + 2 × 28.
A. Strecker, in his work 'Theorien und Experimente zur Bestimmung der
Atomgewichte der Elemente' (Braunschweig, 1859), after summarising the
data relating to the subject, and pointing out the remarkable series of
equivalents--
Cr = 26·2 Mn = 27·6 Fe = 28 Ni = 29 Co = 30 Cu = 31·7 Zn = 32·5
remarks that: 'It is hardly probable that all the above-mentioned
relations between the atomic weights (or equivalents) of chemically
analogous elements are merely accidental. We must, however, leave to the
future the discovery of the _law_ of the relations which appears in these
figures.'[1]
[1] 'Es ist wohl kaum anzunehmen, dass alle im Vorhergehenden
hervorgehobenen Beziehungen zwischen den Atomgewichten (oder
Aequivalenten) in chemischen Verhältnissen einander ähnliche
Elemente bloss zufällig sind. Die Auffindung der in diesen Zahlen
_gesetzlichen_ Beziehungen müssen wir jedoch der Zukunft
überlassen.'
In such attempts at arrangement and in such views are to be recognised
the real forerunners of the periodic law; the ground was prepared for it
between 1860 and 1870, and that it was not expressed in a determinate
form before the end of the decade may, I suppose, be ascribed to the fact
that only analogous elements had been compared. The idea of seeking for a
relation between the atomic weights of all the elements was foreign to
the ideas then current, so that neither the _vis tellurique_ of De
Chancourtois, nor the _law of octaves_ of Newlands, could secure
anybody's attention. And yet both De Chancourtois and Newlands like Dumas
and Strecker, more than Lenssen and Pettenkofer, had made an approach to
the periodic law and had discovered its germs. The solution of the
problem advanced but slowly, because the facts, but not the law, stood
foremost in all attempts; and the law could not awaken a general interest
so long as elements, having no apparent connection with each other, were
included in the same octave, as for example:--
1st octave of | | | | | | | |
Newlands | H | F | Cl | Co & Ni | Br | Pd | I | Pt & Ir
7th Ditto | O | S | Fe | Se | Rh & Ru | Te | Au | Os or Th
Analogies of the above order seemed quite accidental, and the more so as
the octave contained occasionally ten elements instead of eight, and when
two such elements as Ba and V, Co and Ni, or Rh and Ru, occupied one
place in the octave.[2] Nevertheless, the fruit was ripening, and I now
see clearly that Strecker, De Chancourtois, and Newlands stood foremost
in the way towards the discovery of the periodic law, and that they
merely wanted the boldness necessary to place the whole question at such
a height that its reflection on the facts could be clearly seen.
[2] To judge from J. A. R. Newlands's work, _On the Discovery of the
Periodic Law_, London, 1884, p. 149; 'On the Law of Octaves' (from
the _Chemical News_, 12, 83, August 18, 1865).
A third circumstance which revealed the periodicity of chemical elements
was the accumulation, by the end of the sixties, of new information
respecting the rare elements, disclosing their many-sided relations to
the other elements and to each other. The researches of Marignac on
niobium, and those of Roscoe on vanadium, were of special moment. The
striking analogies between vanadium and phosphorus on the one hand, and
between vanadium and chromium on the other, which became so apparent in
the investigations connected with that element, naturally induced the
comparison of V = 51 with Cr = 52, Nb = 94 with Mo = 96, and Ta = 192
with W = 194; while, on the other hand, P = 31 could be compared with S =
32, As = 75 with Se = 79, and Sb = 120 with Te = 125. From such
approximations there remained but one step to the discovery of the law of
periodicity.
The law of periodicity was thus a direct outcome of the stock of
generalisations and established facts which had accumulated by the end of
the decade 1860-1870; it is an embodiment of those data in a more or less
systematic expression. Where, then, lies the secret of the special
importance which has since been attached to the periodic law, and has
raised it to the position of a generalisation which has already given to
chemistry unexpected aid, and which promises to be far more fruitful in
the future and to impress upon several branches of chemical research a
peculiar and original stamp? The remaining part of my communication will
be an attempt to answer this question.
In the first place we have the circumstance that, as soon as the law made
its appearance, it demanded a revision of many facts which were
considered by chemists as fully established by existing experience. I
shall return, later on, briefly to this subject, but I wish now to remind
you that the periodic law, by insisting on the necessity for a revision
of supposed facts, exposed itself at once to destruction in its very
origin. Its first requirements, however, have been almost entirely
satisfied during the last 20 years; the supposed facts have yielded to
the law, thus proving that the law itself was a legitimate induction from
the verified facts. But our inductions from data have often to do with
such details of a science so rich in facts, that only generalisations
which cover a wide range of important phenomena can attract general
attention. What were the regions touched on by the periodic law? This is
what we shall now consider.
The most important point to notice is, that periodic functions, used for
the purpose of expressing changes which are dependent on variations of
time and space, have been long known. They are familiar to the mind when
we have to deal with motion in closed cycles, or with any kind of
deviation from a stable position, such as occurs in
pendulum-oscillations. A like periodic function became evident in the
case of the elements, depending on the mass of the atom. The primary
conception of the masses of bodies, or of the masses of atoms, belongs to
a category which the present state of science forbids us to discuss,
because as yet we have no means of dissecting or analysing the
conception. All that was known of functions dependent on masses derived
its origin from Galileo and Newton, and indicated that such functions
either decrease or increase with the increase of mass, like the
attraction of celestial bodies. The numerical expression of the phenomena
was always found to be proportional to the mass, and in no case was an
increase of mass followed by a recurrence of properties such as is
disclosed by the periodic law of the elements. This constituted such a
novelty in the study of the phenomena of nature that, although it did not
lift the veil which conceals the true conception of mass, it nevertheless
indicated that the explanation of that conception must be searched for in
the masses of the atoms; the more so, as all masses are nothing but
aggregations, or additions, of chemical atoms which would be best
described as chemical individuals. Let me remark, by the way, that though
the Latin word 'individual' is merely a translation of the Greek word
'atom,' nevertheless history and custom have drawn a sharp distinction
between the two words, and the present chemical conception of atoms is
nearer to that defined by the Latin word than by the Greek, although this
latter also has acquired a special meaning which was unknown to the
classics. The periodic law has shown that our chemical individuals
display a harmonic periodicity of properties dependent on their masses.
Now natural science has long been accustomed to deal with periodicities
observed in nature, to seize them with the vice of mathematical analysis,
to submit them to the rasp of experiment. And these instruments of
scientific thought would surely, long since, have mastered the problem
connected with the chemical elements, were it not for a new feature which
was brought to light by the periodic law, and which gave a peculiar and
original character to the periodic function.
If we mark on an axis of abscissæ a series of lengths proportional to
angles, and trace ordinates which are proportional to sines or other
trigonometrical functions, we get periodic curves of a harmonic
character. So it might seem, at first sight, that with the increase of
atomic weights the function of the properties of the elements should also
vary in the same harmonious way. But in this case there is no such
continuous change as in the curves just referred to, because the periods
do not contain the infinite number of points constituting a curve, but a
_finite_ number only of such points. An example will better illustrate
this view. The atomic weights--
Ag = 108 Cd = 112 In = 113 Sn = 118 Sb = 120
Te = 125 I = 127
steadily increase, and their increase is accompanied by a modification of
many properties which constitutes the essence of the periodic law. Thus,
for example, the densities of the above elements decrease steadily, being
respectively--
10·5 8·6 7·4 7·2 6·7 6·4 4·9
while their oxides contain an increasing quantity of oxygen--
Ag_{2}O Cd_{2}O_{2} In_{2}O_{3} Sn_{2}O_{4} Sb_{2}O_{5}
Te_{2}O_{6} I_{2}O_{7}
But to connect by a curve the summits of the ordinates expressing any of
these properties would involve the rejection of Dalton's law of multiple
proportions. Not only are there no intermediate elements between silver,
which gives AgCl, and cadmium, which gives CdCl_{2}, but, according to
the very essence of the periodic law, there can be none; in fact a
uniform curve would be inapplicable in such a case, as it would lead us
to expect elements possessed of special properties at any point of the
curve. The periods of the elements have thus a character very different
from those which are so simply represented by geometers. They correspond
to points, to numbers, to sudden changes of the masses, and not to a
continuous evolution. In these sudden changes destitute of intermediate
steps or positions, in the absence of elements intermediate between, say,
silver and cadmium, or aluminium and silicon, we must recognise a problem
to which no direct application of the analysis of the infinitely small
can be made. Therefore, neither the trigonometrical functions proposed by
Ridberg and Flavitzky, nor the pendulum-oscillations suggested by
Crookes, nor the cubical curves of the Rev. Mr. Haughton, which have been
proposed for expressing the periodic law, from the nature of the case,
can represent the periods of the chemical elements. If geometrical
analysis is to be applied to this subject, it will require to be modified
in a special manner. It must find the means of representing in a special
way, not only such long periods as that comprising
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br,
but short periods like the following:--
Na Mg Al Si P S Cl.
In the theory of numbers only do we find problems analogous to ours, and
two attempts at expressing the atomic weights of the elements by
algebraic formulæ seem to be deserving of attention, although neither of
them can be considered as a complete theory, nor as promising finally to
solve the problem of the periodic law. The attempt of E. J. Mills (1886)
does not even aspire to attain this end. He considers that all atomic
weights can be expressed by a logarithmic function,
15(_n_ - 0·9375^_t_),
in which the variables _n_ and _t_ are _whole numbers_. Thus, for oxygen,
_n_ = 2, and _t_ = 1, whence its atomic weight is = 15·94; in the case of
chlorine, bromine, and iodine, _n_ has respective values of 3, 6, and 9,
whilst _t_ = 7, 6, and 9; in the case of potassium, rubidium, and cæsium,
_n_ = 4, 6, and 9, and _t_ = 14, 18, and 20.
Another attempt was made in 1888 by B. N. Tchitchérin. Its author places
the problem of the periodic law in the first rank, but as yet he has
investigated the alkali metals only. Tchitchérin first noticed the simple
relations existing between the atomic volumes of all alkali metals; they
can be expressed, according to his views, by the formula
A(2 - 0·00535A_n_),
where A is the atomic weight, and _n_ is equal to 8 for lithium and
sodium, to 4 for potassium, to 3 for rubidium, and to 2 for cæsium. If
_n_ remained equal to 8 during the increase of A, the volume would become
zero at A = 46-2/3, and it would reach its maximum at A = 23-1/3. The
close approximation of the number 46-2/3 to the differences between the
atomic weights of analogous elements (such as Cs - Rb, I - Br, and so
on); the close correspondence of the number 23-1/3 to the atomic weight
of sodium; the fact of _n_ being necessarily a whole number, and several
other aspects of the question, induce Tchitchérin to believe that they
afford a clue to the understanding of the nature of the elements; we
must, however, await the full development of his theory before
pronouncing judgment on it. What we can at present only be certain of is
this: that attempts like the two above named must be repeated and
multiplied, because the periodic law has clearly shown that the masses of
the atoms increase abruptly, by steps, which are clearly connected in
some way with Dalton's law of multiple proportions; and because the
periodicity of the elements finds expression in the transition from RX to
RX_{2}, RX_{3}, RX_{4}, and so on till RX_{8}, at which point, the energy
of the combining forces being exhausted, the series begins anew from RX
to RX_{2}, and so on.
While connecting by new bonds the theory of the chemical elements with
Dalton's theory of multiple proportions, or atomic structure of bodies,
the periodic law opened for natural philosophy a new and wide field for
speculation. Kant said that there are in the world 'two things which
never cease to call for the admiration and reverence of man: the moral
law within ourselves, and the stellar sky above us.' But when we turn our
thoughts towards the nature of the elements and the periodic law, we must
add a third subject, namely, 'the nature of the elementary individuals
which we discover everywhere around us.' Without them the stellar sky
itself is inconceivable; and in the atoms we see at once their peculiar
individualities, the infinite multiplicity of the individuals, and the
submission of their seeming freedom to the general harmony of Nature.
Having thus indicated a new mystery of Nature, which does not yet yield
to rational conception, the periodic law, together with the revelations
of spectrum analysis, have contributed to again revive an old but
remarkably long-lived hope--that of discovering, if not by experiment, at
least by a mental effort, the _primary matter_--which had its genesis in
the minds of the Grecian philosophers, and has been transmitted, together
with many other ideas of the classic period, to the heirs of their
civilisation. Having grown, during the times of the alchemists up to the
period when experimental proof was required, the idea has rendered good
service; it induced those careful observations and experiments which
later on called into being the works of Scheele, Lavoisier, Priestley,
and Cavendish. It then slumbered awhile, but was soon awakened by the
attempts either to confirm or to refute the ideas of Prout as to the
multiple proportion relationship of the atomic weights of all the
elements. And once again the inductive or experimental method of studying
Nature gained a direct advantage from the old Pythagorean idea: because
atomic weights were determined with an accuracy formerly unknown. But
again the idea could not stand the ordeal of experimental test, yet the
prejudice remains and has not been uprooted, even by Stas; nay, it has
gained a new vigour, for we see that all which is imperfectly worked out,
new and unexplained, from the still scarcely studied rare metals to the
hardly perceptible nebulæ, have been used to justify it. As soon as
spectrum analysis appears as a new and powerful weapon of chemistry, the
idea of a primary matter is immediately attached to it. From all sides we
see attempts to constitute the imaginary substance _helium_[3] the so
much longed for primary matter. No attention is paid to the circumstance
that the helium line is only seen in the spectrum of the solar
protuberances, so that its universality in Nature remains as problematic
as the primary matter itself; nor to the fact that the helium line is
wanting amongst the Fraunhofer lines of the solar spectrum, and thus does
not answer to the brilliant fundamental conception which gives its real
force to spectrum analysis.
[3] That is, a substance having a wave-length equal to 0·0005875
millimetre.
And finally, no notice is even taken of the indubitable fact that the
brilliancies of the spectral lines of the simple substances vary under
different temperatures and pressures; so that all probabilities are in
favour of the helium line simply belonging to some long since known
element placed under such conditions of temperature, pressure, and
gravity as have not yet been realised in our experiments. Again, the idea
that the excellent investigations of Lockyer of the spectrum of iron can
be interpreted in favour of the compound nature of that element,
evidently must have arisen from some misunderstanding. The spectrum of a
compound certainly does not appear as a sum of the spectra of its
components; and therefore the observations of Lockyer can be considered
precisely as a proof that iron undergoes no other changes at the
temperature of the sun than those which it experiences in the voltaic
arc--provided the spectrum of iron is preserved. As to the shifting of
some of the lines of the spectrum of iron while the other lines maintain
their positions, it can be explained, as shown by M. Kleiber ('Journal of
the Russian Chemical and Physical Society,' 1885, 147), by the relative
motion of the various strata of the sun's atmosphere, and by Zöllner's
laws of the relative brilliancies of different lines of the spectrum.
Moreover, it ought not to be forgotten that if iron were really proved to
consist of two or more unknown elements, we should simply have an
increase in the number of our elements--not a reduction, and still less a
reduction of all of them to one single primary matter.
Feeling that spectrum analysis will not yield a support to the
Pythagorean conception, its modern promoters are so bent upon its being
confirmed by the periodic law, that the illustrious Berthelot, in his
work 'Les origines de l'Alchimie,' 1885, 313, has simply mixed up the
fundamental idea of the law of periodicity with the ideas of Prout, the
alchemists, and Democritus about primary matter.[4] But the periodic law,
based as it is on the solid and wholesome ground of experimental
research, has been evolved independently of any conception as to the
nature of the elements; it does not in the least originate in the idea of
a unique matter; and it has no historical connection with that relic of
the torments of classical thought, and therefore it affords no more
indication of the unity of matter or of the compound character of our
elements, than the law of Avogadro, or the law of specific heats, or even
the conclusions of spectrum analysis. None of the advocates of a unique
matter have ever tried to explain the law from the standpoint of ideas
taken from a remote antiquity when it was found convenient to admit the
existence of many gods--and of a unique matter.
[4] He maintains (on p. 309) that the periodic law requires two new
analogous elements, having atomic weights of 48 and 64, occupying
positions between sulphur and selenium, although nothing of the
kind results from any of the different readings of the law.
When we try to explain the origin of the idea of a unique primary matter,
we easily trace that in the absence of inductions from experiment it
derives its origin from the scientifically philosophical attempt at
discovering some kind of unity in the immense diversity of
individualities which we see around. In classical times such a tendency
could only be satisfied by conceptions about the immaterial world. As to
the material world, our ancestors were compelled to resort to some
hypothesis, and they adopted the idea of unity in the formative material,
because they were not able to evolve the conception of any other possible
unity in order to connect the multifarious relations of matter.
Responding to the same legitimate scientific tendency, natural science
has discovered throughout the universe a unity of plan, a unity of
forces, and a unity of matter, and the convincing conclusions of modern
science compel every one to admit these kinds of unity. But while we
admit unity in many things, we none the less must also explain the
individuality and the apparent diversity which we cannot fail to trace
everywhere. It has been said of old, 'Give us a fulcrum, and it will
become easy to displace the earth.' So also we must say, 'Give us
something that is individualised, and the apparent diversity will be
easily understood.' Otherwise, how could unity result in a multitude?
After a long and painstaking research, natural science has discovered
the individualities of the chemical elements, and therefore it is now
capable not only of analysing, but also of synthesising; it can
understand and grasp generality and unity, as well as the individualised
and the multifarious. The general and universal, like time and space,
like force and motion, vary uniformly; the uniform admit of
interpolations, revealing every intermediate phase. But the
multitudinous, the individualised--such as ourselves, or the chemical
elements, or the members of a peculiar periodic function of the elements,
or Dalton's multiple proportions--is characterised in another way: we see
in it, side by side with a connecting general principle, leaps, breaks of
continuity, points which escape from the analysis of the infinitely
small--an absence of complete intermediate links. Chemistry has found an
answer to the question as to the causes of multitudes; and while
retaining the conception of many elements, all submitted to the
discipline of a general law, it offers an escape from the Indian
Nirvana--the absorption in the universal, replacing it by the
individualised. However, the place for individuality is so limited by the
all-grasping, all-powerful universal, that it is merely a point of
support for the understanding of multitude in unity.
Having touched upon the metaphysical bases of the conception of a unique
matter which is supposed to enter into the composition of all bodies. I
think it necessary to dwell upon another theory, akin to the above
conception--the theory of the compound character of the elements now
admitted by some--and especially upon one particular circumstance which,
being related to the periodic law, is considered to be an argument in
favour of that hypothesis.
Dr. Pelopidas, in 1883, made a communication to the Russian Chemical and
Physical Society on the periodicity of the hydrocarbon radicles, pointing
out the remarkable parallelism which was to be noticed in the change of
properties of hydrocarbon radicles and elements when classed in groups.
Professor Carnelley, in 1886, developed a similar parallelism. The idea
of M. Pelopidas will be easily understood if we consider the series of
hydrocarbon radicles which contain, say, 6 atoms of carbon:--
I. II. III. IV. V.
C_{6}H_{13} C_{6}H_{12} C_{6}H_{11} C_{6}H_{10} C_{6}H_{9}
VI. VII. VIII.
C_{6}H_{8} C_{6}H_{7} C_{6}H_{6}
The first of these radicles, like the elements of the 1st group, combines
with Cl, OH, and so on, and gives the derivatives of hexyl alcohol,
C_{6}H_{13}(OH); but, in proportion as the number of hydrogen atoms
decreases, the capacity of the radicles of combining with, say, the
halogens increases. C_{6}H_{12} already combines with 2 atoms of
chlorine; C_{6}H_{11}, with 3 atoms, and so on. The last members of the
series comprise the radicles of acids: thus C_{6}H_{8}, which belongs to
the 6th group, gives, like sulphur, a bibasic acid,
C_{6}H_{8}O_{2}(OH)_{2}, which is homologous with oxalic acid. The
parallelism can be traced still further, because C_{6}H_{5} appears as a
monovalent radicle of benzene, and with it begins a new series of
aromatic derivatives, so analogous to the derivatives of the aliphatic
series. Let me also mention another example from among those which have
been given by M. Pelopidas. Starting from the alkaline radicle of
monomethylammonium, N(CH_{3})H_{3}, or NCH_{6}, which presents many
analogies with the alkaline metals of the 1st group, he arrives, by
successively diminishing the number of the atoms of hydrogen, at a 7th
group which contains cyanogen, CN, which has long since been compared to
the halogens of the 7th group.
The most important consequence which, in my opinion, can be drawn from
the above comparison is that the periodic law, so apparent in the
elements, has a wider application than might appear at first sight; it
opens up a new vista of chemical evolutions. But, while admitting the
fullest parallelism between the periodicity of the elements and that of
the compound radicles, we must not forget that in the periods of the
hydrocarbon radicles we have a _decrease_ of mass as we pass from the
representatives of the first group to the next, while in the periods of
the elements the mass _increases_ during the progression. It thus becomes
evident that we cannot speak of an identity of periodicity in both cases,
unless we put aside the ideas of mass and attraction, which are the real
corner-stones of the whole of natural science, and even enter into those
very conceptions of simple substances which came to light a full hundred
years later than the immortal principles of Newton.[5]
[5] It is noteworthy that the year in which Lavoisier was born
(1743)--the author of the idea of elements and of the
indestructibility of matter--is later by exactly one century than
the year in which the author of the theory of gravitation and mass
was born (1643 N.S.). The affiliation of the ideas of Lavoisier and
those of Newton is beyond doubt.
From the foregoing, as well as from the failures of so many attempts at
finding in experiment and speculation a proof of the compound character
of the elements and of the existence of primordial matter, it is evident,
in my opinion, that this theory must be classed among mere utopias. But
utopias can only be combated by freedom of opinion, by experiment, and by
new utopias. In the republic of scientific theories freedom of opinions
is guaranteed. It is precisely that freedom which permits me to criticise
openly the widely-diffused idea as to the unity of matter in the
elements. Experiments and attempts at confirming that idea have been so
numerous that it really would be instructive to have them all collected
together, if only to serve as a warning against the repetition of old
failures. And now as to new utopias which may be helpful in the struggle
against the old ones, I do not think it quite useless to mention a
_phantasy_ of one of my students who imagined that the weight of bodies
does not depend upon their mass, but upon the character of the motion of
their atoms. The atoms, according to this new utopian, may all be
homogeneous or heterogeneous, we know not which; we know them in motion
only, and that motion they maintain with the same persistence as the
stellar bodies maintain theirs. The weights of atoms differ only in
consequence of their various modes and quantity of motion; the heaviest
atoms may be much simpler than the lighter ones: thus an atom of mercury
may be simpler than an atom of hydrogen--the manner in which it moves
causes it to be heavier. My interlocutor even suggested that the view
which attributes the greater complexity to the lighter elements finds
confirmation in the fact that the hydrocarbon radicles mentioned by
Pelopidas, while becoming lighter as they lose hydrogen, change their
properties periodically in the same manner as the elements change theirs,
according as the atoms grow heavier.
The French proverb, _La critique est facile, mais l'art est difficile_,
however, may well be reversed in the case of all such ideal views, as it
is much easier to formulate than to criticise them. Arising from the
virgin soil of newly-established facts, the knowledge relating to the
elements, to their masses, and to the periodic changes of their
properties has given a motive for the formation of utopian hypotheses,
probably because they could not be foreseen by the aid of any of the
various metaphysical systems, and exist, like the idea of gravitation, as
an independent outcome of natural science, requiring the acknowledgment
of general laws, when these have been established with the same degree of
persistency as is indispensable for the acceptance of a thoroughly
established fact. Two centuries have elapsed since the theory of
gravitation was enunciated, and although we do not understand its cause,
we still must regard gravitation as a fundamental conception of natural
philosophy, a conception which has enabled us to perceive much more than
the metaphysicians did or could with their seeming omniscience. A hundred
years later the conception of the elements arose; it made chemistry what
it now is; and yet we have advanced as little in our comprehension of
simple substances since the times of Lavoisier and Dalton as we have in
our understanding of gravitation. The periodic law of the elements is
only twenty years old; it is not surprising, therefore, that, knowing
nothing about the causes of gravitation and mass, or about the nature of
the elements, we do not comprehend the _rationale_ of the periodic law.
It is only by collecting established laws--that is, by working at the
acquirement of truth--that we can hope gradually to lift the veil which
conceals from us the causes of the mysteries of Nature and to discover
their mutual dependency. Like the telescope and the microscope, laws
founded on the basis of experiment are the instruments and means of
enlarging our mental horizon.
In the remaining part of my communication I shall endeavour to show, and
as briefly as possible, in how far the periodic law contributes to
enlarge our range of vision. Before the promulgation of this law the
chemical elements were mere fragmentary, incidental facts in Nature;
there was no special reason to expect the discovery of new elements, and
the new ones which were discovered from time to time appeared to be
possessed of quite novel properties. The law of periodicity first enabled
us to perceive undiscovered elements at a distance which formerly was
inaccessible to chemical vision; and long ere they were discovered new
elements appeared before our eyes possessed of a number of well-defined
properties. We now know three cases of elements whose existence and
properties were foreseen by the instrumentality of the periodic law. I
need but mention the brilliant discovery of _gallium_, which proved to
correspond to eka-aluminium of the periodic law, by Lecoq de Boisbaudran;
of _scandium_, corresponding to ekaboron, by Nilson; and of _germanium_,
which proved to correspond in all respects to ekasilicon, by Winkler.
When, in 1871, I described to the Russian Chemical Society the
properties, clearly defined by the periodic law, which such elements
ought to possess, I never hoped that I should live to mention their
discovery to the Chemical Society of Great Britain as a confirmation of
the exactitude and the generality of the periodic law. Now that I have
had the happiness of doing so, I unhesitatingly say that, although
greatly enlarging our vision, even now the periodic law needs further
improvements in order that it may become a trustworthy instrument in
further discoveries.[6]
[6] I foresee some more new elements, but not with the same certitude
as before. I shall give one example, and yet I do not see it quite
distinctly. In the series which contains Hg = 204, Pb = 206, and Bi
= 208, we can imagine the existence (at the place VI-11) of an
element analogous to tellurium, which we can describe as
dvi-tellurium, Dt, having an atomic weight of 212, and the property
of forming the oxide DtO_{3}. If this element really exists, it
ought in the free state to be an easily fusible, crystalline,
non-volatile metal of a grey colour, having a density of about 9·3,
capable of giving a dioxide, DtO_{2}, equally endowed with feeble
acid and basic properties. This dioxide must give on active
oxidation an unstable higher oxide, DtO_{3}, which should resemble
in its properties PbO_{2} and Bi_{2}O_{5}. Dvi-tellurium hydride,
if it be found to exist, will be a less stable compound than even
H_{2}Te. The compounds of dvi-tellurium will be easily reduced, and
it will form characteristic definite alloys with other metals.
I will venture to allude to some other matters which chemistry has
discerned by means of its new instrument, and which it could not have
made out without a knowledge of the law of periodicity, and I will
confine myself to simple substances and to oxides.
Before the periodic law was formulated the atomic weights of the elements
were purely empirical numbers, so that the magnitude of the equivalent,
and the atomicity, or the value in substitution possessed by an atom,
could only be tested by critically examining the methods of
determination, but never directly by considering the numerical values
themselves; in short, we were compelled to move in the dark, to submit to
the facts, instead of being masters of them. I need not recount the
methods which permitted the periodic law at last to master the facts
relating to atomic weights, and I would merely call to mind that it
compelled us to modify the valencies of _indium_ and _cerium_, and to
assign to their compounds a different molecular composition.
Determinations of the specific heats of these two metals fully confirmed
the change. The trivalency of _yttrium_, which makes us now represent its
oxide as Y_{2}O_{3} instead of as YO, was also foreseen (in 1870) by the
periodic law, and it has now become so probable that Clève, and all other
subsequent investigators of the rare metals, have not only adopted it,
but have also applied it without any new demonstration to substances so
imperfectly known as those of the cerite and gadolinite group, especially
since Hillebrand determined the specific heats of lanthanum and didymium
and confirmed the expectations suggested by the periodic law. But here,
especially in the case of didymium, we meet with a series of difficulties
long since foreseen through the periodic law, but only now becoming
evident, and chiefly arising from the relative rarity and insufficient
knowledge of the elements which usually accompany didymium.
Passing to the results obtained in the case of the rare elements
_beryllium_, _scandium_, and _thorium_, it is found that these have many
points of contact with the periodic law. Although Avdéeff long since
proposed the magnesia formula to represent beryllium oxide, yet there was
so much to be said in favour of the alumina formula, on account of the
specific heat of the metals and the isomorphism of the two oxides, that
it became generally adopted and seemed to be well established. The
periodic law, however, as Brauner repeatedly insisted ('Berichte,' 1878,
872; 1881, 53), was against the formula Be_{2}O_{3}; it required the
magnesia formula BeO--that is, an atomic weight of 9--because there was
no place in the system for an element like beryllium having an atomic
weight of 13·5. This divergence of opinion lasted for years, and I often
heard that the question as to the atomic weight of beryllium threatened
to disturb the generality of the periodic law, or, at any rate, to
require some important modifications of it. Many forces were operating in
the controversy regarding beryllium, evidently because a much more
important question was at issue than merely that involved in the
discussion of the atomic weight of a relatively rare element: and during
the controversy the periodic law became better understood, and the mutual
relations of the elements became more apparent than ever before. It is
most remarkable that the victory of the periodic law was won by the
researches of the very observers who previously had discovered a number
of facts in support of the trivalency of beryllium. Applying the higher
law of Avogadro, Nilson and Petterson have finally shown that the density
of the vapour of the beryllium chloride, BeCl_{2}, obliges us to regard
beryllium as bivalent in conformity with the periodic law.[7] I consider
the confirmation of Avdéeff's and Brauner's view as important in the
history of the periodic law as the discovery of scandium, which, in
Nilson's hands, confirmed the existence of ekaboron.
[7] Let me mention another proof of the bivalency of beryllium which
may have passed unnoticed, as it was only published in the Russian
chemical literature. Having remarked (in 1884) that the density of
such solutions of chlorides of metals, MCl_{_n_}, as contain 200
mols. of water (or a large and constant amount of water) regularly
increases as the molecular weight of the dissolved salt increases,
I proposed to one of our young chemists, M. Burdakoff, that he
should investigate beryllium chloride. If its molecule be BeCl_{2}
its weight must be = 80; and in such a case it must be heavier than
the molecule of KCl = 74·5, and lighter than that of MgCl_{2}, =
93. On the contrary, if beryllium chloride is a trichloride,
BeCl_{3} = 120, its molecule must be heavier than that of CaCl_{2}
= 111, and lighter than that of MnCl_{2} = 126. Experiment has
shown the correctness of the former formula, the solution BeCl_{2}
+ 200H_{2}O having (at 15°/4°) a density of 1·0138, this being a
higher density than that of the solution KCl + 200H_{2}O (=
1·0121), and lower than that of MgCl_{2} + 200H_{2}O (= 1·0203).
The bivalency of beryllium was thus confirmed in the case both of
the dissolved and the vaporised chloride.
The circumstance that _thorium_ proved to be quadrivalent, and Th = 232,
in accordance with the views of Chydenius and the requirements of the
periodic law, passed almost unnoticed, and was accepted without
opposition, and yet both thorium and uranium are of great importance in
the periodic system, as they are its last members, and have the highest
atomic weights of all the elements.
The alteration of the atomic weight of _uranium_ from U = 120 into U =
240 attracted more attention, the change having been made on account of
the periodic law, and for no other reason. Now that Roscoe, Rammelsberg,
Zimmermann, and several others have admitted the various claims of the
periodic law in the case of uranium, its high atomic weight is received
without objection, and it endows that element with a special interest.
While thus demonstrating the necessity for modifying the atomic weights
of several insufficiently known elements, the periodic law enabled us
also to detect errors in the determination of the atomic weights of
several elements whose valencies and true position among other elements
were already well known. Three such cases are especially noteworthy:
those of tellurium, titanium and platinum. Berzelius had determined the
atomic weight of _tellurium_ to be 128, while the periodic law claimed
for it an atomic weight below that of iodine, which had been fixed by
Stas at 126·5, and which was certainly not higher than 127. Brauner then
undertook the investigation, and he has shown that the true atomic weight
of tellurium is lower than that of iodine, being near to 125. For
_titanium_ the extensive researches of Thorpe have confirmed the atomic
weight of Ti = 48, indicated by the law, and already foreseen by Rose,
but contradicted by the analyses of Pierre and several other chemists. An
equally brilliant confirmation of the expectations based on the periodic
law has been given in the case of the series osmium, iridium, platinum,
and gold. At the time of the promulgation of the periodic law, the
determinations of Berzelius, Rose, and many others gave the following
figures:--
Os = 200; Ir = 197; Pt = 198; Au = 196.
The expectations of the periodic law[8] have been confirmed, first, by
new determinations of the atomic weight of _platinum_ (by Seubert,
Dittmar, and M'Arthur, which proved to be near to 196 (taking O = 16, as
proposed by Marignac, Brauner, and others); secondly, by Seubert having
proved that the atomic weight of _osmium_ is really lower than that of
platinum, being near to 191; and thirdly, by the investigations of Krüss,
Thorpe and Laurie, proving that the atomic weight of _gold_ exceeds that
of platinum, and approximates to 197. The atomic weights which were thus
found to require correction were precisely those which the periodic law
had indicated as affected with errors; and it has been proved, therefore,
that the periodic law affords a means of testing experimental results. If
we succeed in discovering the exact character of the periodic
relationships between the increments in atomic weights of allied elements
discussed by Ridberg in 1885, and again by Bazaroff in 1887, we may
expect that our instrument will give us the means of still more closely
controlling the experimental data relating to atomic weights.
[8] I pointed them out in the _Liebig's Annalen_, Supplement Band.,
viii. 1871, p. 211.
Let me next call to mind that, while disclosing the variation of chemical
properties,[9] the periodic law, has also enabled us to systematically
discuss many of the physical properties of elementary bodies, and to show
that these properties are also subject to the law of periodicity. At the
Moscow Congress of Russian Naturalists in August, 1869, I dwelt upon the
relations which existed between density and the atomic weight of the
elements. The following year Professor Lothar Meyer, in his well-known
paper,[10] studied the same subject in more detail, and thus contributed
to spread information about the periodic law. Later on, Carnelley,
Laurie, L. Meyer, Roberts-Austen, and several others applied the periodic
system to represent the order in the changes of the magnetic properties
of the elements, their melting points, the heats of formation of their
haloid compounds, and even of such mechanical properties as the
coefficient of elasticity, the breaking stress, &c., &c. These
deductions, which have received further support in the discovery of new
elements endowed not only with chemical but even with physical
properties, which were foreseen by the law of periodicity, are well
known; so I need not dwell upon the subject, and may pass to the
consideration of oxides.[11]
[9] Thus, in the typical small period of
Li, Be, B, C, N, O, F,
we see at once the progression from the alkali metals to the acid
non-metals, such as are the halogens.
[10] _Liebig's Annalen_, Supplement Band., vii. 1870.
[11] A distinct periodicity can also be discovered in the spectra of
the elements. Thus the researches of Hartley, Ciamician, and
others have disclosed, first, the homology of the spectra of
analogous elements: secondly, that the alkali metals have simpler
spectra than the metals of the following groups; and thirdly, that
there is a certain likeness between the complicated spectra of
manganese and iron on the one hand, and the no less complicated
spectra of chlorine and bromine on the other hand, and their
likeness corresponds to the degree of analogy between those
elements which is indicated by the periodic law.
In indicating that the gradual increase of the power of elements of
combining with oxygen is accompanied by a corresponding decrease in their
power of combining with hydrogen, the periodic law has shown that there
is a limit of oxidation, just as there is a well-known limit to the
capacity of elements for combining with hydrogen. A single atom of an
element combines with at most four atoms of either hydrogen or oxygen;
and while CH_{4} and SiH_{4} represent the highest hydrides, so RuO_{4}
and OsO_{4} are the highest oxides. We are thus led to recognise types of
oxides, just as we have had to recognise types of hydrides.[12]
[12] Formerly it was supposed that, being a bivalent element, oxygen
can enter into any grouping of the atoms, and there was no limit
foreseen as to the extent to which it could further enter into
combination. We could not explain why bivalent sulphur, which
forms compounds such as
O O
/ \ / \
S | and S O,
\ / \ /
O O
could not also form oxides such as--
O--O O--O
/ \ / \
S | or S O,
\ / \ /
O--O O--O
while other elements, as, for instance, chlorine, form compounds
such as--
Cl--O--O--O--O--K
The periodic law has demonstrated that the maximum extent to which
different non-metals enter into combination with oxygen is determined by
the extent to which they combine with hydrogen, and that the sum of the
number of equivalents of both must be equal to 8. Thus chlorine, which
combines with 1 atom or 1 equivalent of hydrogen, cannot fix more than 7
equivalents of oxygen, giving Cl_{2}O_{7}; while sulphur, which fixes 2
equivalents of hydrogen, cannot combine with more than 6 equivalents or 3
atoms of oxygen. It thus becomes evident that we cannot recognise as a
fundamental property of the elements the atomic valencies deduced from
their hydrides; and that we must modify, to a certain extent, the theory
of atomicity if we desire to raise it to the dignity of a general
principle capable of affording an insight into the constitution of all
compound molecules. In other words, it is only to carbon, which is
quadrivalent with regard both to oxygen and hydrogen, that we can apply
the theory of constant valency and of bond, by means of which so many
still endeavour to explain the structure of compound molecules. But I
should go too far if I ventured to explain in detail the conclusions
which can be drawn from the above considerations. Still, I think it
necessary to dwell upon one particular fact which must be explained from
the point of view of the periodic law in order to clear the way to its
extension in that particular direction.
The higher oxides yielding salts the formation of which was foreseen by
the periodic system--for instance, in the short series beginning with
sodium--
Na_{2}O, MgO, Al_{2}O_{3}, SiO_{2}, P_{2}O_{5}, SO_{3}, Cl_{2}O_{7},
must be clearly distinguished from the higher degrees of oxidation which
correspond to hydrogen peroxide and bear the true character of peroxides.
Peroxides such as Na_{2}O_{2}, BaO_{2}, and the like have long been
known. Similar peroxides have also recently become known in the case of
chromium, sulphur, titanium, and many other elements, and I have
sometimes heard it said that discoveries of this kind weaken the
conclusions of the periodic law in so far as it concerns the oxides. I do
not think so in the least, and I may remark, in the first place, that all
these peroxides are endowed with certain properties obviously common to
all of them, which distinguish them from the actual, higher, salt-forming
oxides, especially their easy decomposition by means of simple contact
agencies; their incapability of forming salts of the common type; and
their capability of combining with other peroxides (like the faculty
which hydrogen peroxide possesses of combining with barium peroxide,
discovered by Schoene). Again, we remark that some groups are especially
characterised by their capacity of generating peroxides. Such is, for
instance, the case in the sixth group, where we find the well-known
peroxides of sulphur, chromium, and uranium; so that further
investigation of peroxides will probably establish a new periodic
function, foreshadowing that molybdenum and tungsten will assume peroxide
forms with comparative readiness. To appreciate the constitution of such
peroxides, it is enough to notice that the peroxide form of sulphur
(so-called persulphuric acid) stands in the same relation to sulphuric
acid as hydrogen peroxide stands to water:--
H(OH), or H_{2}O, responds to (OH)(OH), or H_{2}O_{2},
and so also--
H(HSO_{4}), or H_{2}SO_{4}, responds to
(HSO_{4})(HSO_{4}), or H_{2}S_{2}O_{8}.
Similar relations are seen everywhere, and they correspond to the
principle of substitutions which I long since endeavoured to represent as
one of the chemical generalisations called into life by the periodic law.
So also sulphuric acid, if considered with reference to hydroxyl, and
represented as follows:--
HO(SO_{2}OH),
has its corresponding compound in dithionic acid--
(SO_{2}OH)(SO_{2}OH), or H_{2}S_{2}O_{6}.
Therefore, also, phosphoric acid, HO(POH_{2}O_{2}), has, in the same
sense, its corresponding compound in the subphosphoric acid of Saltzer:--
(POH_{2}O_{2})(POH_{2}O_{2}), or H_{4}P_{2}O_{6};
and we must suppose that the peroxide compound corresponding to
phosphoric acid, if it be discovered, will have the following
structure:--
(H_{2}PO_{4})_{2} or H_{4}P_{2}O_{8} = 2H_{2}O + 2PO_{3}.[13]
So far as is known at present, the highest form of peroxides is met with
in the peroxide of uranium, UO_{4}, prepared by Fairley;[14] while
OsO_{4} is the highest oxide giving salts. The line of argument which is
inspired by the periodic law, so far from being weakened by the discovery
of peroxides, is thus actually strengthened, and we must hope that a
further exploration of the region under consideration will confirm the
applicability to chemistry generally of the principles deduced from the
periodic law.
[13] In this sense, oxalic acid, (COOH)_{2}, also corresponds to
carbonic acid, OH(COOH), in the same way that dithionic acid
corresponds to sulphuric acid, and subphosphoric acid to
phosphoric; hence, if a peroxide corresponding to carbonic acid be
obtained, it will have the structure of (HCO_{3})_{2}, or
H_{2}C_{2}O_{6} = H_{2}O + C_{2}O_{5}. So also lead must have a
real peroxide, Pb_{2}O_{5}.
[14] The compounds of uranium prepared by Fairley seem to me especially
instructive in understanding the peroxides. By the action of
hydrogen peroxide on uranium oxide, UO_{3}, a peroxide of uranium,
UO_{4},4H_{2}O, is obtained (U = 240) if the solution be acid; but
if hydrogen peroxide act on uranium oxide in the presence of
caustic soda, a crystalline deposit is obtained which has the
composition Na_{4}UO_{8},4H_{2}O, and evidently is a combination
of sodium peroxide, Na_{2}O_{2}, with uranium peroxide, UO_{4}. It
is possible that the former peroxide, UO_{4},4H_{2}O, contains the
elements of hydrogen peroxide and uranium peroxide, U_{2}O_{7}, or
even U(OH)_{6},H_{2}O_{2}, like the peroxide of tin recently
discovered by Spring, which has the constitution
Sn_{2}O_{5},H_{2}O_{2}.
Permit me now to conclude my rapid sketch of the oxygen compounds by the
observation that the periodic law is especially brought into evidence in
the case of the oxides which constitute the immense majority of bodies at
our disposal on the surface of the earth.
The oxides are evidently subject to the law, both as regards their
chemical and their physical properties, especially if we take into
account the cases of polymerism which are so obvious when comparing
CO_{2}, with Si_{_n_}O_{2_n_}. In order to prove this I give the
densities s and the specific volumes v of the higher oxides of two short
periods. To render comparison easier, the oxides are all represented as
of the form R_{2}O_{_n_}. In the column headed [Delta] the differences
are given between the volume of the oxygen compound and that of the
parent element, divided by _n_--that is, by the number of atoms of oxygen
in the compound:--[15]
_s._ _v._ [Delta]
Na_{2}O 2·6 24 -22
Mg_{2}O_{2} 3·6 22 -3
Al_{2}O_{3} 4·0 26 +1·3
Si_{2}O_{4} 2·65 45 5·2
P_{2}O_{5} 2·39 59 6·2
S_{2}O_{6} 1·96 82 8·7
K_{2}O 2·7 35 -55
Sc_{2}O_{3} 3·86 35 0
Li_{2}O_{4} 4·2 38 +5
V_{2}O_{5} 3·49 52 6·7
Cr_{2}O_{6} 2·74 73 9·5
[15] [Delta] thus represents the average increase of volume for each
atom of oxygen contained in the higher salt-forming oxide. The
acid oxides give, as a rule, a higher value of [Delta], while in
the case of the strongly alkaline oxides its value is usually
negative.
I have nothing to add to these figures, except that like relations appear
in other periods as well. The above relations were precisely those which
made it possible for me to be certain that the relative density of
ekasilicon oxide would be about 4·7; germanium oxide, actually obtained
by Winkler, proved, in fact, to have the relative density 4·703.
The foregoing account is far from being an exhaustive one of all that
has already been discovered by means of the periodic law telescope in the
boundless realms of chemical evolution. Still less is it an exhaustive
account of all that may yet be seen, but I trust that the little which I
have said will account for the philosophical interest attached in
chemistry to this law. Although but a recent scientific generalisation,
it has already stood the test of laboratory verification, and appears as
an instrument of thought which has not yet been compelled to undergo
modification; but it needs not only new applications, but also
improvements, further development, and plenty of fresh energy. All this
will surely come, seeing that such an assembly of men of science as the
Chemical Society of Great Britain has expressed the desire to have the
history of the periodic law described in a lecture dedicated to the
glorious name of Faraday.
APPENDIX III
ARGON, A NEW CONSTITUENT OF THE ATMOSPHERE
WRITTEN BY PROFESSOR MENDELÉEFF IN FEBRUARY 1895
The remarks made in Chapter V., Note 16 bis respecting the newly
discovered constituent of the atmosphere are here supplemented by data
(taken from the publications of the Royal Society of London) given by the
discoverers Lord Rayleigh and Professor Ramsay in January 1895, together
with observations made by Crookes and Olszewsky upon the same subject.