Such a system has no degrees of freedom. This means that no physical
condition, pressure or temperature can be varied without destroying a
phase, so that such a system can only exist in equilibrium at one fixed
temperature, with a fixed value for its vapor-pressure.
[Illustration: J. William Gibbs]
For instance, if the system is heated above the fixed temperature, ice
disappears and if the pressure is raised, vapor is condensed. If this
same system of water alone contains but two phases, for instance, liquid
and vapor, F = 1 + 2 − 2 = 1, or there is one degree of freedom. In such
a system, one physical condition such as temperature can be varied
independently, but only one, without destroying a phase. For instance,
the temperature may be raised or lowered, but for every value of
temperature there is a corresponding value for the vapor-pressure. One
is a function of the other. If both values are varied independently, one
phase will disappear, either vapor condensing entirely to water or the
reverse. Finally if the system consists of one phase only, as water
vapor, F = 2, or the system is divariant, which means that at any given
temperature it is possible for vapor to exist at varying pressures.
The illustration which has been given relates to physical equilibrium,
but the rule is applicable to cases involving chemical changes as well.
In comparing the phase-rule with the law of mass action, it will be
noticed that both have to do with equilibrium. The great advantage of
the former is that it is entirely independent of the molecular condition
of the substances in the different phases. For instance, it makes no
difference so far as the application of the rule is concerned, whether a
substance in solution is dissociated, undissociated or combined with the
solvent. In any case, the solution constitutes one phase. On the other
hand, the rule is purely qualitative, giving information only as to
whether a given change in conditions is possible. The law of mass action
is a quantitative expression so that when the value of the constant is
once known, the change can be calculated which takes place in the entire
system if the concentration of one substance is varied. The law,
however, requires a knowledge of the molecular condition of the reacting
substances, which may be uncertain or unknown, and chiefly on this
account it has, like the phase-rule, often only a qualitative
significance.
The phase rule has served as a most valuable means of classifying
systems in equilibrium and as a guide in determining the possible
conditions under which such systems can exist. As illustrations of its
practical application, van’t Hoff used it as an underlying principle in
his investigations on the conditions under which salt deposits have been
formed in nature, and Rooseboom was able by its means to explain the
very complicated relations existing in the alloys of iron and carbon
which form the various grades of wrought iron, steel and cast iron.
_Thermochemistry._—This branch of chemistry has to do with heat evolved
or absorbed in chemical reactions. It is important chiefly because in
many cases it furnishes the only measure we have of the energy changes
involved in reactions. To a great extent, it dates from the discovery by
Hess in 1840 of a fundamental law which states that the heat evolved in
a reaction is the same whether it takes place in one or in several
stages. This law has made it possible to calculate the heat values of a
large number of reactions which cannot be determined by direct
experiment.
Thermochemistry has been developed by a comparatively few men who have
contributed a surprisingly large number of results. Favre and
Silbermann, beginning shortly after 1850, improved the apparatus for
calorimetric determinations, which is called the calorimeter, and
published many results. At about the same time Julius Thomsen, and in
1873 Berthelot, began their remarkable series of publications which
continued until recently. Thomsen’s investigations were published in
1882 in =4= volumes. It is probably safe to say that the greater part of
the data of thermochemistry was obtained by these two investigators. The
bomb calorimeter, an apparatus for determining heat values by direct
combustion, was developed by Berthelot. The recent work of Mixter at
Yale, published in the Journal, and of Richards at Harvard should be
mentioned particularly. Mixter’s work in this field began in 1901 (=12=,
347). Using an improved bomb calorimeter, he has developed a method of
determining the heats of formation of oxides by combustion with sodium
peroxide. By this same method as well as by direct combustion in oxygen,
he has obtained results which appear to equal or excel in accuracy any
which have ever been obtained in his field of work. Richards’s work has
consisted largely of improvements in apparatus. He developed the
so-called adiabatic calorimeter which practically eliminates one of the
chief errors in thermal work caused by the heating or cooling effect of
the surroundings. This modification is being generally adopted where
extremely accurate work is required.
_Organic Chemistry._
One hundred years ago qualitative tests for a few organic compounds were
known, the elements usually occurring in them were recognized, and some
of them had been analyzed quantitatively, but organic chemistry was far
less advanced than inorganic, and almost the whole of its enormous
development has taken place during our period.
Berzelius made a great advance in the subject by establishing the fact,
which had been doubted previously, that the elements in organic
compounds are combined in constant, definite proportions. In 1823 Liebig
brought to light the exceedingly important fact of isomerism by showing
that silver fulminate had the same percentage composition as silver
cyanate, a compound of very different properties. Isomeric compounds
with identical molecular weight as well as the same composition have
since been found in very many cases, and they have played a most
important part in determining the arrangements of atoms in molecules.
They have been found to be very numerous in many cases. For instance,
three pentanes with the formula C_{5}H_{12}are known, all that are
possible according to theory, and in each case the structure of the
molecule has been established. On theoretical grounds it has been
calculated that 802 isomeric compounds with the formula C_{13}H_{28} are
possible, while with more complex formulas the numbers of isomers may be
very much greater.
A particularly interesting case of isomerism was observed by Wöhler in
1828, when he found that ammonium cyanate changes spontaneously into
urea
(NH_{4}CNO → N_{2}H_{4}CO).
This was the first synthesis of an organic compound from inorganic
material, and it overthrew the prevailing view that vital forces were
essential in the formation of organic substances. A great many natural
organic compounds have been made artificially since that time, and some
of them, such as artificial alizarin, indigo, oil of wintergreen, and
vanillin, have more or less fully replaced the natural products. The
preparation of a vast number of compounds not known in nature, many of
which are of practical importance as medicines, dyes, explosives, etc.,
has been another great achievement of organic chemistry.
The development of our present formulas for organic compounds, by means
of which in many cases the relative positions of the atoms can be shown
with the greatest confidence, has been gradual. Formulas based on the
dualistic idea of Berzelius were used for some time, type-formulas, with
the employment of compound radicals, came later, the substitution of
atoms or groups of atoms for others in chemical reactions came to be
recognized, but one of the most important steps was the recognition of
the quadrivalence of carbon and the general application of valency to
atoms by Kekulé about 1858. This led directly to the use of modern
structural formulas which have been of the greatest value in the
theoretical interpretation of organic reactions. It was Kekulé also who
proposed the hexagonal ring-formula for benzene, C_{6}H_{6}, which led
to exceedingly important theoretical and practical developments. The
details of the formulas for many other rings and complex structures have
been established since that time, and there is no doubt that the
remarkable achievements in organic chemistry during the past sixty years
have been much facilitated by the use of these formulas.
Many important researches in organic chemistry have been carried out in
the United States, and the activity in this direction has greatly
increased in recent years. In this connection the large amount of work
of this kind accomplished in the Sheffield Laboratory, at present under
the guidance of Professor T. B. Johnson, should be mentioned.
It has happened that comparatively few publications on organic chemistry
have appeared in the Journal, but it may be stated that the preparation
of chloroform and its physiological effects were described by Guthrie
(=21=, 64, 1832). Unknown to him, it had been prepared by Souberain, a
French chemist, the previous year, but the former was the first to
describe its physiological action. Silliman gave a sample to Doctor Eli
Ives of the Yale Medical School, who used it to relieve a case of
asthma. This was the first use of chloroform in medical practice (=21=,
405, 1832). Guthrie also described in the Journal (=21=, 284, 1832) his
new process for converting potato starch into glucose, a method which is
essentially the same as that used to-day in converting cornstarch into
glucose. Lawrence Smith (=43=, 301, 1842 _et seq._), Horsford (=3=, 369,
1847 _et seq._), Sterry Hunt (=7=, 399, 1849), Carey Lea (=26=, 379,
1858 _et seq._), Remsen (=5=, 179, 1873 _et seq._), and others have
contributed articles on organic chemistry.
_Agricultural Chemistry._
Until near the middle of the nineteenth century, it was believed that
plants, like animals, used organic matter for food, and depended chiefly
upon the humus of the soil for their growth. This view was held even
long after it was known that plant leaves absorb carbon dioxide and give
off oxygen, and after the ashes of plants had been accurately analyzed.
This incorrect view was overthrown by the celebrated German chemist,
Liebig, who made many investigations upon the subject, and, properly
interpreting previous knowledge, published a book in 1840 upon the
application of chemistry to agriculture and physiology in which he
maintained that the nutritive materials of all green plants are
inorganic substances, namely, carbon dioxide, water, ammonia (nitrates),
sulphates, phosphates, silica, lime, magnesia, potash, iron, and
sometimes common salt. He drew the vastly important conclusion that the
effective fertilization of soils depends upon replenishing the inorganic
substances that have been exhausted by the crops.
The fundamental principles set forth by Liebig have been confirmed, and
it has been found that the fertilizing constituents most commonly
lacking in soils are nitrogen compounds, phosphates, and potassium
salts, so that these have formed the important constituents of
artificial fertilizers. Liebig himself found that humus is valuable in
soils, because it absorbs and retains the soluble salts.
The foundation established by Liebig in regard to artificial fertilizers
has led to an enormous application of these materials, much to the
advantage of the world’s food supply.
It was Liebig’s belief, in accordance with the prevailing views, that
decay and putrefaction as well as alcoholic and other fermentations were
spontaneous processes, and when the eminent French chemist, Pasteur, in
1857, explained fermentation as directly caused by yeast, an
epoch-making discovery which led to the explanation of decay and
putrefaction by bacterial action and to the germ-theory of disease, the
explanation was violently opposed by Liebig and other German chemists.
Pasteur’s view prevailed, however, and since that time it has been found
that various kinds of bacteria are responsible for the formation of
ammonia from nitrogenous organic matter and also for the change of
ammonia into the nitrates that are available as plant-food.
The long-debated question as to the availability of atmospheric nitrogen
for plant-food was settled in 1886 by the discovery of Hellriegel that
bacteria contained in nodules on the roots, especially of leguminous
plants, are capable of bringing nitrogen into combination and furnishing
it to the plants.
No more than an allusion can be made to agricultural experiment stations
where soils, fertilizers, foods and other products are examined, and
where other problems connected with agriculture are studied.
The late S. W. Johnson of Yale studied with Liebig and subsequently did
much service for agricultural chemistry in this country, by his
investigations, his teaching, and his writings. His book, “How Crops
Grow,” published in 1868, gave an excellent account of the principles of
agricultural chemistry. He did much to bring about the establishment of
agricultural experiment stations in this country, and for a long time he
was the director of the Connecticut Station.
In the Journal, as early as 1827, Amos Eaton (=12=, 370) published a
simple method for the mechanical analysis of soils to determine their
suitability for wheat-culture, and Hilgard, between 1872 and 1874,
described an elaborate study of soil-analysis. J. P. Norton, a Yale
professor, in 1847 (=3=, 322) published an investigation on the analysis
of the oat, which was awarded a prize of fifty sovereigns by a Scotch
agricultural society, while Johnson, Atwater, and others have
contributed articles on the analysis of various farm products.
_Industrial Acids and Alkalies._
One hundred years ago sulphuric acid was manufactured on a comparatively
very small scale in lead chambers. In 1818, an English manufacturer of
the acid introduced the modern feature of using pyrites in the place of
brimstone, while the Gay-Lussac tower in 1827 and the Glover tower in
1859 began to be applied as great improvements in the chamber process.
Within about twenty years the contact process, employing platinized
asbestos, has replaced the old chamber process to a large extent. It has
the advantage of producing the concentrated acid, or the fuming acid,
directly.
During our period the manufacture of sulphuric acid has increased
enormously. Very large quantities of it have been used in connection
with the Leblanc soda process in its rapid development. It came to be
employed extensively for absorbing ammonia in the illuminating-gas
industry, which was in its infancy one hundred years ago. New industries
such as the manufacture of “superphosphates” as artificial fertilizers,
the refining of petroleum, the manufacture of artificial dyestuffs and
many other modern chemical products have greatly increased the demand
for it, while its employment in the production of nitric and other
acids, and for many other purposes not already mentioned, has been very
great.
The manufacture of nitric acid has been greatly extended during our
period on account of its employment for producing explosives, artificial
dyestuffs, and for many other purposes. Chile saltpeter became available
for making it about 1852. This acid has been manufactured recently from
atmospheric nitrogen and oxygen by combining them by the aid of powerful
electric discharges. This process has been used chiefly in Norway where
water-power is abundant, as it requires a large expenditure of energy. A
still more recent method for the production of nitric acid depends upon
the oxidation of ammonia by air with the aid of a contact substance,
such as platinized asbestos.
The production of ammonia, which was very small a hundred years ago, has
been vastly increased in connection with the development of the
illuminating-gas industry and the employment of by-product coke ovens.
This substance is very extensively used in refrigerating machines and
also in a great many chemical operations, including the Solvay soda
process. Ammonium salts are of great importance also as fertilizers in
agriculture. The conversion of atmospheric nitrogen into ammonia on a
commercial scale is a recent achievement. It has been accomplished by
heating calcium carbide, an electric-furnace product made from lime and
coke, with nitrogen gas, thus producing calcium cyanamide, and then
treating this cyanamide with water under proper conditions. Another
method devised by Haber consists in directly combining nitrogen and
hydrogen gases under high pressure with the aid of a contact substance.
Leblanc’s method for obtaining sodium carbonate from sodium chloride by
first converting the latter into the sulphate by means of sulphuric acid
and then heating the sulphate with lime and coal in a furnace was
invented as early as 1791, but it was not rapidly developed and did not
gain a foothold in England until 1826 on account of a high duty on salt
up to that time. Afterwards the process flourished greatly in connection
with the sulphuric acid industry upon which it depended, and with the
bleaching-powder industry which utilized the hydrochloric acid
incidentally produced by it, and, of course, in connection with soap
manufacture and many other industries in which the soda itself was
employed.
About 1866 the Solvay process appeared as a rival to the Leblanc
process. This depends upon the precipitation of sodium bicarbonate from
salt solutions by means of carbon dioxide and ammonia, with the
subsequent recovery of the ammonia. It has displaced the older process
to a large extent, and it is carried on extensively in this country, for
instance, at Syracuse, New York.
Other processes for soda depend upon the electrolysis of sodium chloride
solutions. In this case caustic soda and chlorine are the direct
products, and the chlorine thus produced and liquefied by pressure in
steel cylinders, has become an important commercial article.
In earlier times wood-ashes were the source of potash and potassium
salts. Wurtz in the Journal (=10=, 326, 1850) suggested the availability
of New Jersey greensand as a source of potash and showed how this
mineral could be decomposed, but it does not appear that this mineral
has ever been utilized for the purpose. About 1861 the German
potash-salt deposits began to be developed, and these have since become
the chief source of this material. At present many efforts are being
made to obtain potassium compounds from other sources, such as brines,
cement-kiln dust, and feldspar and other minerals but thus far the
results have not satisfied the demand.
_Conclusion._
This account of chemical progress has given only a limited view of small
portions of the subject, because the amount of available material is so
vast in comparison with the space allowed for its presentation. Since
the Journal has published comparatively little organic chemistry, it was
decided to make room for a better presentation of other things by giving
only a brief discussion of this exceedingly active and important branch
of the science. For similar reasons industrial and metallurgical
chemistry, and other branches besides, in spite of their great growth
and importance, have been neglected, except for some incidental
references to them, and some account of a few of the more important
industrial chemicals.
It appears that we have much reason to be proud of the advances in
chemistry that have been made during the Journal’s period, and of the
part that the Journal has taken in connection with them, and there seems
to be no doubt that this progress has not diminished during more recent
times.
The present tendency of chemical research is evidently towards a still
greater development of organic chemistry, and an increased application
of physics and mathematics to chemical theory and practice.
The very great improvements that have been made in chemical education,
both in the number of students and the quality of instruction, during
the period under discussion, and particularly in rather recent times,
gives promise for excellent future progress.
_Note._
Footnote 153:
It appears that the most accurate experimental demonstration ever made
of this law was that of E. W. Morley, published in the Journal (=41=,
220, 276, 1891). He showed that 2·0002 volumes of hydrogen combine
with one volume of oxygen.
XI
A CENTURY’S PROGRESS IN PHYSICS