ESTABLISHMENT OF GENERAL PRINCIPLES OF CHEMICAL SCIENCE
(_continued_)--PERIOD OF DAVY AND BERZELIUS.
_Humphry Davy_, 1778-1829. _Johann Jacob Berzelius_, 1779-1848.
We may roughly date the period of chemical advance during which the
connections between chemistry and other branches of natural knowledge were
recognized and studied, as beginning with the first year of this century,
and as continuing to our own day.
The elaboration of the atomic theory was busily carried on during the
second and third decades of this century; to this the labour of the Swedish
chemist Berzelius largely contributed.
That there exist many points of close connection between chemical and
electrical science was also demonstrated by the labours of the same
chemist, and by the brilliant and impressive discoveries of Sir Humphry
Davy.
A system of classification of chemical elements and compounds was
established by the same great naturalists, and many inroads were made into
the domain of the chemistry of bodies of animal and vegetable origin.
The work of Berzelius and Davy, characterized as it is by thoroughness,
clearness and definiteness, belongs essentially to the modern era of
chemical advance; but I think we shall better preserve the continuity of
our story if we devote a chapter to a consideration of the work of these
two renowned naturalists before entering on our review of the time
immediately preceding the present, as typical workers in which time I have
chosen Liebig and Dumas.
In the last chapter we found that the foundations of the atomic theory had
been laid, and the theory itself had been applied to general problems of
chemical synthesis, by Dalton. In giving, in that chapter, a short sketch
of the modern molecular theory, and in trying to explain the meaning of the
term "molecule" as contrasted with "atom," I necessarily carried the reader
forward to a time considerably later than the first decade of this century.
We must now retrace our steps; and in perusing the account of the work of
Berzelius and Davy given in the present chapter, the reader must endeavour
to have in his mind a conception of atom analogous to the mental picture
formed by Dalton (see pp. 135, 136); he must regard the term as applicable
to element and compound alike; he must remember that the work of which he
reads is the work of those who are striving towards a clear conception of
the atom, and who are gradually rising to a recognition of the existence of
more than one order of small particles, by the regular putting together of
which masses of matter are constituted.
No materials, so far as I am aware, exist from which a life of Berzelius
can be constructed. I must therefore content myself with giving a mere
enumeration of the more salient points in his life. Of his chemical work
abundant details are fortunately to be found in his own "Lehrbuch," and in
the works and papers of himself and his contemporaries.
* * * * *
JOHANN JACOB BERZELIUS was the son of the schoolmaster of Waefersunda, a
village near Linkoping, in East Gothland, Sweden. He was born in August
1779--he was born, that is, a few years after Priestley's discovery of
oxygen; at the time when Lavoisier had nearly completed his theory of
combustion; when Dalton was endeavouring to keep the unruly youth of
Eaglesfield in subjection; and when Black, having established the existence
of fixed air and the theory of latent heat, was the central figure in the
band of students who were enlarging our knowledge of Nature in the Scottish
capital.
Being left an orphan at the age of nine, the young Berzelius was brought up
by his grandfather, who appears to have been a man of education and sense.
After attending school at Linkoping, he entered the University of Upsala as
a student of medicine. Here he soon began to show a taste for chemistry. It
would appear that few or no experiments were then introduced into his
lectures by the Professor of Chemistry at Upsala; little encouragement was
given to pursue chemical experiments, and so Berzelius had to trust to his
own labours for gaining an acquaintance with practical chemistry. Having
thus made considerable progress in chemistry, and being on a visit to the
mineral baths of Medevi, he seized the opportunity to make a very thorough
analysis of the waters of this place, which were renowned in Sweden for
their curative properties. The publication of this analysis marks the first
appearance of Berzelius as an author.
He graduated as M.B. in 1801, and a year or two later presented his
dissertation, entitled "The Action of Galvanism on Organic Bodies," as a
thesis for the degree of Doctor of Medicine. This thesis, like that of
Black, published about half a century earlier, marks an important stage in
the history of chemistry. These and other publications made the young
doctor famous; he was called to Stockholm to be extraordinary (or
assistant) Professor of Chemistry in the medical school of that capital.
Sometimes practising medicine in order to add to his limited income, but
for the most part engaged in chemical research, he remained in Stockholm
for nearly fifty years, during most of which time the laboratory of
Berzelius in the Swedish capital was regarded as one of the magnetic poles
of the chemical world. To this point came many of the great chemists who
afterwards enriched the science by their discoveries. Woehler, H. and G.
Rose, Magnus, Gmelin, Mitscherlich and others all studied with Berzelius.
He visited England and France, and was on terms of intimacy and in
correspondence with Davy, Dalton, Gay-Lussac, Berthollet and the other men
who at that period shed so much lustre on English and French science.
It is said that Berzelius was so much pleased with the lectures of Dr.
Marcet at Guy's Hospital, that on his return from his visit to England in
1812, he introduced much more liveliness and many more experimental
illustrations into his own lectures.
At the age of thirty-one, Berzelius was chosen President of the Stockholm
Academy of Sciences; a few years later he was elected a Foreign Fellow of
the Royal Society, which society bestowed on him the Copley Medal in 1836.
He was raised to the rank of a baron by the King of Sweden, being allowed
as a special privilege to retain his own name.
In the year 1832 Berzelius resigned his professorship, and in the same year
he married. During the remainder of his life, he continued to receive
honours of all kinds, but he never for a moment forsook the paths of
science. After the death of Davy, in 1829, he was recognized as the
leading European chemist of his age; but, although firm in his own
theoretical views, he was ready to test these views by appealing to Nature.
The very persistency with which he clung to a conception established on
some solid experimental basis insured that new light would be thrown on
that conception by the researches of those chemists who opposed him.
Probably no chemist has added to the science so many carefully determined
facts as Berzelius; he was always at work in the laboratory, and always
worked with the greatest care. Yet the appliances at his command were what
we should now call poor, meagre, and utterly inadequate. Professor Woehler
of Goettingen, who in the fulness of days and honours has so lately gone
from amongst us, recently gave an account of his visit to Berzelius in the
year 1823. Woehler had taken his degree as Doctor of Medicine at Heidelberg,
and being anxious to prosecute the study of chemistry he was advised by his
friends to spend a winter in the laboratory of the Swedish professor.
Having written to Berzelius and learned that he was willing to allow him
working room in his laboratory, the young student set out for Stockholm.
After a journey to Luebeck and a few days' passage in a small
sailing-vessel, he arrived in the Swedish capital.
Knocking at the door of the house pointed out as that of Berzelius, he
tells us that his heart beat hard as the door was opened by a tall man of
florid complexion. "It was Berzelius himself," he exclaims. Scarcely
believing that he was in the very room where so many famous discoveries had
been made, he entered the laboratory. No water, no gas, no draught-places,
no ovens were to be seen; a couple of plain tables, a blowpipe, a few
shelves with bottles, a little simple apparatus, and a large water-barrel
whereat Anna, the ancient cook of the establishment, washed the laboratory
dishes, completed the furnishings of this room, famous throughout Europe
for the work which had been done in it. In the kitchen which adjoined, and
where Anna cooked, was a small furnace and a sand bath for heating
purposes.
In this room many great discoveries were made. Among these we may note the
separation of the element columbium in 1815, and of selenion in 1818; the
discovery of the new earth thoria in 1828; the elucidation of the
properties of yttrium and cerium about 1820, of uranium in 1823, and of the
platinum metals in 1828; the accurate determination of the atomic weights
of the greater number of the elements; the discovery of "sulphur salts" in
1826-27, and the proof that silica is an acid, and that most of the "stony"
minerals are compounds of this acid with various bases.
But we shall better learn the value of some of these discoveries by taking
a general review of the contributions to chemical science of the man who
spent most of his life at work in that room in Stockholm.
The German chemist Richter, in the first or second year of this century,
had drawn attention to the fact that when two neutral compounds, such as
nitrate of potash and chloride of lime, react chemically, the substances
produced by this reaction are also neutral. All the potash combined with
nitric acid in one salt changes places with all the lime combined with
muriatic acid in the other salt; therefore, said Richter, these different
quantities of potash and lime are neutralized by the same quantity of
nitric acid; and, hence, these amounts of potash and lime are chemically
_equivalent_, because these are the amounts which perform the same
reaction, viz. neutralization of a fixed quantity of acid. If then careful
analyses were made of a number of such neutral compounds as those named,
the _equivalents_ of all the commoner "bases" and "acids"[9] might be
calculated.
Richter's own determinations of the equivalents of acids and bases were not
very accurate, but Berzelius was impressed with the importance of this
work. The year before the appearance of Dalton's "New System" (_i.e._ in
1807), he began to prepare and carefully analyze series of neutral salts.
As the work was proceeding he became acquainted with the theory of Dalton,
and at once saw its extreme importance. For some time Berzelius continued
to work on the lines laid down by Dalton, and to accumulate data from which
the atomic weights of elements might be calculated; but he soon
perceived--as the founder of the theory had perceived from the very
outset--that the fundamental conception of each atom of an element as being
a distinct mass of matter weighing more or less than the atom of every
other element, and of each atom of a compound as being built up of the
atoms of the elements which compose that compound,--Berzelius, I say,
perceived that these conceptions must remain fruitless unless means were
found for determining the number of elementary atoms in each compound atom.
We have already learned the rules framed by the founder of the atomic
theory for his guidance in attempting to solve this problem. Berzelius
thought those rules insufficient and arbitrary; he therefore laid down two
general rules, on the lines of which he prosecuted his researches into
chemical synthesis.
"One atom of one element combines with one, two, three, or more atoms of
another element." This is practically the same as Dalton's definitions of
binary, ternary, etc., compounds (p. 132). "Two atoms of one element
combine with three and five atoms of another element." Berzelius here
recognizes the existence of compound atoms of a more complex structure than
any of those recognized by Dalton.
Berzelius further extended the conception of atom by applying it to groups
of elements formed, according to him, by the combination of various
compound atoms. To his mind every compound atom appeared as built up of
two parts; each of these parts might be an elementary atom, or might be
itself built up of several elementary atoms, yet in the Berzelian theory
each acted as a definite whole. So far as the building up of the complex
atom went, each of the two parts into which this atom could be divided
acted as if it were a simple atom.
If we suppose a patch of two shades of red colour to be laid on a smooth
surface, and alongside of this a patch of two shades of yellow colour, and
if we suppose the whole mass of colour to be viewed from a distance such
that one patch appears uniformly red and the other uniformly yellow, we
shall have a rough illustration of the Berzelian compound atom. To the
observer the whole mass of colour appears to consist of two distinct
patches of contrasted colours; but let him approach nearer, and he
perceives that what appeared to be a uniform surface of red or yellow
really consists of two patches of unlike shades of red or of yellow. The
whole mass of colour represents the compound atom; broadly it consists of
two parts--the red colour represents one of the constituent atoms, the
yellow colour represents the other constituent atom; but on closer
examination the red atom, so to speak--and likewise the yellow atom--is
found to consist of parts which are less unlike each other than the whole
red atom is unlike the whole yellow atom.
We shall have to consider in more detail the reasoning whereby Berzelius
arrived at this conception of every compound atom as a _dual_ structure
(see pp. 209-212). At present I wish to notice this conception as lying at
the root of most of the work which he did in extending and applying the
Daltonian theory. I wish to insist on the fact that the atomic theory could
not advance without methods being found for determining the number of
elementary atoms in a compound atom, without clear conceptions being gained
of every compound atom as a structure, and without at least attempts being
made to learn the laws in accordance with which that structure was built.
Before the atomic weight of oxygen could be determined it was necessary
that the number of oxygen and of hydrogen atoms in the atom of water should
be known; otherwise all that could be stated was, the atomic weight of
oxygen is a simple multiple of 8. Berzelius did much to advance chemical
science by the introduction and application of a few simple rules whereby
he determined the number of elementary atoms in various compound atoms. But
as the science advanced, and as more facts came to be known, the Berzelian
rules were found to be too narrow and too arbitrary; chemists sought for
some surer and more generally applicable method than that which Berzelius
had introduced, and the imperious demand for this method at last forced
them to recognize the importance of the great generalization of the Italian
naturalist Avogadro, which they had possessed since the year 1811, but the
meaning of which they had so long failed to understand.
Berzelius made one great step in the direction of recognizing Avogadro's
distinction between atom and molecule when he accepted Gay-Lussac's
generalization that "equal volumes of gases contain equal numbers of
atoms:" but he refused to apply this to other than elementary gases. The
weights of the volumes of elementary gases which combined were, for
Berzelius, also the weights of the atoms of these elements. Thus, let the
weight of one volume of hydrogen be called 1, then two volumes of hydrogen,
weighing 2, combine with one volume of oxygen, weighing 16, to form two
volumes of water vapour; therefore, said Berzelius, the atom of water
consists of two atoms of hydrogen and one atom of oxygen, and the atom of
the latter element is sixteen times heavier than the atom of the former.
Three volumes of hydrogen, weighing 3, combine with one volume of nitrogen,
weighing 14, to form two volumes of ammonia; therefore, said Berzelius, the
atom of ammonia consists of three atoms of hydrogen combined with one atom
of nitrogen, and the nitrogen atom is fourteen times heavier than the atom
of hydrogen.
While Berzelius was applying these rules to the determination of the atomic
weights of the elements, and was conducting the most important series of
analyses known in the annals of the science, two great physico-chemical
discoveries were announced.
In the year 1818 the "_law of isomorphism_" was stated by Mitscherlich:
"Compounds the atoms of which contain equal numbers of elementary atoms,
similarly arranged, have the same crystalline form." As thus stated, the
law of isomorphism affirms that if two compounds crystallize in the same
form, the atoms of these compounds are built up of the same number of
elementary atoms--however different may be the nature of the elements in
the compounds--and that these elementary atoms are similarly arranged. This
statement was soon found to be too absolute, and was accordingly modified;
but to go into the history of the law of isomorphism would lead us too far
from the great main path of chemical advance, the course of which we are
seeking to trace.
Berzelius at once accepted Mitscherlich's law, as an aid in his researches
on atomic weights. The help to be derived from this law may be illustrated
thus: let us assume that two compounds have been obtained exhibiting
identity of crystalline form; let it be further assumed that the number of
elementary atoms in the atom of one of these compounds is known; it
follows, by the law of isomorphism, that the number of elementary atoms in
the atom of the other is known also. Let the two compounds be _sulphate of
potash_ and _chromate of potash_; let it be assumed that the atom of the
first named is known to consist of two atoms of potassium, one atom of
sulphur, and four atoms of oxygen; and that the second substance is known
to be a compound of the elements potassium, chromium and oxygen; then the
atom of the second compound contains, by Mitscherlich's law, two atoms of
potassium, one atom of chromium and four atoms of oxygen: hence the
relative weight of the atom of chromate of potash can be determined, and
hence the relative weight of the atom of chromium can also be determined.
A year after the announcement of Mitscherlich's law, the following
generalization was stated to hold good, by two French naturalists, Dulong
and Petit:--"The atoms of all solid elements have the same capacity for
heat."
If the amount of heat required to raise the temperature of one grain of
water through one degree be called _one unit of heat_, then the capacity
for heat of any body other than water is the number of units of heat
required to raise the temperature of one grain of that substance through
one degree. Each chemical substance, elementary and compound, has its own
capacity for heat; but, instead of comparing the capacities for heat of
equal weights, Dulong and Petit compared the capacities for heat of weights
representing the weights of the atoms of various elements. Thus, equal
amounts of heat are required to raise, through the same interval of
temperature, fifty-six grains of iron, one hundred and eight grains of
silver, and sixty-three and a half grains of copper; but the weights of the
atoms of these three elements are in the proportion of 56:108:63-1/2.
Dulong and Petit based their generalization on measurements of the
capacities for heat of thirteen elements; further research has shown that
their statement most probably holds good for all the solid elements. Here
then was a most important instrument put into the hands of the chemist.
It is only necessary that the atomic weight of one solid element should be
certainly known, and that the amount of heat required to raise through one
degree the number of grains of that element expressed by its atomic weight
should also be known; then the number which expresses the weight, in
grains, of any other solid element which is raised through one degree by
the same amount of heat, likewise expresses the relative weight of the atom
of that element. Thus, suppose that the atomic weight of silver is known to
be 108, and suppose that six units of heat are required to raise the
temperature of one hundred and eight grains of this metal through one
degree; then suppose it is found by experiment that six units of heat
suffice to raise the temperature of two hundred and ten grains of bismuth
through one degree, it follows--according to the law of Dulong and
Petit--that 210 is the atomic weight of bismuth.
The modified generalization of Gay-Lussac--"Equal volumes of _elementary_
gases contain equal numbers of atoms;" the laws of "isomorphism" and of
"atomic heat;" and the two empirical rules stated on p. 163;--these were
the guides used by Berzelius in interpreting the analytical results which
he and his pupils obtained in that memorable series of researches, whereby
the conceptions of Dalton were shown to be applicable to a wide range of
chemical phenomena.
The fixity of composition of chemical compounds has now been established; a
definite meaning has been given to the term "element;" the conception of
"atom" has been gained, but much remains to be done in the way of rendering
this conception precise; and fairly good, but not altogether satisfactory
methods have been introduced by which the relative weights of the atoms of
elements and compounds may be determined. At this time chemists are busy
preparing and describing new compounds, and many new elements are also
being discovered; the need of classification begins to be felt more and
more.
In the days of Berzelius and Davy strenuous efforts were made to obtain
some generalizations by the application of which the many known elements
and compounds might be divided into groups. It was felt that a
classification might be founded on the composition of compounds, or perhaps
on the properties of the same compounds. These two general principles
served as guides in most of the researches then instituted; answers were
sought to these two questions: Of what elements is this compound composed?
and, What can this compound do; how does it react towards other bodies?
Lavoisier, as we know, regarded oxygen as the characteristic element of all
_acids_. This term _acid_ implies the possession, by all the substances
denoted by it, of some common property; let us shortly trace the history of
this word in chemistry.
Vinegar was known to the Greeks and Romans, and the names which they gave
this substance tell us that sourness was to them its characteristic
property. They knew that vinegar effervesced when brought into contact with
chalky earths, and that it was able to dissolve many substances--witness
the story of Cleopatra's draught of the pearl dissolved in vinegar. Other
substances possessed of these properties--for instance oil of vitriol and
spirits of salt--as they became known, were classed along with vinegar; but
no attempts were made to clearly define the properties of these bodies till
comparatively recent times.
The characteristics of an acid substance enumerated by Boyle are--solvent
power, which is exerted unequally on different bodies; power of turning
many vegetable blues to red, and of restoring many vegetable colours which
had been destroyed by alkalis; power of precipitating solid sulphur from
solutions of this substance in alkalis, and the power of acting on alkalis
to produce substances without the properties of either acid or alkali.
But what, one may ask, is an alkali, of which mention is so often made by
Boyle?
From very early times it had been noticed that the ashes which remained
when certain plants were burned, and the liquid obtained by dissolving
those ashes in water, had great cleansing powers; that they removed oily
matter, fat and dirt from cloth and other fabrics. The fact that an aqueous
solution of these ashes affects the coloured parts of many plants was also
noticed in early times. As progress was made in chemical knowledge
observers began to contrast the properties of this plant-ash with the
properties of acids. The former had no marked taste, the latter were always
very sour; the former turned some vegetable reds to blue, the latter turned
the blues to red; a solution of plant-ash had no great solvent action on
ordinary mineral matter, whereas this matter was generally dissolved by an
acid. In the time of the alchemists, who were always seeking for the
principles or essences of things, these properties of acids were attributed
to _a principle of acidity_, while the properties of plant-ash and
substances resembling plant-ash were attributed to a _principle of
alkalinity_ (from Arabic _alkali_, or _the ash_).
In the seventeenth century the distinction between acid and alkali was made
the basis of a system of chemical medicine. The two principles of acidity
and alkalinity were regarded as engaged in an active and never-ending
warfare. Every disease was traced to an undue preponderance of one or other
of these principles; to keep these unruly principles in quietness became
the aim of the physician, and of course it was necessary that the physician
should be a chemist, in order that he might know the nature and habits of
the principles which gave him so much trouble.
Up to this time the term "alkali" had been applied to almost any substance
having the properties which I have just enumerated; but this group of
substances was divided by Van Helmont and his successors into _fixed
alkali_ and _volatile alkali_, and fixed alkali was further subdivided into
_mineral alkali_ (what we now call soda) and _vegetable alkali_ (potash).
About the same time acids were likewise divided into three groups;
_vegetable_, _animal_, and _mineral acids_. To the properties by which
alkali was distinguished, viz. cleansing power and action on vegetable
colouring matters, Stahl (the founder of the phlogistic theory) added that
of combining with acids. When an acid (that is, a sour-tasting substance
which dissolves most earthy matters and turns vegetable blues to red) is
added to an alkali (that is, a substance which feels soap-like to the
touch, which does not dissolve many earthy matters, and which turns many
vegetable reds to blue) the properties of both acid and alkali disappear,
and a new substance is produced which is not characterized by the
properties of either constituent. The new substance, as a rule, is without
action on earthy matters or on vegetable colours; it is not sour, nor is it
soapy to the touch like alkali; it is _neutral_. It is _a salt_. But,
although Stahl stated that an alkali is a substance which combines with an
acid, it was not until a century later that these three--alkali, acid,
salt--were clearly distinguished.
But the knowledge that a certain group of bodies are sour and dissolve
minerals, etc., and that a certain other group of bodies are nearly
tasteless and do not dissolve minerals, etc., was evidently a knowledge of
only the outlying properties of the bodies; it simply enabled a term to be
applied to a group of bodies, which term had a definite connotation.
_Why_ are acids acid, and _why_ are alkalis alkaline?
Acids are acid, said Becher (latter part of seventeenth century), because
they all contain the same principle, viz. the primordial acid. This
primordial acid is more or less mixed with earthy matter in all actual
acids; it is very pure in spirits of salt.
Alkalis are alkaline, said Basil Valentine (beginning of the sixteenth
century), because they contain a special kind of matter, "the matter of
fire."
According to other chemists (_e.g._ J. F. Meyer, 1764), acids owe their
acidity to the presence of a sharp or biting principle got from fire.
Acids, alkalis and salts _all_ contain, according to Stahl (beginning of
the eighteenth century), more or less _primordial acid_. The more of this a
substance contains, the more acid it is; the less of this it contains, the
more alkaline it is.
All these attempted explanations recognize that similar properties are to
be traced to similarity of composition; but the assertion of the existence
of a "primordial acid," or of "the matter of fire," although undoubtedly a
step in advance, was not sufficiently definite (unless it was supplemented
by a distinct account of the properties of these principles) to be accepted
when chemical knowledge became accurate.
The same general consideration, founded on a large accumulation of facts,
viz. that similarity of properties is due to similarity of composition,
guided Lavoisier in his work on acids. He found the "primordial acid" of
Stahl, and the "biting principle" of Meyer, in the element oxygen.
I have already (p. 91) shortly traced the reasoning whereby Lavoisier
arrived at the conclusion that oxygen is _the acid-producer_; here I would
insist on the difference between his method and that of Basil Valentine,
Stahl and the older chemists. _They_ carried into the domain of natural
science conceptions obtained from, and essentially belonging to the domain
of metaphysical or extra-physical speculation; _he_ said that oxygen is the
acidifier, because all the compounds of this element which he actually
examined were possessed of the properties included under the name acid. We
know that Lavoisier's conclusion was erroneous, that it was not founded on
a sufficiently broad basis of facts. The conception of an acidifying
principle, although that principle was identified with a known element, was
still tainted with the vices of the alchemical school. We shall see
immediately how much harm was done by the assertion of Lavoisier, "All
acids contain oxygen."
In Chapter II. (pp. 32-37) we traced the progress of knowledge regarding
alkalis from the time when the properties of these bodies were said to be
due to the existence in them of "matter of fire," to the time when Black
had clearly distinguished and defined caustic alkali and carbonated alkali.
The truly philosophical character, and at the same time the want of
enthusiasm, of Black become apparent if we contrast his work on alkali with
that of Lavoisier on acid. Black did not hamper the advance of chemistry by
finding a "principle of alkalinity;" but neither did he give a full
explanation of the fact that certain bodies are alkaline while others are
not. He set himself the problem of accurately determining the differences
in composition between burnt (or caustic) and unburnt (or mild) alkali, and
he solved the problem most successfully. He showed that the properties of
mild alkalis differ from those of caustic alkalis, because the composition
of the former differs from that of the latter; and he showed exactly
wherein this difference of composition consists, viz. in the possession or
non-possession of fixed air.
Strange we may say that this discovery did not induce Black to prosecute
the study of caustic alkalis: surely he would have anticipated Davy, and
have been known as the discoverer of potassium and sodium.
In the time of Stahl the name "salt" was applied, as we have learned, to
the substance produced by the union of an acid with an alkali; but the same
word was used by the alchemists with an altogether different signification.
Originally applied to the solid matter obtained by boiling down sea-water,
and then extended to include all substances which, like this solid matter,
are very easily dissolved by water and can be recovered by boiling down
this solution, "salt" was, in the sixteenth and seventeenth centuries, the
name given to one of the hypothetical principles or elements. Many kinds of
matter were known to be easily dissolved by water; the common possession of
these properties was sought to be accounted for by saying that all these
substances contained the same principle, namely, _the principle of salt_. I
have already tried to indicate the reasoning whereby Boyle did so much to
overthrow this conception of salt. He also extended our knowledge of
special substances which are now classed as salts. The chemists who came
after Boyle gradually reverted to the older meaning of the term "salt,"
adopting as the characteristics of all substances placed in this class,
ready solubility in water, fusibility, or sometimes volatility, and the
possession of a taste more or less like that of sea-salt.
Substances which resembled salts in general appearance, but were insoluble
in water, and very fixed in the fire, were called "earths"; and, as was
generally done in those days, the existence of a primordial earth was
assumed, more or less of which was supposed to be present in actual earths.
This recognition of the possibility of more or less of the primordial earth
being present in actually occurring earths, of course necessitated the
existence of various kinds of earth. The earths were gradually
distinguished from each other; lime was recognized as a substance distinct
from baryta, baryta as distinct from alumina, etc.
Stahl taught that one essential property of an earth was fusibility by
fire, with production of a substance more or less like glass. This property
was possessed in a remarkable degree by quartz or silica. Hence silica was
regarded as the typical earth, until Berzelius, in 1815, proved it to be an
acid. But the earths resembled alkalis, inasmuch as they too combined with,
and so neutralized, acids.
There is an alkali hidden in every earth, said some chemists.
An alkali is an earth refined by the presence of acid and combustible
matter, said others.
Earths thus came to be included in the term "alkali," when that term was
used in its widest acceptation. But a little later it was found that some
of the earths were thrown down in the solid form from their solutions in
acids by the addition of alkalis; this led to a threefold division, thus--
Earths <----> Alkaline earths <----> Alkalis
Insoluble Somewhat soluble Very soluble in
in water. in water. water.
The distinction at first drawn between "earth" and "alkali" was too
absolute; the intermediate group of "alkaline earths" served to bridge over
the gap between the extreme groups.
"In Nature," says Wordsworth, "everything is distinct, but nothing defined
into absolute independent singleness."
At this stage of advance, then, an earth is regarded as differing from an
alkali in being insoluble, or nearly insoluble in water; in not being soapy
to the touch, and not turning vegetable reds to blue: but as resembling an
alkali, in that it combines with and neutralizes an acid; and the product
of this neutralization, whether accomplished by an alkali or by an earth,
is called a salt. To the earth or alkali, as being the foundation on which
the salt is built, by the addition of acid, the name of _base_ was given by
Rouelle in 1744.
But running through every conception which was formed of these
substances--acid, alkali, earth, salt--we find a tendency, sometimes
forcibly marked, sometimes feebly indicated, but always present, to
consider salt as a term of much wider acceptation than any of the others.
An acid and an alkali, or an acid and an earth, combine to form a salt; but
the salt could not have been thus produced unless the acid, the alkali and
the earth had contained in themselves some properties which, when combined,
form the properties of the salt.
The acid, the alkali, the earth, each is, in a sense, a salt. The perfect
salt is produced by the coalescence of the saltness of the acid with the
saltness of the alkali. This conception finds full utterance in the names,
once in common use, of _sal acidum_ for acid, _sal alkali_ for alkali, and
_sal salsum_ or _sal neutrum_ for salt. All are salts; at one extreme
comes that salt which is marked by properties called acid properties, at
the other extreme comes the salt distinguished by alkaline properties, and
between these, and formed by the union of these, comes the middle or
neutral salt.
It is thus that the nomenclature of chemistry marks the advances made in
the science. "What's in a name?" To the historical student of science,
almost everything.
We shall find how different is the meaning attached in modern chemistry to
these terms, _acid salt_, _alkaline salt_, _neutral salt_, from that which
our predecessors gave to their _sal acidum_, _sal alkali_, and _sal
neutrum_.
We must note the appearance of the term _vitriol_, applied to the solid
salt-like bodies obtained from acids and characterized by a glassy lustre.
By the middle of last century the vitriols were recognized as all derived
from, or compounded of, sulphuric acid (oil of vitriol) and metals; this
led to a subdivision of the large class of neutral salts into (1) metallic
salts produced by the action of sulphuric acid on metals, and (2) neutral
salts produced by the action of earths or alkalis on acids generally.
To Rouelle, a predecessor of Lavoisier, who died four years before the
discovery of oxygen, we owe many accurate and suggestive remarks and
experiments bearing on the term "salt." I have already mentioned that it
was he who applied the word "base" to the alkali or earth, or it might be
metal, from which, by the action of acid, a salt is built up. He also
ceased to speak of an acid as _sal acidum_, or of an alkali as _sal
alkali_, and applied the term "salt" exclusively to those substances which
are produced by the action of acids on bases. When the product of such an
action was neutral--that is, had no sour taste, no soapy feeling to the
touch, no action on vegetable colours, and no action on acids or bases--he
called that product _a neutral salt_; when the product still exhibited some
of the properties of acid, _e.g._ sourness of taste, he called it _an acid
salt_; and when the product continued to exhibit some of the properties of
alkali, _e.g._ turned vegetable reds to blue, he called it _an alkaline
salt_.
Rouelle also proved experimentally that an acid salt contains more
acid--relatively to the same amount of base--than a neutral salt, and that
an alkaline salt contains more base--relatively to the same amount of
acid--than a neutral salt; and he proved that this excess of acid, or of
base, is chemically united to the rest of the salt--is, in other words, an
essential part of the salt, from which it cannot be removed without
changing the properties of the whole.
But we have not as yet got to know why certain qualities connoted by the
term "acid" can be affirmed to belong to a group of bodies, why certain
other, "alkaline," properties belong to another group, nor why a third
group can be distinguished from both of these by the possession of
properties which we sum up in the term "earthy." Surely there must be some
peculiarity in the composition of these substances, common to all, by
virtue of which all are acid. The atom of an acid is surely composed of
certain elements which are never found in the atom of an alkali or an
earth; or perhaps the difference lies in the number, rather than in the
nature of the elements in the acid atoms, or even in the arrangement of the
elementary atoms in the compound atom of acid, of alkali, and of earth.
I think that our knowledge of salt is now more complete than our knowledge
of either acid, alkali, or earth. We know that a salt is formed by the
union of an acid and an alkali or earth; if, then, we get to know the
composition of acids and bases (_i.e._ alkalis and earths), we shall be
well on the way towards knowing the composition of salts.
And now we must resume our story where we left it at p. 176. Lavoisier had
recognized oxygen as the acidifier; Black had proved that a caustic alkali
does not contain carbonic acid.
Up to this time metallic calces, and for the most part alkalis and earths
also, had been regarded as elementary substances. Lavoisier however proved
calces to be compounds of metals and oxygen; but as some of those calces
had all the properties which characterized earths, it seemed probable that
all earths are metallic oxides, and if all earths, most likely all alkalis
also. Many attempts were made to decompose earths and alkalis, and to
obtain the metal, the oxide of which the earth or the alkali was supposed
to be. One chemist thought he had obtained a metal by heating the earth
baryta with charcoal, but from the properties of his metal we know that he
had not worked with a pure specimen of baryta, and that his supposed
metallic base of baryta was simply a little iron or other metal, previously
present in the baryta, or charcoal, or crucible which he employed.
But if Lavoisier's view were correct--if all bases contained oxygen--it
followed that all salts are oxygen compounds. Acids all contain oxygen,
said Lavoisier; this was soon regarded as one of the fundamental facts of
chemistry. Earths and alkalis are probably oxides of metals; this before
long became an article of faith with all orthodox chemists. Salts are
produced by the union of acids and bases, therefore all salts contain
oxygen: the conclusion was readily adopted by almost every one.
When the controversy between Lavoisier and the phlogistic chemists was at
its height, the followers of Stahl had taunted Lavoisier with being unable
to explain the production of hydrogen (or phlogiston as they thought)
during the solution of metals in acids; but when Lavoisier learned the
composition of water, he had an answer sufficient to quell these taunts.
The metal, said Lavoisier, decomposes the water which is always present
along with the acid, hydrogen is thus evolved, and the metallic calx or
oxide so produced dissolves in the acid and forms a salt. If this
explanation were correct--and there was an immense mass of evidence in its
favour and apparently none against it--then all the salts produced by the
action of acids on metals necessarily contained oxygen.
The Lavoisierian view of a salt, as a compound of a metallic oxide--or
base--with a non-metallic oxide--or acid--seemed the only explanation which
could be accepted by any reasonable chemist: in the early years of this
century it reigned supreme.
But even during the lifetime of its founder this theory was opposed and
opposed by the logic of facts. In 1787 Berthollet published an account of
experiments on prussic acid,--the existence and preparation (from Prussian
blue) of which acid had been demonstrated three or four years before by the
Swedish chemist Scheele--which led him to conclude this compound to be a
true acid, but free from oxygen. In 1796 the same chemist studied the
composition and properties of sulphuretted hydrogen, and pronounced this
body to be an acid containing no oxygen.
But the experiments and reasoning of Berthollet were hidden by the masses
of facts and the cogency of argument of the Lavoisierian chemists.
The prevalent views regarding acids and bases were greatly strengthened by
the earlier researches of Sir Humphry Davy, in which he employed the
voltaic battery as an instrument in chemical investigation. Let us now
consider some of the electro-chemical work of this brilliant chemist.
In the spring of the year 1800 the electrical battery, which had recently
been discovered by Volta, was applied by Nicholson and Carlisle to effect
the decomposition of water. The experiments of these naturalists were
repeated and confirmed by Davy, then resident at Bristol, who followed up
this application of electricity to effect chemical changes by a series of
experiments extending from 1800 to 1806, and culminating in the Bakerian
Lecture delivered before the Royal Society in the latter year.
The history of Davy's life during these years, years rich in results of the
utmost importance to chemical science, will be traced in the sequel;
meanwhile we are concerned only with the results of his chemical work.
The first Bakerian Lecture of Humphry Davy, "On some Chemical Agencies of
Electricity," deserves the careful study of all who are interested in the
methods of natural science; it is a brilliant example of the
disentanglement of a complex natural problem.
Volta and others had subjected water to the action of a current of
electricity, and had noticed the appearance of acid and alkali at the
oppositely electrified metallic surfaces. According to some experimenters,
the acid was nitrous, according to others, muriatic acid. One chemist
asserted the production of a new and peculiar body which he called _the
electric acid_. The alkali was generally said to be ammonia.
When Davy passed an electric current through distilled water contained in
glass vessels, connected by pieces of moist bladder, cotton fibre, or
other vegetable matters, he found that nitric and hydrochloric acids were
formed in the water surrounding the positively electrified plate or pole,
and soda around the negatively electrified pole, of the battery.
When the same piece of cotton fibre was repeatedly used for making
connection between the glass vessels, and was washed each time in dilute
nitric acid, Davy found that the production of muriatic acid gradually
ceased; hence he traced the formation of this acid to the presence of the
animal or vegetable substance used in the experiments.
Finding that the glass vessels were somewhat corroded, and that the greater
the amount of corrosion the greater was the amount of soda making its
appearance around the negative pole, he concluded that the soda was
probably a product of the decomposition of the glass by the electric
current; he therefore modified the experiment. He passed an electric
current through distilled water contained in small cups of agate,
previously cleaned by boiling in distilled water for several hours, and
connected by threads of the mineral asbestos, chosen as being quite free
from vegetable matter; alkali and acid were still produced. The experiment
was repeated several times with the same apparatus; acid and alkali were
still produced, but the alkali decreased each time. The only conclusion to
be drawn was that the alkali came from the water employed. Two small cups
of gold were now used to contain the water; a very small amount of alkali
appeared at the negative pole, and a little nitric acid at the positive
pole. The quantity of acid slowly increased as the experiment continued,
whereas the quantity of alkali remained the same as after a few minutes'
action of the electric current. The production of alkali is probably due,
said Davy, to the presence in the water of some substance which is not
removed by distillation in a glass retort. By boiling down in a silver dish
a quantity of the water he had used, a very small amount of solid matter
was obtained, which after being heated was distinctly alkaline. Moreover
when a little of this solid matter was added to the water contained in the
two golden cups, there was a sudden and marked increase in the amount of
alkali formed around the negative pole. Another quantity of the water which
he had used was again distilled in a silver retort, and a little of the
distillate was subjected to electrolysis as before. No alkali appeared. A
little piece of glass was placed in the water; alkali quickly began to
form. Davy thus conclusively proved that the alkali produced during the
electrolysis (_i.e._ decomposition by the electric current) of water is not
derived from the water itself, but from mineral impurities contained in the
water, or in the vessel in which the water is placed during the experiment.
But the production of nitric acid around the positive pole was yet to be
accounted for.
Before further experiments could be made it was necessary that Davy should
form an hypothesis--that he should mentally connect the appearance of the
nitric acid with some other phenomenon sufficient to produce this
appearance; he could then devise experiments which would determine whether
the connection supposed to exist between the two phenomena really did exist
or not.
Now, of the constituents of nitric acid--nitrogen, hydrogen and oxygen--all
except the first named are present in pure water; nitrogen is present in
large quantity in the ordinary atmosphere. It was only necessary to assume
that some of the hydrogen and oxygen produced during the electrolysis of
water seized on and combined with some of the nitrogen in the air which
surrounded that water, and the continual production of nitric acid during
the whole process of electrolysis was explained.
But how was this assumption to be proved or disproved? Davy adopted a
method frequently made use of in scientific investigations:--remove the
assumed cause of a phenomenon; if the phenomenon ceases to be produced, the
assumed cause is probably the real cause. Davy surrounded the little gold
cups containing the water to be electrolysed with a glass jar which he
connected with an air-pump; he exhausted most of the air from the jar and
then passed the electric current through the water. Very little nitric acid
appeared. He now again took out most of the air from the glass jar,
admitted some hydrogen to supply its place, and again pumped this out. This
process he repeated two or three times and then passed the electric
current. _No_ acid appeared in the water. He admitted air into the glass
vessel; nitric acid began to be produced. Thus he proved that whenever air
was present in contact with the water being electrolysed, nitric acid made
its appearance, and when the air was wholly removed the acid ceased to be
produced. As he had previously shown that the production of this acid was
not to be traced to impurities in the water, to the nature of the vessel
used to contain the water, or to the nature of the material of which the
poles of the battery were composed, the conclusion was forced upon him that
the production of nitric acid in the water, and the presence of ordinary
air around the water invariably existed together; that if one of these
conditions was present, the other was also present--in other words, that
one was the cause of the other.
The result of this exhaustive and brilliant piece of work is summed up by
Davy in these words: "It seems evident then that water, chemically pure, is
decomposed by electricity into gaseous matter alone, into oxygen and
hydrogen."
From the effects of the electric current on glass, Davy argued that other
earthy compounds would probably undergo change under similar conditions. He
therefore had little cups of gypsum made, in which he placed pure water,
and passed an electric current through the liquid. Lime was formed around
the negative, and sulphuric acid around the positive pole. Using similar
apparatus, he proved that the electric current decomposes very many
minerals into an earthy or alkaline base and an acid.
Picturing to himself the little particles of a salt as being split by the
electric current each into two smaller particles, one possessed of acid and
the other of alkaline properties, Davy thought it might be possible to
intercept the progress of these smaller particles, which he saw ever
travelling towards the positive and negative poles of the battery. He
accordingly connected these small glass vessels by threads of washed
asbestos; in one of the outer vessels he placed pure water, in the other an
aqueous solution of sulphate of potash, and in the central vessel he placed
ammonia. The negative pole of the battery being immersed in the sulphate of
potash, and the positive pole in the water, it was necessary for the
particles of sulphuric acid--produced by the decomposition of the sulphate
of potash--to travel through the ammonia in the central vessel before they
could find their way to the positive pole. Now, ammonia and sulphuric acid
cannot exist in contact--they instantly combine to form sulphate of
ammonia; the sulphuric acid particles ought therefore to be arrested by the
ammonia. But the sulphuric acid made its appearance at the positive pole
just as if the central vessel had contained water. It seemed that the
mutual attraction ordinarily exerted between sulphuric acid and ammonia was
overcome by the action of the electric current. Ammonia would generally
present an insuperable barrier to the progress of sulphuric acid, but the
electrical energy appeared to force the acid particles over this barrier;
they passed towards their goal as if nothing stood in their way.
Experiments are now multiplied by Davy, and the general conclusion drawn is
that "Hydrogen, the alkaline substances, the metals and certain metallic
oxides are attracted by negatively electrified metallic surfaces, and
repelled by positively electrified metallic surfaces; and contrariwise,
that oxygen and acid substances are attracted by positively electrified
metallic surfaces, and repelled by negatively electrified metallic
surfaces; and these attractive and repulsive forces are sufficiently
energetic to destroy or suspend the usual operation of chemical
affinity."[10]
To account for this apparent suspension of the ordinary chemical laws, Davy
supposes that chemical compounds are continually decomposed and re-formed
throughout the liquid which is subjected to the electrical action. Thus, in
the experiment with water, ammonia and sulphate of potash, he supposes that
the sulphuric acid and ammonia do combine in the central vessel to form
sulphate of ammonia, but that this compound is again decomposed, by the
electrical energy, into sulphuric acid--which passes on towards the
positive pole--and ammonia--which remains in the central vessel--ready to
combine with more sulphuric acid as that comes travelling onwards from its
source in the vessel containing sulphate of potash to its goal in the
vessel containing water.
The eye of the philosopher had pierced beneath the apparent stability of
the chemical systems which he studied. To his vision there appeared in
those few drops of water and ammonia and sulphate of potash a never-ceasing
conflict of contending forces; there appeared a continual shattering and
rebuilding of the particles of which the masses were composed. The whole
was at rest, the parts were in motion; the whole was constant in chemical
composition, the composition of each particle was changed a thousand times
in the minutest portion of every second. To the mind of Davy, the
electrolysis of every chemical compound was a new application of the great
law established by Newton--"To every action there is an equal and opposite
reaction."
Each step made in chemical science since Davy's time has but served to
emphasize the universality of this principle of action and reaction, a
principle which has been too much overlooked in the chemical text-books,
but the importance of which recent researches are beginning to impress on
the minds of chemists.
It is the privilege of the philosophic student of Nature to penetrate the
veil with which she conceals her secrets from the vulgar gaze. To him are
shown sights which "eye hath not seen," and by him are perceived sounds
which "ear hath not heard." Each drop of water is seen by him not only to
be built up of myriads of small parts, but each particle is seen to be in
motion; many particles are being decomposed into still smaller particles of
matter, different in properties from the original particles, but as the
original particles are at the same time being reproduced, the continued
existence of the drop of water with the properties of water is to him the
result of the mutual action and reaction of contending forces. He knows
that rest and permanence are gained, not by the cessation of action, but by
the continuance of conflict; he knows that in the realm of natural
phenomena, stable equilibrium is the resultant of the action of opposite
forces, and that complete decomposition occurs only when one force becomes
too powerful or another becomes too weak.
Pursuing the train of thought initiated by the experiments which I have
described, Davy entered upon a series of researches which led him to
consider every chemical substance as possessing definite electrical
relations towards every other substance. "As chemical attraction between
two bodies seems to be destroyed by giving one of them an electrical state
different from that which it naturally possessed--that is, by bringing it
into a state similar to the other--so it may be increased by exalting its
natural energy." Thus zinc, a metal easily oxidized, does not combine with
oxygen when negatively electrified, whereas silver, a metal oxidized with
difficulty, readily combines with oxygen when positively electrified.
Substances in opposite electrical states appear to combine chemically, and
the greater the electrical difference the greater the readiness with which
chemical combination is effected. Electrical energy and chemical attraction
or _affinity_ are evidently closely connected; perhaps, said Davy, they are
both results of the same cause.
Thus Davy arrived at the conception of a system of bodies as maintained in
equilibrium by the mutual actions and reactions of both chemical and
electrical forces; by increasing either of these a change is necessarily
produced in the other. Under certain electrical conditions the bodies will
exert no chemical action on one another, but such action may be started by
changing these electrical conditions, or, on the other hand, by changes in
the chemical relations of the bodies a change in the electrical relations
may be induced. Thus Davy found that if plates of copper and sulphur are
heated, the copper exhibits a positive and the sulphur a negative
electrical condition; that these electrical states become more marked as
temperature rises, until the melting point of sulphur is reached, when the
copper and sulphur combine together chemically and produce sulphide of
copper.
When water is electrolysed, Davy looked on the oppositely electrified
metallic plates in the battery as striving to attain a state of
equilibrium; the negatively electrified zinc strives to gain positive
electricity from the copper, which strives to gain negative electricity
from the zinc. The water he regarded as the carrier of these electricities,
the one in this direction, the other in that. In thus acting as a carrier,
the water is itself chemically decomposed, with production of hydrogen and
oxygen; but this chemical rearrangement of some of the substances which
composed the original system (of battery and water) involves a fresh
disturbance of electrical energy, and so the process proceeds until the
whole of the water is decomposed or the whole of the copper or zinc plate
is dissolved in the battery. If the water were not chemically decomposed,
Davy thought that the zinc and copper in the battery would quickly attain
the state of electrical equilibrium towards which they continually strive,
and that the current would therefore quickly cease.
Davy thought that "however strong the natural electrical energies of the
elements of bodies may be, yet there is every probability of a limit to
their strength; whereas the powers of our artificial instruments seem
capable of indefinite increase." By making use of a very powerful battery,
he hoped to be able to decompose substances generally regarded as simple
bodies.
Taking a wide survey of natural phenomena, he sees these two forces, which
we call chemical and electrical, everywhere at work, and by their mutual
actions upholding the material universe in equilibrium. In the outbreaks
of volcanoes he sees the disturbance of this equilibrium by the undue
preponderance of electrical force; and in the formation of complex minerals
beneath the surface of the earth, he traces the action of those chemical
attractions which are ever ready to bring about the combination of
elements, if they are not held in check by the opposing influence of
electrical energy.
We shall see how the great and philosophical conception of Davy was used by
Berzelius, and how, while undoubtedly gaining in precision, it lost much in
breadth in being made the basis of a rigid system of chemical
classification.
Davy's hope that the new instrument of research placed in the hands of
chemists by Volta would be used in the decomposition of supposed simple
substances was soon to be realized. A year after the lecture "On some
Chemical Agencies of Electricity," Davy was again the reader of the
Bakerian Lecture; this year (1807) it was entitled, "On some New Phenomena
of Chemical Change produced by Electricity, particularly the Decomposition
of the Fixed Alkalis; and the Exhibition of the New Substances which
constitute their Bases; and on the General Nature of Alkaline Bodies."
In his first experiments on the effect of the electrical current on potash
and soda, Davy used strong aqueous solutions of these alkalis, with the
result that hydrogen and oxygen only were evolved. He then passed the
current through melted potash kept liquid during the operation by the use
of a spirit-lamp, the flame of which was fed with oxygen. Much light was
evolved, and a great flame appeared at the negative pole; on changing the
direction of the current, "aeriform globules, which inflamed in the air,
rose through the potash."
On the 6th of October 1807, a piece of potash was placed on a disc of
platinum, which was made the negative pole of a very powerful battery; a
platinum wire brought into contact with the upper surface of the potash
served as the positive pole. When the current was passed, the potash became
hot and soon melted; gas was evolved at the upper surface, and at the lower
(negative) side "there was no liberation of elastic fluid, but small
globules, having a high metallic lustre, and being precisely similar in
visible characters to quicksilver appeared, some of which burst with
explosion and bright flame as soon as they were formed, and others
remained, and were merely tarnished, and finally covered by a white film
which formed on their surfaces."
When Davy saw these metallic globules burst through the crust of fusing
potash, we are told by one of his biographers, "he could not contain his
joy, he actually bounded about the room in ecstatic delight; and some
little time was required for him to compose himself sufficiently to
continue the experiment."
This was the culminating point of the researches in which he had been
continuously engaged for about six years. His interest and excitement were
intense; the Bakerian Lecture was written "on the spur of the occasion,
before the excitement of the mind had subsided," yet, says his
biographer--and we may well agree with him--"yet it bears proof only of the
maturest judgment; the greater part of it is as remarkable for experimental
accuracy as for logical precision." But "to every action there is an equal
and opposite reaction:" immediately after the delivery of the lecture, Davy
was prostrated by a severe attack of illness, which confined him to bed for
nine weeks, and was very nearly proving fatal.
That the phenomenon just described was really the decomposition of potash,
and the production of the metal of which this substance is an oxygenized
compound, was proved by obtaining similar results whether plates of silver,
copper, or gold, or vessels of plumbago, or even charcoal, were used to
contain the potash, or whether the experiment was conducted in the air, or
in a glass vessel from which air had been exhausted, or in glass tubes
wherein the potash was confined by mercury. The decomposition of potash was
followed within a few days by that of soda, from which substance metallic
globules were obtained which took fire when exposed to the air.
But the analysis of potash and soda was not sufficient for Davy; he
determined to accomplish the synthesis of these substances. For this
purpose he collected small quantities of the newly discovered metals, by
conducting the electrolysis of potash and soda under experimental
conditions such that the metals, as soon as produced, were plunged under
the surface of naphtha, a liquid which does not contain oxygen, and which
protected them from the action of the surrounding air.
A weighed quantity of each metal was then heated in a stream of pure dry
oxygen, the products were collected and weighed, and it was found that
solutions of these products in water possessed all the properties of
aqueous solutions of potash and soda.
The new metals were now obtained in larger quantity by Davy, and their
properties carefully determined by him; they were named _potassium_ and
_sodium_ respectively. They were shown to possess all those properties
which were generally accepted as characteristic of metal, except that of
being heavy. The new metals were extremely light, lighter than water. For
some time it was difficult to convince all chemists that a metal could be a
very light substance. We are assured that a friend of Davy, who was shown
potassium for the first time, and was asked what kind of substance he
supposed it to be, replied, "It is metallic, to be sure;" "and then,
balancing it on his finger, he added in a tone of confidence, 'Bless me,
how heavy it is!'"
Davy argued that since the alkalis, potash and soda, were found to be
oxygen compounds of metals, the earths would probably also be found to be
metallic oxides. In the year 1808 he succeeded in decomposing the three
earths, lime, baryta and strontia, and in obtaining the metals _calcium_,
_barium_ and _strontium_, but not in a perfectly pure condition, or in any
quantity. He also got evidence of the decomposition of the earths silica,
alumina, zirconia and beryllia, by the action of powerful electric
currents, but he did not succeed in obtaining the supposed metallic bases
of these substances.
So far Davy's discoveries had all tended to confirm the generally accepted
view which regarded alkalis and earths as metallic oxides. But we found
that the outcome of these views was to regard all salts--and among these,
of course, common salt--as oxygen compounds.[11] Acids were oxygen
compounds, bases were oxygen compounds, and as salts were produced by the
union of acids with bases, they, too, must necessarily be oxygen compounds.
Berthollet had thrown doubt on the universality of Lavoisier's name
"oxygen," _the_ acidifier, but he had not conclusively proved the existence
of any acid which did not contain oxygen.
The researches of Davy naturally led him to consider the prevalent views
regarding acids, bases and salts.
Muriatic (or as we now call it hydrochloric) acid had long been a
stumbling-block to the thorough-going Lavoisierian chemists. Oxygen could
not be detected in it, yet it ought to contain oxygen, because oxygen is
the acidifier. Of course, if muriatic acid contains oxygen, the
salts--muriates--produced by the action of this acid on alkalis and earths
must also contain oxygen. Many years before this time the action of
muriatic acid on manganese ore had been studied by the Swedish chemist
Scheele, who had thus obtained a yellow-coloured gas with a very strong
smell. Berthollet had shown that when a solution of this gas in water is
exposed to sunlight, oxygen is evolved and muriatic acid is produced. The
yellow gas was therefore supposed to be, and was called, "oxidized muriatic
acid," and muriatic acid was itself regarded as composed of oxygen and an
unknown substance or _radicle_.
In 1809 Gay-Lussac and Thenard found that one volume of hydrogen united
with one volume of the so-called oxidized muriatic acid to form muriatic
acid; the presence of hydrogen in this acid was therefore proved.
When Davy began (1810-11) to turn his attention specially to the study of
salts, he adopted the generally accepted view that muriatic acid is a
compound of oxygen and an unknown radicle, and that by the addition of
oxygen to this compound oxidized muriatic acid is produced. But unless Davy
could prove the presence of oxygen in muriatic acid he could not long hold
the opinion that oxygen was really a constituent of this substance. He
tried to obtain direct evidence of the presence of oxygen, but failed. He
then set about comparing the action of muriatic acid on metals and metallic
oxides with the action of the so-called oxidized muriatic acid on the same
substances. He showed that salt-like compounds were produced by the action
of oxidized muriatic acid either on metals or on the oxides of these
metals, oxygen being evolved in the latter cases; and that the same
compounds and water were produced by the action of muriatic acid on the
same metallic oxides.
These results were most easily and readily explained by assuming the
so-called oxidized muriatic acid to be an elementary substance, and
muriatic acid to be a compound of this element with hydrogen. To the new
element thus discovered--for he who establishes the elementary nature of a
substance may almost be regarded as its discoverer--Davy gave the name of
_chlorine_, suggested by the yellow colour of the gas (from Greek, =
_yellow_). He at once began to study the analogies of chlorine, to find by
experiment which elements it resembled, and so to classify it. Many metals,
he found, combined readily with chlorine, with evolution of heat and light.
It acted, like oxygen, as a supporter of combustion; it was, like oxygen,
attracted towards the negative pole of the voltaic battery; its compound
with hydrogen was an acid; hence said Davy chlorine, like oxygen, is a
supporter of combustion and also an acidifier.
But it was very hard to get chemists to adopt these views. As Bacon says,
"If false facts in Nature be once on foot, what through neglect of
examination, the countenance of antiquity, and the use made of them in
discourse, they are scarce ever retracted."
Chemists had long been accustomed to systems which pretended to explain all
chemical facts. The phlogistic theory, which had tyrannized over chemistry,
had been succeeded by the Lavoisierian chemistry, which recognized one
acidifier, and this also the one supporter of combustion. To ascribe these
properties to any element other than oxygen appeared almost profane.
But when Davy spoke of chlorine as an acidifier, he did not use this word
in the same sense as that in which it was employed by the upholders of the
oxygen theory of acids; he simply meant to express the fact that a compound
containing chlorine as one of its constituents, but not containing oxygen,
was a true acid. When Gay-Lussac attempted to prove that hydrogen is an
_alkalizing principle_, Davy said, "This is an attempt to introduce into
chemistry a doctrine of occult qualities, and to refer to some mysterious
and inexplicable energy what must depend upon a peculiar corpuscular
arrangement." And with regard to Gay-Lussac's strained use of analogies
between hydrogen compounds and alkalis, he says, "The substitution of
analogy for fact is the bane of chemical philosophy; the legitimate use of
analogy is to connect facts together, and to guide to new experiments."
But Davy's facts were so well established, and his experiments so
convincing, that before two or three years had passed, most chemists were
persuaded that chlorine was an element--_i.e._ a substance which had never
been decomposed--and that muriatic acid was a compound of this element with
hydrogen.
Berzelius was among the last to adopt the new view. Woehler tells us that in
the winter of 1823, when he was working in the laboratory of Berzelius,
Anna, while washing some basins, remarked that they smelt strongly of
oxidized muriatic acid: "Now," said Berzelius, "listen to me, Anna. Thou
must no longer say 'oxidized muriatic acid,' but 'chlorine;' that is
better."
This work on chlorine was followed up, in 1813, by the proof that the class
of acidifiers and supporters of combustion contains a third elementary
substance, viz. iodine. As Davy's views regarding acids and salts became
developed, he seems to have more and more opposed the assumption that any
one element is especially to be regarded as the acidifying element; but at
the same time he seems to admit that most, if not all, acids contain
hydrogen. Such oxides as sulphur trioxide, nitrogen pentoxide, etc., do not
possess acid properties except in combination with water. But he of course
did not say that all hydrogen compounds are acids; he rather regarded the
possession by a substance of acid properties as dependent, to a great
extent, on the nature of the elements other than hydrogen which it
contained, or perhaps on the arrangement of all the elements in the
particles of the acid. He regarded the hydrogen in an acid as capable of
replacement by a metal, and to the metallic derivative--as it might be
called--of the acid, thus produced, he gave the name of "salt." An acid
might therefore be a compound of hydrogen with one other element--such were
hydrochloric, hydriodic, hydrofluoric acids--or it might be a compound of
hydrogen with two or more elements, of which one might or might not be
oxygen--such were hydrocyanic acid and chloric or nitric acid. If the
hydrogen in any of these acids were replaced by a metal a salt would be
produced. A salt might therefore contain no oxygen, _e.g._ chloride or
iodide of potassium; but in most cases salts did contain oxygen, _e.g._
chlorate or nitrate of potassium.
Acids were thus divided into oxyacids (or acids which contain oxygen) and
acids containing no oxygen; the former class including most of the known
acids. The old view of salts as being compounds of acids (_i.e._ oxides of
the non-metallic elements) and bases (_i.e._ oxides of metals) was
overthrown, and salts came to be regarded as metallic derivatives of acids.
From this time, these terms--acids, salts, bases--become of less importance
than they formerly were in the history of chemical advance.
In trying to explain Davy's electro-chemical theory I have applied the word
_affinity_ to the mutual action and reaction between two substances which
combine together to form a chemical compound. It is now necessary that we
should look a little more closely into the history of this word _affinity_.
Oil and water do not mix together, but oil and potash solution do; the
former may be said not to have, and the latter to have, an affinity one for
the other. When sulphur is heated, the yellow odourless solid, seizing upon
oxygen in the air, combines with it to produce a colourless strongly
smelling gas. Sulphur and oxygen are said to have strong affinity for each
other.
If equal weights of lime and magnesia be thrown into diluted nitric acid,
after a time it is found that some of the lime, but very little of the
magnesia, is dissolved. If an aqueous solution of lime be added to a
solution of magnesia in nitric acid, the magnesia is precipitated in the
form of an insoluble powder, while the lime remains dissolved in the acid.
It is said that lime has a stronger affinity for nitric acid than magnesia
has. Such reactions as these used to be cited as examples of _single
elective affinity_--single, because one substance combined with one other,
and elective, because a substance seemed to choose between two others
presented to it, and to combine with one to the exclusion of the other.
But if a neutral solution of magnesia in sulphuric acid is added to a
neutral solution of lime in nitric acid, sulphate of lime and nitrate of
magnesia are produced. The lime, it was said, leaves the nitric and goes
to the sulphuric acid, which, having been deserted by the magnesia, is
ready to receive it; at the same time the nitric acid from which the lime
has departed combines with the magnesia formerly held by the sulphuric
acid. Such a reaction was said to be an instance of _double affinities_.
The chemical changes were caused, it was said, by the simultaneous affinity
of lime for sulphuric acid, which was greater than its affinity for nitric
acid, and the affinity of magnesia for nitric acid, which was greater than
its affinity for sulphuric acid.
If a number of salts were mixed, each base--supposing the foregoing
statements to be correct--would form a compound with that acid for which it
had the greatest affinity. It should then be possible to draw up tables of
affinity. Such tables were indeed prepared. Here is an example:--
_Sulphuric Acid._
Baryta. Lime.
Strontia. Ammonia.
Potash. Magnesia.
Soda.
This table tells us that the affinity of baryta for sulphuric acid is
greater than that of strontia for the same acid, that of strontia greater
than that of potash, and so on. It also tells that potash will decompose a
compound of sulphuric acid and soda, just as soda will decompose a compound
of the same acid with lime, or strontia will decompose a compound with
potash, etc.
But Berthollet showed in the early years of this century that a large
quantity of a body having a weak affinity for another will suffice to
decompose a small quantity of a compound of this other with a third body
for which it has a strong affinity. He showed, that is, that the formation
or non-formation of a compound is dependent not only on the so-called
affinities between the constituents, but also on the relative quantities of
these constituents. Berthollet and other chemists also showed that affinity
is much conditioned by temperature; that is, that two substances which show
no tendency towards chemical union at a low temperature may combine when
the temperature is raised. He, and they, also proved that the formation or
non-formation of a compound is much influenced by its physical properties.
Thus, if two substances are mixed in solution, and if by their mutual
action a substance can be produced which is insoluble in the liquids
present, that substance is generally produced whether the affinity between
the original pair of substances be strong or weak.
The outcome of Berthollet's work was that tables of affinity became almost
valueless. To say that the affinity of this body for that was greater than
its affinity for a third body was going beyond the facts, because the
formation of this or that compound depended on many conditions much more
complex than those connoted by the term "affinity." Yet the conception of
affinity remained, although it could not be applied in so rigorous a way as
had been done by the earlier chemists. If an element, A, readily combines
with another element, B, under certain physical conditions, but does not,
under the same conditions, combine with a third element, C, it may still be
said that A and B have, and A and C have not, an affinity for each other.
This general conception of affinity was applied by Berzelius to the atoms
of elements. Affinity, said Berzelius, acts between unlike atoms, and
causes them to unite to form a compound atom, unlike either of the original
atoms; cohesion, on the other hand, acts between like atoms, causing them
to hold together without producing any change in their properties. Affinity
varies in different elements. Thus the affinity of gold for oxygen is very
small; hence it is that gold is found in the earth in the metallic state,
while iron, having a great affinity for oxygen, soon rusts when exposed to
air, or when buried in the earth. Potassium and sodium have great
affinities for oxygen, chlorine, etc.; yet the atoms of potassium and
sodium do not themselves combine. The more any elements are alike
chemically the smaller is their affinity for each other; the more any
elements are chemically unlike the greater is their mutual affinity; but
this affinity is modified by circumstances. Thus, said Berzelius, if equal
numbers of atoms of A and B, having equal or nearly equal affinity for C,
mutually react, compound atoms, AC and BC, will be produced, but atoms of A
and B will remain. The amounts of AC and BC produced will be influenced by
the greater or less affinity of A and B for C; but if there be a greater
number of A than of B atoms, a greater amount of AC than of BC will be
produced. In these cases all the reacting substances and the products of
the actions are supposed to be liquids; but BC, if a solid substance, will
be produced even if the affinity of A for C is greater than that of B for
C.
In some elements, Berzelius taught, affinity slumbers, and can be awakened
only by raising the temperature. Thus carbon in the form of coal has no
affinity for oxygen at ordinary temperatures; it has remained for ages in
the earth without undergoing oxidation; but when coal is heated the
affinities of carbon are awakened, combination with oxygen occurs, and heat
is produced.
But why is it that certain elementary atoms exhibit affinity for certain
others? It depends, said Berzelius, on the electrical states of these
atoms. According to the Berzelian theory, every elementary atom has
attached to it a certain quantity of electricity, part of which is positive
and part negative. This electricity is accumulated at two points on each
atom, called respectively the positive pole and the negative pole; but in
each atom one of these electricities so much preponderates over the other
as to give the whole atom the character of either a positively or a
negatively electrified body. When two atoms combine chemically the positive
electricity in one neutralizes the negative electricity in the other. As we
know that similar electricities repel, and opposite electricities attract
each other, it follows that a markedly positive atom will exhibit strong
affinity for a markedly negative atom, less strong affinity for a feebly
negative, and little or no affinity for a positively electrified atom; but
two similarly electrified atoms may exhibit affinity, because in every
positive atom there is some negative electricity, as in every negative atom
there is some positive electricity. Thus, in the atoms of copper and zinc
positive electricity predominates, said Berzelius, but the zinc atoms are
more positive than those of copper; hence, when the metals are brought into
contact the negative electricity of the copper atoms is attracted and
neutralized by the positive electricity of the zinc atoms, combination
takes place, and the compound atom is still characterized by a predominance
of positive electricity.
Hence Berzelius identified "electrical polarity" with chemical affinity.
Every atom was regarded by him as _both_ positively _and_ negatively
electrified; but as one of these electricities was always much stronger
than the other, every atom regarded as a whole appeared to be _either_
positively _or_ negatively electrified. Positive atoms showed affinity for
negative atoms, and _vice versa_. As a positive atom might become more
positive by increasing the temperature of the atom, so might the affinity
of this atom for that be more marked at high than at low temperatures.
Now, if two elementary atoms unite, the compound atom must--according to
the Berzelian views--be characterized either by positive or negative
electricity. This compound atom, if positive, will exhibit affinity for
other compound atoms in which negative electricity predominates; if
negative, it will exhibit affinity for other positively electrified
compound atoms. If two compound atoms unite chemically, the complex atom so
produced will, again, be characterized by one or other of the two
electricities, and as it is positive or negative, so will it exhibit
affinity for positively or negatively electrified complex atoms. Thus
Berzelius and his followers regarded every compound atom, however complex,
as essentially built up of two parts, one of which was positively and the
other negatively electrified, and which were held together chemically by
virtue of the mutual attractions of these electricities; they regarded
every compound atom as a _dual_ structure. The classification adopted by
Berzelius was essentially a dualistic classification. His system has always
been known in chemistry as _dualism_.
Berzelius divided compound atoms (we should now say molecules) into three
groups or orders--
_Compound atoms of the first order_, formed by the immediate combination of
atoms of two, or in organic compounds of three, elementary substances.
_Compound atoms of the second order_, formed by the combination of atoms of
an element with atoms of the first order, or by the combination of two or
more atoms of the first order.
_Compound atoms of the third order_, formed by combination of two or more
atoms of the second order.
When an atom of the third order was decomposed by an electric current, it
split up, according to the Berzelian teaching, into atoms of the second
order--some positively, others negatively electrified. When an atom of the
second order was submitted to electrolysis, it decomposed into atoms of the
first order--some positively, others negatively electrified.
Berzelius said that a base is an electro-positive oxide, and an acid is an
electro-negative oxide. The more markedly positive an oxide is, the more
basic it is; the more negative it is, the more is it characterized by acid
properties.
One outcome of this teaching regarding acids and bases was to overthrow the
Lavoisierian conception of oxygen as the acidifying element. Some oxides
are positive, others negative, said Berzelius; but acids are characterized
by negative electricity, therefore the presence of oxygen in a compound
does not always confer on that compound acid properties.
We have already seen that silica was regarded by most chemists as a typical
earth; but Berzelius found that in the electrolysis of compounds of silica,
this substance appeared at the positive pole of the battery--that is, the
atom of silica belonged to the negatively electrified order of atoms.
Silica was almost certainly an oxide; but electro-negative oxides are, as a
class, acids; therefore silica was probably an acid. The supposition of
the acid character of silica was amply confirmed by the mineralogical
analyses and experiments of Berzelius. He showed that most of the earthy
minerals are compounds of silica with electro-positive metallic oxides, and
that silica plays the part of an acid in these minerals; and in 1823 he
obtained the element silicon, the oxide of which is silica. On this basis
Berzelius reared a system of classification in mineralogy which much aided
the advance of that branch of natural science.
By the work of Berzelius and Davy the Lavoisierian conception of acid has
now been much modified and extended; it has been rendered less rigid, and
is therefore more likely than before to be a guide to fresh discoveries.
The older view of acid and alkali was based, for the most part, on a
qualitative study of the reactions of chemical substances: bodies were
placed in the same class because they were all sour, or all turned
vegetable blues to red, etc. This was followed by a closer study of the
composition of substances, and by attempts to connect the properties of
these substances with their composition; but when this attempt resulted in
the promulgation of the dictum that "oxygen is the acidifying principle,"
it began to be perceived that a larger basis of fact must be laid before
just conclusions could be drawn as to the connections between properties
and composition of substances. This larger basis was laid by the two
chemists whose work we have now reviewed. Of the life of one of these men
I have already given such a sketch as I can from the materials available to
me; of the life of the other we happily possess ample knowledge. Let us now
consider the main features of this life.
* * * * *
HUMPHRY DAVY, the eldest son of Robert and Grace Davy, was born at
Penzance, in Cornwall, on December 17, 1778, eight months that is before
the birth of Berzelius. His parents resided on a small property which had
belonged to their ancestors for several generations. Surrounded by many
kind friends by whom he was much thought of, the boy appears to have passed
a very happy childhood. Even at the age of five his quickness and
penetration were marked by those around him, and at school these continued
to be his predominant characteristics. Nurtured from his infancy in the
midst of beautiful and romantic scenery, and endowed with great observing
power and a lively imagination, young Davy seemed destined to be one of
those from whose lips is "poured the deathless singing;" all through life
he was characterized by a strongly marked poetic temperament.
Humphry Davy was held in much esteem by his school friends as a composer of
valentines and love letters, as a daring and entertaining teller of
stories, and as a successful fireworks manufacturer. Such a combination of
qualities would much endear him to his boy-companions. We are told that at
the age of eight he used to mount on an empty cart, around which a circle
of boys would collect to be entertained by the wonderful tales of the
youthful narrator.
Finishing his school education at the age of fifteen, he now began his own
education of himself. In 1795 he was apprenticed to a surgeon and
apothecary (afterwards a physician), in Penzance, with whom he learned the
elements of medical science; but his time during the years which he spent
under Mr. Borlase was much occupied in shooting, fishing, searching for
minerals and geological specimens, composing poetry, and pursuing
metaphysical speculations. He was now, as through life, an enthusiastic
lover of Nature; his mind was extremely active, ranging over the most
diverse subjects; he was full of imagination, and seemed certain to
distinguish himself in any pursuit to which he should turn his attention.
During the next three or four years Davy indulged freely in speculations in
all manner of subjects; he started, as people generally do when young, from
general principles and followed these out to many conclusions. Even in his
study of physiology and other branches of science, he appears at this time
to have adopted the speculative rather than the experimental method; but
unlike most youthful metaphysicians he was ready to give up an opinion
whenever it appeared to him incorrect. By the time he reached the age of
twenty he had discarded this method of seeking for truth, and was ever
afterwards distinguished by his careful working out of facts as the
foundation for all his brilliant theories.
Davy appears to have begun the study of chemistry about 1798 by reading
Lavoisier's "Elements of Chemistry," the teachings of which he freely
criticized. About this time Mr. Gregory Watt came to live at Penzance as a
lodger with Davy's mother, and with him the young philosopher had much talk
on chemical and other scientific subjects. He also became acquainted with
Mr. Davies Gilbert--who was destined to succeed Davy as President of the
Royal Society--and from him he borrowed books and received assistance of
various kinds in his studies.
It was during these years that Davy made experiments on heat, which were
published some years later, and which are now regarded as laying the
foundations of the modern theory according to which heat is due to the
motions of the small parts of bodies. He arranged two brass plates so that
one should carry a block of ice which might be caused to revolve in contact
with the other plate; the plates were covered by a glass jar, from which he
exhausted the air by means of a simple syringe of his own contrivance; the
machine being placed on blocks of ice the plates were caused to revolve.
The ice inside the jar soon melted; Davy concluded that the heat required
to melt this ice could only be produced by the friction of the ice and
brass, and that therefore heat could not be any form of ponderable matter.
In the year 1798 Davy was asked to go to Bristol as superintendent of the
laboratory of a new Pneumatic Institution started by Dr. Beddoes for the
application of gases to the treatment of diseases. Davy had corresponded
with Beddoes before this time regarding his experiments on heat, and the
latter seems to have been struck with his great abilities and to have been
anxious to secure him as experimenter for his institution. Davy was
released from his engagements with Mr. Borlase, and, now about twenty years
of age, set out for his new home, having made as he says all the
experiments he could at Penzance, and eagerly looking forward to the better
appliances and incitements to research which he hoped to find at Bristol.
The Pneumatic Institution was supported by subscriptions, for the most part
from scientific men. It was started on a scientific basis. Researches were
to be made on gases of various kinds with the view of applying these as
remedies in the alleviation of disease. An hospital for patients, a
laboratory for experimental research, and a lecture theatre were provided.
At this time many men of literary and intellectual eminence resided in
Bristol; among these were Coleridge and Southey. Most of these men were
visitors at the house of Dr. Beddoes, and many distinguished men came from
various parts of the county to visit the institution. Davy thus entered on
a sphere of labour eminently suited for the development of his genius. With
ample mechanical appliances for research, with plenty of time at his
disposal, surrounded by an atmosphere of inquiry and by men who would
welcome any additions he could make to the knowledge of Nature, and being
at the same time not without poetic and imaginative surroundings, by which
he was ever spurred onwards in the pursuit of truth--placed in these
circumstances, such an enthusiastic and diligent student of science as Davy
could not but obtain results of value to his fellows. The state of chemical
science at this time was evidently such as to incite the youthful worker.
The chains with which Stahl and his successors had so long bound the limbs
of the young science had been broken by Lavoisier; and although the French
school of chemistry was at this time dominant, and not disinclined to treat
as ignorant any persons who might differ from its teaching, yet there was
plenty of life in the cultivators of chemistry. The controversy between
Berthollet and Proust was about to begin; the Lavoisierian views regarding
acids and salts were not altogether accepted by Gay-Lussac, Thenard and
others; and from the laboratory of Berzelius there was soon to issue the
first of those numerous researches which drew the attention of every
chemist to the capital of Sweden. The voltaic battery had been discovered,
and had opened up a region of possibilities in chemistry.
Davy began his researches at the institution by experiments with nitrous
oxide, a gas supposed by some people at that time to be capable of
producing most harmful effects on the animal system. He had to make many
experiments before he found a method for preparing the pure gas, and in the
course of these experiments he added much to the stock of chemical
knowledge regarding the compounds of nitrogen and oxygen. Having obtained
fairly pure nitrous oxide, he breathed it from a silk bag; he experienced a
"sensation analogous to gentle pressure on all the muscles;... the objects
around me became dazzling and my hearing more acute;... at last an
irresistible propensity to action was indulged in.... I recollect but
indistinctly what followed; I know that my motions were various and
violent." Southey and Coleridge breathed the gas; the poets only laughed a
little. Encouraged by the results of these experiments, Davy proceeded to
prepare and breathe nitric oxide--whereby he was rendered very ill--and
then carburetted hydrogen--which nearly killed him.
In his chemical note-book about this time, Davy says, "The perfection of
chemical philosophy, or the laws of corpuscular motion, must depend on the
knowledge of all the simple substances, their mutual attractions, and the
ratio in which the attractions increase or diminish with increase or
diminution of temperature.... The first step towards these laws will be the
decomposition of those bodies which are at present undecompounded." And in
the same note-book he suggests methods which he thinks might effect the
decomposition of muriatic and boric acids, the alkalis and earths. Here
are the germs of his future work.
After about eight months' work at Bristol he published a volume of
"Researches," which contained a great many new facts, and was characterized
by vigour and novelty of conception. These researches had been carried out
with intense application; each was struck off at a red heat. His mind
during this time was filled with vast scientific conceptions, and he began
also to think of fame. "An active mind, a deep ideal feeling of good, and a
look towards future greatness," he tells us, sustained him.
Count Rumford, the founder of the Royal Institution in London, was anxious
to obtain a lecturer on chemistry for the Institution. Davy was strongly
recommended, and after a little arrangement--concerning which Davy says in
a letter, "I will accept of no appointment except on the sacred terms of
independence"--he was appointed Assistant Lecturer on Chemistry and
Director of the Laboratory. About a year later his official designation was
changed to Professor of Chemistry. This appointment opened up a great
sphere of research; "the sole and uncontrolled use of the apparatus of the
institution for private experiments" was to be granted him, and he was
promised "any apparatus he might need for new experiments."
He had now the command of a good laboratory; he had not to undergo the
drudgery of systematic teaching, but was only required to give lectures to
a general audience. Before leaving Bristol he had commenced experiments on
the chemical applications of the voltaic battery; these he at once followed
up with the better apparatus now at his command. The results of this
research, and his subsequent work on the alkalis and on muriatic acid and
chlorine, have been already described. The circumstances of Davy's life had
hitherto been most favourable; how nobly he had availed himself of these
circumstances was testified by the work done by him.
His first lecture was delivered in the spring of 1801, and at once he
became famous. A friend of Davy says, "The sensation created by his first
course of lectures at the Institution, and the enthusiastic admiration
which they obtained, is scarcely to be imagined. Men of the first rank and
talent, the literary and the scientific, the practical and the theoretical,
blue-stockings and women of fashion, the old and the young--all crowded,
eagerly crowded the lecture-room. His youth, his simplicity, his natural
eloquence, his chemical knowledge, his happy illustrations and
well-conducted experiments, excited universal attention and unbounded
applause. Compliments, invitations and presents were showered upon him in
abundance from all quarters; his society was courted by all, and all
appeared proud of his acquaintance." One of his biographers says of these
lectures, "He was always in earnest, and when he amused most, amusement
appeared most foreign to his object. His great and first object was to
instruct, and in conjunction with this, maintain the importance and dignity
of science; indeed, the latter, and the kindling a taste for scientific
pursuits, might rather be considered his main object, and the conveying
instruction a secondary one."
The greatest pains were taken by Davy in the composition and rehearsal of
his lectures, and in the arrangement of experiments, that everything should
tend towards the enlightenment of his audience. Surrounded by a brilliant
society, invited to every fashionable entertainment, flattered by admirers,
tempted by hopes of making money, Davy remained a faithful and enthusiastic
student of Nature. "I am a lover of Nature," he writes at this time to a
friend, "with an ungratified imagination. I shall continue to search for
untasted charms, for hidden beauties. My _real_, my _waking_ existence, is
amongst the objects of scientific research. Common amusements and
enjoyments are necessary to me only as dreams to interrupt the flow of
thoughts too nearly analogous to enlighten and vivify."
During these years (_i.e._ from 1802 to 1812) he worked for the greater
part of each day in the laboratory. Every week, almost every day, saw some
fresh discovery of importance. He advanced from discovery to discovery. His
work was characterized by that vast industry and extreme rapidity which
belong only to the efforts of genius. Never, before or since, has chemical
science made such strides in this country.
In 1803 Davy was elected a Fellow, and in 1807 one of the secretaries of
the Royal Society. In 1812 he retired from the professorship of chemistry
at the Royal Institution; in the same year he was made a knight.
The next two or three years were mostly spent in travelling abroad with his
wife--he had married a widow lady, Mrs. Apreece, in 1812. During his visit
to Paris he made several experiments on the then recently discovered
iodine, and proved this substance to be an element.
The work which Davy had accomplished in the seventeen years that had now
elapsed since he began the study of chemistry, whether we consider it
simply as a contribution to chemical science, or in the light of the
influence it exerted on the researches of others, was of first-rate
importance; but a fresh field now began to open before him, from which he
was destined to reap the richest fruits. In the autumn of 1815 his
attention was drawn to the subject of fire-damp in coal-mines. As he passed
through Newcastle, on his return from a holiday spent in the Scottish
Highlands, he examined various coal-mines and collected samples of
fire-damp; in December of the same year his _safety-lamp_ was perfected,
and soon after this it was in the hands of the miner.
The steps in the discovery of this valuable instrument were briefly these.
Davy established the fact that fire-damp is a compound of carbon and
hydrogen; he found that this gas must be mixed with a large quantity of
ordinary air before the mixture becomes explosive, that the temperature at
which this explosion occurs is a high one, and that but little heat is
produced during the explosion; he found that the explosive mixture could
not be fired in narrow metallic tubes, and also that it was rendered
non-explosive by addition of carbonic acid or nitrogen. He reasoned on
these facts thus: "It occurred to me, as a _considerable_ heat was required
for the inflammation of the fire-damp, and as it produced in burning a
comparatively _small degree_ of heat, that the effect of carbonic acid and
azote, and of the surfaces of small tubes, in preventing its explosion,
depended on their cooling powers--upon their lowering the temperature of
the exploding mixture so much that it was no longer sufficient for its
continuous inflammation." He at once set about constructing a lamp in which
it should be impossible for the temperature of ignition of a mixture of
fire-damp and air to be attained, and which therefore, while burning, might
be filled with this mixture without any danger of an explosion. He
surrounded the flame of an oil-lamp with a cylinder of fine wire-gauze;
this lamp when brought into an atmosphere containing fire-damp and air
could not cause an explosion, because although small explosions might occur
in the interior of the wire cylinder, so much heat was conducted away by
the large metallic surface that the temperature of the explosive atmosphere
outside the lamp could not attain that point at which explosion would
occur.
In 1818 Sir Humphry Davy was made a baronet, in recognition of his great
services as the inventor of the safety-lamp; and in 1820 he was elected to
the most honourable position which can be held by a man of science in this
country, he became the President of the Royal Society.
For seven years he was annually re-elected president, and during that time
he was the central figure in the scientific society of England. During
these years he continued his investigations chiefly on electro-chemical
subjects and on various branches of applied science. In 1826 his health
began to fail. An attack of paralysis in that year obliged him to
relinquish most of his work. He went abroad and travelled in Italy and the
Tyrol, sometimes strong enough to shoot or fish a little, or even to carry
on electrical experiments; sometimes confined to his room, or to gentle
exercise only. He resigned the presidentship of the Royal Society in 1827.
In 1828 he visited Rome, where he was again attacked by paralysis, and
thought himself dying, but he recovered sufficiently to attempt the journey
homeward. At Geneva he became very ill, and expired in that city on the
29th of May 1829.
During these later years of illness and suffering, his intense love of and
delight in Nature were very apparent; he returned again to the simple
tastes and pleasures of his early days. His intimate knowledge of natural
appearances and of the sights and sounds of country life is conspicuous in
the "Salmonia, or Days of Fly-fishing," written during his later years.
Sir Humphry Davy was emphatically a genius. He was full of eager desire to
know the secrets of the world in which he lived; he looked around him with
wonder and delight, ever conscious of the vastness of the appearances which
met his gaze; an exuberance of life and energy marked his actions;
difficulties were encountered by him only to be overcome; he was depressed
by no misfortunes, deterred by no obstacles, led aside from his object by
no temptations, and held in bondage by no false analogies.
His work must ever remain as a model to the student of science. A thorough
and careful foundation of fact is laid; on this, hypotheses are raised, to
be tested first by reasoning and argument, then by the tests of the
laboratory, which alone are final. Analogies are seized; hints are eagerly
taken up, examined, and acted on or dismissed. As he works in the
laboratory, we see his mind ranging over the whole field of chemical
knowledge, finding a solution of a difficulty here, or guessing at a
solution there; combining apparently most diverse facts; examining
phenomena which appear to have no connection; never dwelling too long on an
hypothesis which cannot yield some clue to the object of research, but
quickly discovering the road which will lead to the wished-for solution.
Like so many great experimenters Davy accomplished wonders with little
apparatus. When he went abroad for the first time he took with him two
small boxes, one twenty, and the other twelve inches long, by about seven
inches wide and four deep. With the apparatus contained in these boxes he
established the elementary nature of iodine, and made a rough estimation of
its atomic weight; he determined many of its analogies with chlorine,
proving that, like chlorine, it is markedly electro-negative, and that its
compounds are decomposed by chlorine; he accomplished the synthesis of
hydriodic acid, and approximately determined the composition of iodide of
nitrogen. But when it was necessary to employ delicate or powerful
apparatus, he was able by the use of that also to obtain results of primary
importance. The decomposition of potash, soda, baryta, lime and strontia
could not have been effected had he not had at his command the resources of
a well-furnished laboratory.
Davy has had no successor in England. Much useful and some brilliant work
has been done by English chemists since his day, but we still look back to
the first quarter of the century as the golden age of chemistry in this
country. On the roll wherein are written the names of England's greatest
sons, there is inscribed but a single chemist--Humphry Davy.
* * * * *
I carried on the account of the work of Davy's great contemporary,
Berzelius, to the time when he had fairly established dualistic views of
the structure of chemical compounds, and when, by the application of a few
simple rules regarding the combinations of elementary atoms, he had largely
extended the bounds of the atomic theory of Dalton.
Berzelius also did important work in the domain of organic chemistry. By
numerous analyses of compounds of animal and vegetable origin, he clearly
established the fact that the same laws of combination, the same fixity of
composition, and the same general features of atomic structure prevail
among the so-called organic as among the inorganic compounds. In doing this
he broke down the artificial barrier which had been raised between the two
branches of the science, and so prepared the way for modern chemistry,
which has won its chief triumphs in the examination of organic compounds.
By the many and great improvements which he introduced into analytical
chemistry, and by the publication of his "Textbook of Chemistry," which
went through several editions in French and German, and also of his yearly
report on the advance of chemistry, Berzelius exerted a great influence on
the progress of his favourite science. Woehler tells us that when the spring
of the year came, at which time his annual report had to be prepared,
Berzelius shut himself up in his study, surrounded himself with books, and
did not stir from the writing-table until the work was done.
In his later days Berzelius was much engaged in controversy with the
leaders of the new school, the rise and progress of which will be traced in
the next chapter, but throughout this controversy he found time to add many
fresh facts to those already known. He continued his researches until his
death in 1848.
The work of the great Swedish chemist is characterized by thoroughness in
all its parts: to him every fact appeared to be of importance; although now
perhaps only an isolated fact, he saw that some day it would find a place
in a general scheme of classification. He worked in great measure on the
lines laid down by Dalton and Davy; the enormous number and accuracy of his
analyses established the law of multiple proportions on a sure basis, and
his attempts to determine the constitution of compound atoms, while
advancing the atomic theory of Dalton, drew attention to the all-important
distinction between atom and molecule, and so prepared chemists for the
acceptance of the generalization of Avogadro. The electro-chemical
conceptions of Davy were modified by Berzelius; they were shorn of
something of their elasticity, but were rendered more suited to be the
basis of a rigid theory.
* * * * *
At the close of this transition period from the Lavoisierian to the modern
chemistry, we find analytical chemistry established as an art; we find the
atomic theory generally accepted, but we notice the existence of much
confusion which has arisen from the non-acceptance of the distinction made
by Avogadro between atom and molecule; we find the analogies between
chemical affinity and electrical energy made the basis of a system of
classification which regards every compound atom (or molecule) as built up
of two parts, in one of which positive, and in the other negative
electricity predominates; and accompanying this system of classification we
find that an acid is no longer regarded as necessarily an oxygen compound,
but rather as a compound possessed of certain properties which are probably
due to the arrangement of the elementary atoms, among which hydrogen
appears generally to find a place; we find that salts are for the most part
regarded as metallic derivatives of acids; and we find that by the
decomposition of the supposed elementary substances, potash, soda, lime,
etc., the number of the elements has been extended, the application of a
new instrument of research has been brilliantly rewarded, and the
Lavoisierian description of "element" as the "attained, not the attainable,
limit of research" has been emphasized.
FOOTNOTES:
[9] The history and meaning of these terms is considered on p. 171, _et
seq._
[10] For an explanation of this expression, "chemical affinity," see p.
206, _et seq._
[11] These views have been already explained on pp. 182, 183.