[_Concluded._]
It is evident, from what has been already said, that chemistry
and physics are near akin--indeed, they can hardly be separated.
Avogadro’s law and spectrum analysis are but two illustrations of the
relationship, but many other examples are equal to them in importance.
Take, for instance, the action of light upon chemical substances;
it may provoke union of elements, or effect the decomposition
of compounds; upon the latter phenomenon the art of photography
depends. That salts of silver are chemically changed by light was
the fundamental observation, and upon this fact most photographic
processes, though not all, are founded. Thus light, working as a
chemist in the laboratory of the photographic plate, has become the
useful servant of all arts, all sciences, and all industries--an
indispensable aid to invention and research. On this theme a volume
might be written; a bare reference to it must be sufficient here.
Still another branch of chemistry, recently developed but essentially
an extension of the theory of valence, is also due to the study
of optical relations. That different crystalline bodies differ in
their behavior toward polarized light has long been known, and the
polariscope is recognized as an instrument of great value in chemical
research. To the analysis and valuation of sugars and sirups it is most
effectively applied, and commercial transactions of great magnitude
depend in part upon its testimony. Here is practical utility, but the
development of theory is what concerns us now.
The discovery of isomerism, of the fact that very different compounds
might contain the same elements united in the same proportions, was
easily interpreted by the theory of valence in a fairly complete and
satisfactory way. In the structural formulæ the different atomic
groupings were clearly shown, but with one essential limitation--the
arrangement was in a single plane. That is, the linking of the atoms
was considered, but not their relations to tridimensional space.
For the study of reactions, for the classification of compounds,
the structural symbols sufficed; but human thought is not so easily
satisfied, and more was soon required. One class of isomers was
unexplained, and an explanation was demanded.
A typical example of the difficulty was offered by tartaric acid, which
exists in two forms differing crystallographically and optically. One
form, dissolved in water, twisted a ray of polarized light to the left,
the other produced a rotation to the right, while the crystals of the
two acids, similar in all other respects, also showed a right- and
left-handedness in the arrangement of their planes. The crystal of one
variety resembled the other as would its reflection in a mirror--the
same, but reversed. These differences, discovered by Pasteur as long
ago as 1848, the theory of valence could not explain; to interpret
them, and other similar cases, the arrangement of the atoms in space
had to be considered.
In 1874 two chemists, Van t’Hoff and Lebel, working independently,
offered a solution of the problem, and stereochemistry, the chemistry
of molecular structure in three dimensions, was founded. They proposed,
in effect, to treat the carbon atom essentially as a tetrahedron, the
four angles corresponding to the four units of valence or bonds of
affinity. They then studied the linking or union of such tetrahedra,
and found that with their aid the formulæ for tartaric acid could be
developed in different ways, showing right- and left-handed atomic
groupings. Other similar compounds were equally explicable. Thus the
definite conception of a tridimensional, geometric atom led to a new
development of structural formulæ, from which many discoveries have
already proceeded. The fruitfulness of the speculation vindicates its
use, but it is only the first step in a method of research which must
in time be applied to all of the chemical elements. Probably the study
of crystalline form will be connected with these chemico-structural
expressions, and from the union some greater generalizations will be
born. From the geometry of the crystal to the geometry of the molecule
there must be some legitimate transition. With all their utility,
our present conceptions of chemical structure are incomplete; they
represent only portions or special phases of some great general law,
but so far as they go, properly used, they are valid.
But light is not the only physical force involved in chemical changes;
heat and electricity are far more important. Heat, in particular, is
essential to every chemical operation; it provokes combination and
effects decomposition; it appears in one reaction and vanishes in
another; apart from thermal phenomena the science of chemistry could
not exist. From the very beginnings of chemistry this interdependence
has been recognized, and its study has led to notable discoveries and
to great enlargements of resource. In the theory of phlogiston the
connection between heat and chemical change was crudely stated, and
when Lavoisier saw that combustion was oxidation, thermochemistry began
to exist.
In every chemical change a definite amount of heat is either liberated
or absorbed--a distinct, measurable quantity. This fact was established
by Hess in 1840, and since then the thermal values of many reactions
have been determined, notably by Thomsen in Denmark and Berthelot
in France. The data are already numerous, but as yet they have not
been co-ordinated into any general law. They are in great measure the
raw material with which some future scholar is to build. One fact,
however, is already clear--namely, that the heat of formation or
of combustion of any compound is conditioned by its structure. Two
isomeric substances may differ widely in their calorific constants, an
observation which has repeatedly been verified. Thus the conception of
structure, of atomic grouping, appears again a chief factor in a set of
unsolved problems.
In the relations of chemistry to heat perhaps the greatest advances
have been made in the extension of our resources, particularly in
regard to the development and control of temperatures. At the beginning
of the century the range of temperatures available to the chemist was
narrowly limited--from the freezing point of mercury at one end to the
heat of a blast furnace at the other. His command of heat and cold are
now vastly greater than then, and the steps which have been taken are
worth tracing.
At the lower end of the scale the greatest progress has been made
through the liquefaction of gases. When a liquid evaporates, heat is
absorbed, or, reversely stated, cold is produced, and the more rapid
the evaporation the greater is the cooling effect. A command of more
volatile liquids is therefore a command of cold, and the liquefied
gases represent the extreme limit of our power in that direction.
Near the beginning of the century, by combining cold and pressure,
sulphurous acid and chlorine were reduced to the liquid state. In
1823 Faraday succeeded in liquefying still other gases, and in 1835
Thilorier went even further and reduced carbonic acid to a snowlike
solid. Liquid chlorine, sulphurous acid, and carbonic acid, stored in
strong cylinders of steel, are now commercial products, manufactured
and sold in large quantities like any other merchandise. They can be
transported to long distances and kept indefinitely, to the great
convenience of chemists and the furtherance of research.
In 1845 Faraday published the results of further investigations, when
it appeared that all but six of the known gases had been reduced to the
liquid state. Through cold and pressure lower and lower temperatures
were gained, each step forward having given a foothold from which a
new advance was possible. In 1877 Pictet and Cailletet simultaneously
succeeded in liquefying four of the supposedly permanent gases;
nitrogen yielded to the attacks of Wroblevsky and Olzewski in 1883,
and hydrogen alone remained unconquered. In 1898 Dewar overcame this
last obstacle, and in the year following he actually reduced hydrogen
to an icelike solid which melts at only eighteen degrees centigrade
above the theoretical absolute zero. Every gas has been liquefied,
and probably the lowest degree of cold attainable by man has been
reached. Within the century the work began and ended; the future can
only improve the working methods and utilize the new resources. Liquid
ammonia has long been used in the manufacture of artificial ice and for
direct refrigeration; liquid air, with its temperature of two hundred
centigrade degrees below zero, is now almost a commercial product,
obtainable in quantity, but with its possibilities of usefulness as yet
practically undeveloped. The infant Hercules will doubtless find no
lack of tasks to do, each one more arduous and more helpful to man than
any labor of his mythical prototype.
To the chemist the possibilities thus opened are innumerable. Pictet
has shown that at the low temperatures which are now easily attainable
all chemical action stops, even the most energetic substances lying
in contact with each other quiet and inert. A greater control of the
more violent chemical reactions is therefore within reach, doubtless to
be utilized in many ways yet unforeseen. At the highest temperatures,
also, chemical union ceases, and compounds are decomposed; each
reaction is possible only within a limited thermal range, of which the
beginning and the end are measurable. From future measurements of this
sort new laws will surely be discovered.
The first step in the upward scale of temperatures was taken in 1802,
when Robert Hare, an American, invented the oxyhydrogen blowpipe.
With this instrument platinum, hitherto infusible, was melted, a
result of great importance to chemists. Apart from recent electrical
applications, platinum finds its chief use in the construction of
chemical apparatus, and Hare’s invention was therefore of great
assistance to chemical research. Later in the century electrical
currents were utilized as producers of great heat, until in the
very modern device of the electric furnace the range of available
temperatures has been at least doubled. Temperatures of three to four
thousand degrees of the centigrade scale are now at the disposal of the
chemist, and these are manageable in compact apparatus at very moderate
cost. Cheap aluminum is one of the products of this new instrument,
and the extraordinary abrasive substance, carborundum, is another. New
industries have been created by the electric furnace, and in the hands
of Moissan it has yielded scientific results of great interest and
remarkable variety. The rarest metals can now be separated from their
oxides with perfect ease, and new compounds, obtainable in no other
way, the furnace has placed at our disposal. This field of research is
now barely opened; from it the twentieth century should gather a rich
harvest.
With electricity also chemistry is nearly allied, and along some lines
the two branches of science have been curiously intertwined. Like
the other physical forces, electricity may either provoke or undo
combination, and, like heat, it may itself be generated as a product of
chemical action. The voltaic pile and the galvanic battery owe their
currents to chemical change, and it is only since the middle of the
century that any other source of electric energy has become available
for practical purposes. It is not surprising, therefore, that many
thinkers should have sought to identify chemical and electric force,
the two have so much in common. It was with the galvanic current that
Davy decomposed the alkalies, and since his day other electro-chemical
decompositions have been studied in great number, to the development
of important industries. To the action of the current upon metallic
solutions we owe the electrotype and all our processes for
electroplating, and these represent only the beginnings of usefulness.
Even now, almost daily, advances are being made in the practical
applications of electrolysis, and the forward movement is likely to
continue throughout the coming century. From the curiously reversible
chemical reactions of the secondary battery the automobile derives
its power, and here again we find a field for invention so large that
its limits are beyond our sight. From every peak that science can
scale new ranges come into view. The solution of one problem always
creates another, and this fact gives to scientific investigation its
chief interest. We gain, only to see that more gain is possible; the
opportunity for advance is infinite. Forever and ever thought can reach
out into the unknown, and never need to weep because there are no more
worlds to conquer.
It was the study of electro-chemical changes which led Berzelius to
his electro-chemical theory of combination, and then to the dualistic
theory, which has already been mentioned. In or about the year 1832,
when the Berzelian doctrines were at the summit of their fame, Faraday
showed that the chemical power of a current was directly proportioned
to the quantity of electricity which passed, and this led him to
believe that chemical affinity and electric energy were identical.
Electrolysis, the electrical decomposition of compounds in solution,
was a special object of his attention, and by quantitative methods he
found that the changes produced could be stated in terms of chemical
equivalents or combining numbers. One equivalent weight of zinc
consumed in the galvanic battery yields a current which will deposit
one equivalent of silver from its solution, or which, decomposing
water, will liberate one equivalent each of oxygen and hydrogen. All
electro-chemical changes followed this simple law, which gave new
emphasis to the atomic theory, and furnished a new means for measuring
the combining numbers.
In the early days of electro-chemistry the products of electrolysis
were studied in the light of the dualistic theory. But as chemical
investigation along other lines overthrew this hypothesis, a closer
examination of electrolytic reactions became necessary. Electrical
decompositions were dualistic in character, but the dualism was
not that taught by Berzelius. When a salt, dissolved in water, is
decomposed by the current it is separated into two parts, which Faraday
called its _ions_; in Berzelian terms these were in most cases oxides,
but this conclusion fitted only a part of the facts, and finally was
abandoned. Whatever the _ions_ might be, they were not ordinary oxides.
Many and long were the investigations bearing upon this subject
before a satisfactory settlement was reached. The phenomena observed
in solutions, raised still another question, that of the nature of
solution itself, and this is not yet fully answered. Two lines of
study, however, have converged, within recent years, to some remarkable
conclusions, the latest large development of chemical theory.
It has long been known that solutions of salts do not freeze so
easily as pure water, and also that their boiling points are higher.
In 1883 Raoult discovered a remarkable relation between the freezing
point of a solution and the molecular weight of the substance
dissolved, a relation which has since been elaborately studied by
many investigators. From either the freezing-point depression or the
elevation of the boiling point the molecular weight of a soluble
compound can now be calculated, and many uncertain molecular weights
have thus been determined.
Another phenomenon connected with solutions, which has received much
attention, is that of osmotic pressure. A salt in solution exercises
a definite pressure, quantitatively measurable, which is curiously
analogous to the pressure exerted by gases. In a gas the molecules are
widely separated, and move about with much freedom. In a very dilute
solution the molecules of a salt are similarly separated, and are also
comparatively free to move. The kinetic theory of gases, therefore, is
now paralleled by a kinetic theory of solutions, founded by Van t’Hoff
in 1887, which is now generally accepted. All the well-established laws
connecting pressure, temperature, and volume among gases find their
equivalents in the phenomena exhibited by solutions. In Avogadro’s
law we learn that equal volumes of gases, under like conditions of
temperature and pressure, contain equal numbers of molecules. According
to the new generalizations, equal volumes of different solutions, _if
they exert the same osmotic pressure_, also contain equal numbers
of molecules. The parallelism is perfect. With these relations the
freezing- and boiling-point phenomena are directly connected.
But, both for gases and for solutions some apparent anomalies existed.
Certain compounds, when vaporized, seemed not to conform to Avogadro’s
law, and called for explanation. This proved to be simple, and was
supplied by the fact that the anomalous compound, as such, did not
exist as vapor, but was split up, _dissociated_, into other things. For
instance, ammonium chloride, above a certain temperature, is decomposed
into a mixture of two gases--hydrochloric acid and ammonia--which, on
cooling, reunite and reproduce the original compound. Twice as much
vapor as is required by theory, and specifically half as heavy, is
produced by this transformation, which is only one of a large class,
all well understood.
In the case of solutions it was found that certain compounds, notably
the acids, alkalies, and metallic salts, caused a depression of
freezing point which was twice as great as ought to be expected.
This fact was illuminated by the phenomena observed in gases, and
soon it was seen that here too a splitting up of molecules, a true
dissociation, occurred. These anomalous solutions, moreover, were
electrolytes--that is, they conducted electricity and underwent
electrolytic decompositions--while normal substances, especially
solutions of carbon compounds, such as sugar, were not.
Van t’Hoff’s discoveries went far, but one more step was needed, and
this was taken by Arrhenius in 1888. Electrolytic compounds, when
dissolved, are actually dissociated into their _ions_, partially so in
a strong solution, entirely so in one which is infinitely dilute, a
statement which leads to some extraordinary conclusions. For instance,
the _ions_ of common salt are sodium and chlorine. In a dilute solution
the salt itself ceases to exist, while atoms of sodium and atoms of
chlorine wander about, chemically separated from each other but still
in equilibrium. Sodium sulphate may be regarded as made up of two
parts--sodium and an acid radicle which contains one atom of sulphur to
four of oxygen--and these parts, its _ions_, are severed apart during
solution to move about independent of each other.
This theory of Arrhenius, the theory of electrolytic dissociation, is
supported by many facts, and fits in well with the kinetic theory of
Van t’Hoff. Electrolysis is no longer to be considered as a separating
process, but rather as a sorting of the _ions_, which receive different
electrical charges and concentrate at the two electrical poles. The
phenomena of freezing and boiling points in solutions, and of the
absorption of heat when solid salts are dissolved, all harmonize with
the conclusions which have been reached. A complete theory of solutions
is yet to be proposed; but these new doctrines, which are true so far
as they go, represent a long step in the right direction. A final
theory will include them, but they are not likely to be set aside.
As we near the end of the century we find one more discovery to note,
from a most unexpected quarter--the discovery of new gases in the
atmosphere. In 1893 Lord Rayleigh was at work upon new determinations
of density, with regard to the more important gases. In the case of
nitrogen an anomaly appeared: nitrogen obtained from the atmosphere
was found to be very slightly heavier than that prepared from chemical
sources, but the difference was so slight that it might almost have
been ignored. To Rayleigh, however, such a procedure was inadmissible,
and he sought for an explanation of his results. Joining forces with
Ramsay, the observed discrepancies were hunted down, and in 1894 the
discovery of argon was announced. Ramsay soon found in certain rare
minerals another new gas--helium--whose spectral lines had previously
been noted in the spectrum of the sun; and still later, working with
liquid air, he discovered four more of these strange elements--krypton,
xenon, neon, and metargon. By extreme accuracy of measurement this
chain of discovery was started, and, as some one has aptly said, it
represents the triumph of the third decimal. A noble dissatisfaction
with merely approximate data was the motive which initiated the work.
To the chemist these new gases are sorely puzzling. They come from a
field which was thought to be exhausted, and cause us to wonder why
they were not found before. The reason for the oversight is plain:
the gases are devoid of chemical properties, at least none have yet
been certainly observed. They are colorless, tasteless, odorless,
inert; so far they have been found to be incapable of union with other
elements; apart from some doubtful experiments of Berthelot, they form
no chemical compounds. Under the periodic law they are difficult to
classify; they seem to belong nowhere; they simply exist, unsocial,
alone. Only by their density, their spectra, and some physical
properties can these intractable new forms of matter be identified.
In a sketch like this a host of discoveries must remain unnoticed,
and others can be barely mentioned. The isolation of fluorine and
the manufacture of diamonds by Moissan, the synthesis of sugars by
Fischer, the discovery of soluble forms of silver by Carey Lea--all
these achievements and many more must be passed over. Something,
however, needs to be said upon the utilitarian aspects of chemistry,
and concerning its influence upon other sciences. Portions of this
field have been touched in the preceding pages; the interdependence of
chemistry and physics is already evident; other subjects now demand our
attention.
Medicine and physiology are both debtors to chemistry for much of
their advancement, and in more than one way. From the chemist medicine
has received a host of new remedies, some new processes, and advanced
methods for the diagnosis of disease. The staining of tissues for
identification under the microscope is effected by chemical agents,
the analysis of urine helps to identify disorders of the kidneys;
nitrous oxide, chloroform, ether, and cocaine almost abolish pain. The
disinfection of the sick-room and the antiseptic methods which go far
toward the creation of modern surgery all depend upon chemical products
whose long list increases year by year. Crude drugs are now replaced
by active principles discovered in the laboratory--morphine, quinine,
and the like--and instead of the bulky, nauseous draughts of olden
time, the invalid is given tasteless capsules of gelatin or compressed
tablets of uniform strength and more accurately graded power. A great
part of physiology consists of the study of chemical processes, the
transformation of compounds within the living organism, and practically
all this advance is the creation of the nineteenth century. Modern
bacteriology, at least in its practical applications, began with a
chemical discussion between Liebig and Pasteur as to the nature of
fermentation: step by step the field of exploration has enlarged; as
the result of the investigations we have preventive medicine, more
perfect sanitation, and antiseptic surgery. The ptomaines which cause
disease and the antitoxins which prevent it are alike chemical in
their nature, and were discovered by chemical methods. Physiology
without chemistry could not exist; even the phenomena of respiration
were meaningless before the discovery of oxygen. The human body is a
chemical laboratory, and without the aid of the chemist its mysteries
can not be unraveled.
To agriculture also chemistry is a potent ally, whose value can
hardly be overrated. It has created fertilizers and insecticides for
the use of the farmer and taught their intelligent use, and in the
many experiment stations of the world it is daily discovering facts
or principles which are practically applicable to agriculture. The
beet-sugar industry was developed by chemical researches and chemical
methods; the arts of the dairy have been chemically improved; the
food of all civilized nations is better and more abundant than it was
before the chemist gave his aid to its production. Adulteration, always
practiced, is now easily detected by chemical analysis, and, though the
evil still exists, the remedy for it is in sight. To Liebig, who gave
to agricultural chemistry its first great impulse forward, mankind is
indebted to an amount which is beyond all computation.
In manufactures the influence of chemistry is seen at every turn.
When the century began, probably no industrial establishment in the
world dreamed of maintaining a chemical laboratory; to-day, hundreds
are well equipped and often heavily manned for the sole benefit of
the intelligent manufacturer. Coal gas is a chemical product; its
by-products are ammonia and coal tar; from the latter, as we have
seen, hundreds of useful substances, the discoveries of the last half
century, are prepared. Better and cheaper soap and glass owe their
existence to chemical improvement in the making of alkalies; chemical
bleaching has replaced the tedious action of sunlight and dew; chemical
dyestuffs give our modern fabrics nearly all their hues. Metallurgy is
almost wholly a group of chemical processes; every metal is extracted
from its ores by methods which rest on chemical foundations; analyses
of fuel, flux, and product go on side by side with the smelting. The
cyanide and chlorination processes for gold, the Bessemer process for
steel, are apt illustrations of the advances in chemical metallurgy;
but before these come into play the dynamite of the miner, another
chemical invention, must have done its work underground. For rare
minerals, the mere curiosities of twenty years ago, uses have been
found; from monazite we obtain the oxides which form the mantle of the
Welsbach burner; from beauxite, aluminum is made. The former waste
products of many an industry have also revealed unsuspected values, and
chemistry has the sole honor of their discovery.
In education, chemistry has steadily grown in importance, until a
single university may have need of as many as twenty chemists in its
teaching staff, teaching not only what is already known, but also the
art of research. As a disciplinary study, chemistry ranks high in the
college curriculum, and it opens the way to a new learned profession,
equal in rank with those of more ancient standing.
For the material advancement of mankind the nineteenth century has
done more than all the preceding ages combined, and science has been
the chief instrument of progress. Scientific methods, experimental
investigation, have replaced the old empiricism, and no man can
imagine where the forward movement is to end. Hitherto research
has been sporadic, individual, unorganized; but fruitful beyond
all anticipation. In the future it should become more systematic,
better organized, richer in facilities. Through laboratories equipped
for research alone the twentieth century must work, and chemistry
is entitled to its fair share of the coming opportunities. The
achievements of the chemist, great as they have been during this
century, are but a beginning; the larger possibilities are ahead. The
greatest laws are yet undiscovered; the invitation of the unknown was
never more distinct than now.