theoretical proposition, advanced by Prout (1815). Assuming that the
atomic weight of hydrogen was one, Prout’s hypothesis asserted that the
atomic weights of all other elementary bodies were multiples of that
of hydrogen. The most rigid investigations of recent years have shown
that Prout’s hypothesis is untenable; but the remarkable fact still
remains, that in a great many cases the atomic weights of the elements
are almost whole numbers, or differ from whole numbers by almost a half
unit.
The determination of the atomic weights of the various elements
during the past one hundred years has been worked on by hundreds of
chemists whose names it would be impracticable to mention. The most
important of them are Berzelius, Cooke, Cleve, Delafontaine, Dumas,
Hermann, Marchand, Marignac (1817), Morley, Noyes, Pelouse (1807–1867),
Richards, Schneider, Stas (1813–1891), and Thompson. Of all these
workers Stas, a Belgian chemist, is perhaps the most renowned. Among
those mentioned, Cooke, Morley, Noyes, Delafontaine, and Richards are
citizens of the United States.
From the less than thirty elements which were known at the beginning
of the century, there are known to-day seventy-two with certainty,
and perhaps one or two more whose identity has not yet been fully
established. The chemists who have become most renowned by the
discovery of elementary bodies are: Cavendish, Scheele, Berzelius,
Wöhler (1800–1882), Davy, Gay-Lussac, Priestly, Bunsen (b. 1811),
Crookes (b. 1832), and Ramsay.
The following elements, twenty-eight in number, were known before 1800:
ELEMENTS KNOWN BEFORE 1800.
1. Copper Known to Ancients.
2. Gold ” ” ”
3. Iron ” ” ”
4. Lead ” ” ”
5. Silver ” ” ”
6. Tin ” ” ”
7. Carbon ” ” ”
(But three forms not identified until 1786–1800.)
8. Mercury Known to Ancients.
9. Antimony Fifteenth Century.
10. Bismuth ” ”
11. Zinc ” ”
12. Phosphorus 1669
13. Arsenic (Isolated) 1697
” (Studied) 1733
14. Cobalt 1733
15. Platinum 1735–1748
16. Nickel 1751
17. Hydrogen 1766
18. Nitrogen 1772
19. Oxygen 1774
20. Manganese (Studied in compounds,
isolated at unknown date) 1774
21. Barium 1774
22. Tungsten 1781–1785
23. Molybdenum 1782
24. Tellurium 1782–1798
25. Strontium 1790
26. Yttrium 1794
27. Chromium 1797
28. Beryllium 1798
Four additional elements were known to exist before that date, but they
had not been isolated and identified. These are:—
ELEMENTS KNOWN BUT NOT ISOLATED OR EXAMINED BEFORE 1800.
Chlorine {Compound known 1774
{Isolated and studied 1810
Titanium {Known in compounds 1791
{Isolated 1824
Uranium {Known in compounds 1789
{Isolated 1824
Zirconium {Known in compounds 1789
{Isolated 1824
The following elements, forty-nine in number, have been discovered
since 1800:—
ELEMENTS DISCOVERED SINCE 1800.
1. Niobium 1801
2. Vanadium 1801
3. Tantalum. Studied about 1802–1803
(Not yet isolated.)
4. Cerium 1803
5. Iridium 1803
6. Osmium 1803
7. Palladium 1803
8. Rhodium 1803
9. Potassium 1807
10. Sodium 1807
11. Calcium 1808
12. Boron 1808
13. Silicon 1810
14. Iodine 1812
15. Cadmium 1817
16. Lithium 1817
17. Selenium 1817
18. Bromine 1826
19. Aluminium 1827
20. Thorium 1828
21. Ruthenium 1828–1845
22. Magnesium 1830
23. Lanthanum 1839
24. Terbium. Studied about 1839
(Not yet isolated.)
25. Erbium 1843
26. Neodymium 1843
27. Praseodymium 1843
28. Rubidium 1860
29. Cæsium 1860
30. Thallium 1861
31. Indium 1863
32. Gallium 1875
33. Decipium. (Name given in 1878 to
mixture of Samarium and Decipium.)
Isolated 1878
34. Ytterbium 1878
35. Thulium. (Name given by Cleve in
1879 to a metal in Gadolinite.
Has not yet been isolated, and
elementary nature is disputed.)
36. Scandium. Known since 1879
(Not yet isolated.)
37. Germanium 1885
38. Samarium. (A name given to a metal
found in Gadolinite. Elementary
nature very doubtful.)
39. Holmium. (Not yet isolated.)
40. Argon 1895
41. Helium 1896
42. Metargon 1898
43. Krypton 1898
44. Neon 1898
45. Polonium 1898
46. Coronium 1898
47. Xenon 1898
48. Monium 1898
49. Etherion (?) 1898
50. Gadolinium (?) 1885
51. Radium (?) 1898
The date in each case is that of the discovery. Numbers 49, 50, and
51 are not yet sufficiently well known to justify being considered
elements, and are therefore properly followed by an interrogation point.
II. PHYSICAL CHEMISTRY.
In strictly physical chemistry the relations of electricity and heat
to chemical action have been extensively developed during the century.
The specific heats of the elements and of most of their compounds have
been carefully determined, and thermo and physical chemistry under the
leadership of such master minds as Berthollet, Thompson, Van’t Hoff,
and Ostwald have been brought to the highest degree of perfection.
The chemist now does not consider that he knows any body until he knows
thoroughly its relations to heat and to electricity. The action of
light must also be included, but this subject will be more thoroughly
discussed under graphic chemistry.
The nature of solutions has also been developed by the studies of
Ostwald and Van’t Hoff, and as a result of these studies, a flood of
light has been thrown upon the constitution of compound bodies.
In the development of physical chemistry, attention should be directed
to the help afforded by Newlands (1864) and Mendelejeff (1869) and
others, showing that the elements form groups which tend to recur with
a periodicity which is sufficiently definite to enable the investigator
to foretell to some extent the properties of the elements which have
never yet been discovered, and whose existence is necessary in order to
fill up the gaps in existing groups.
By this method the existence, atomic weight and properties of scandium,
gallium, and germanium were foretold years before their discovery.
Such actual realization of a scientific-prophetic method is one of
the strongest indications of the basis of fact upon which it rests.
Although a rigid application of the principles of the periodic law is
not possible, yet its discovery and elaboration mark one of the great
forward steps of chemical philosophy.
If we regard any material system by itself, i.e., independently of any
other system or influence by which it may be surrounded, we recognize
it as consisting of essentially two things,—matter and energy. A
precise definition of either matter or energy is difficult, if not
impossible; but what is connoted by these names is sufficiently well
understood by their well-known properties. Both energy and matter are
essential to each and every system. They are coexistent. In the light
of human experience, we cannot conceive of one existing without the
other; and in the study of any material system, consideration of one of
these components without the other can only be regarded as incomplete.
But, for the sake of convenience, this has been the practice, and,
generally speaking, chemists have concerned themselves with matter
changes of equilibria, while physicists have more especially directed
their attention to energy equilibria. The object of the physical
chemist is to follow equilibria changes in given systems, having due
regard for both the matter and energy involved.
Berthollet may be regarded as the first true physical chemist, on
account of his classical views on mass action. Largely because the time
was not ripe for it, his views were not generally adopted.
A quarter of a century later (1867), Guldberg and Waage gave a precise
mathematical expression of the law, but still it attracted very little
attention from investigators. A tremendous impetus was given to the
subject by the electrolytic dissociation theory of Arrhenius (1887),
and the extension of the additive laws of gases to dilute solutions,
by Van’t Hoff (1885). This was but a comparatively small field in
the subject, but it stimulated activity along the whole line, the
wonderful increase of our knowledge concerning the velocity or rates of
reaction, the heat changes involved, and the marvelous development of
electrolytic chemistry being pertinent instances.
The generalization of Gibbs, known as the phase rule (1876), which
accurately states the condition for equilibrium in the system, and
the Theorem of Le Chatelier (1884), that any change in the factors
of equilibrium from outside is followed by a reverse change within
the system, together with the mass law, now give us a consistent
theoretical foundation for the subject. In general terms, it may be
said that all chemistry, at least all theoretical chemistry, properly
belongs to the province of physical chemistry, and the title, while in
many ways convenient, is misleading.
III. ORGANIC CHEMISTRY.
Compounds containing carbon enter into all the products of a living
cell. For this reason the chemistry of carbon compounds came to be
known as organic chemistry. This should not be taken as a definition,
however, without limitations. Many of the compounds containing carbon
are not known to enter into living tissue in any way, and their
connection with it is very remote and not essential. On the other
hand, it should be remembered that many organic compounds, and those
even of most importance, contain some other element,—nitrogen, for
example,—as the significant one.
While nearly all the known elements can enter into organic compounds,
the vast majority of such substances are composed of but very few. For
instance, those classes of which sugar, starch, the fats, etc., are
examples, contain only carbon, oxygen, and hydrogen. With nitrogen,
sulphur, and phosphorus added to these elements, almost the entire
range of organic chemistry is covered. Organic chemistry, therefore,
differs from inorganic chemistry in that, while the number of compounds
is much larger, the number of elements involved is very limited.
[Illustration: MICHAEL FARADAY.]
Berzelius may be regarded as having founded organic chemistry in the
beginning of this century. As a result of his analyses of the salts
of organic acids, he clearly demonstrated that the laws of definite
and multiple proportions hold equally for organic compounds and for
inorganic ones. The work of this master was ably furthered by Liebig
(1803–1873), who devised most elegant methods for the analytical
investigation of organic compounds, methods which are in use to-day
without any essential change.
Very soon, however, it was found that organic compounds existed having
the same percentage composition, but quite dissimilar properties,
physical and chemical, as, for instance, sugar and starch. Other
striking examples are Faraday’s discovery (1825) of a compound
identical in composition with ethylene, but wholly different in
properties; and Wöhler’s classical synthesis (1828) of urea by the
transformation of ammonium cyanate. Similar facts in the domain of
inorganic chemistry, though now well known, were at that time wanting,
and thus this most fruitful idea, designated as isomerism, was
introduced into the science.
The next great step was the introduction of the theory of radicles,
first suggested tentatively by Berzelius (1810), but put forward in
a definite way as one of the results of the classical investigation
on benzoyl by Liebig and Wöhler (1832). That is to say, a group of
elements, or radicle, can pass through a series of compounds, from one
to the other, as though the group were one single element. For years
this idea was the guiding principle in chemical investigations, and
was most useful in aiding the classification of chemical compounds and
bringing order out of the chaos of accumulating observations.
But the search for radicles was in a sense a vain one. We now know
that _no_ radicle exists as such by itself. Meanwhile, Dumas and
his pupil Laurent had introduced and developed the theory of types,
whereby all chemical compounds could be classified under four types,
which marked a distinct step in advance. Laurent, together with his
colleague Gerhardt (1816–1856), recognized the shortcomings of both
the radicle and type theories in their earlier forms, and showed
their inter-relation, when modified so as to do away with certain
inconsistencies.
Dumas had before this demonstrated the theory of substitution
(1834),—that is, that in certain compounds one or more of the elements
can be driven out and replaced by others without changing the essential
characteristics of the compound. For instance, chloracetic acid,
in which part of the hydrogen of acetic acid has been replaced by
chlorine, contains all the essential characteristics of acetic acid; in
fact, some of them—its acidic properties, for example—being markedly
accentuated. This theory was fiercely assailed at first, notably by
Liebig. Like all theories of science, it was in the beginning pushed
to the extreme, and put forward to explain things to which it was
not applicable. It gradually came to demonstrate its own right to
existence, largely as a result of the work of Laurent and Gerhardt,
and made its influence felt in the exposition of their ideas, to which
reference has just been made.
The development of these theories, about the middle of the century, was
greatly hastened by the work of many brilliant investigators, notably
Wurtz (1817–1884), Hofmann (1818–1892), Williamson (1824–), Kolbe
(1818–1884), and Frankland (1825–) among others.
Kekulé proposed a new type, marsh gas or methane. Shortly afterwards,
his well-known formula for benzene, the starting-point and foundation
of the vast class of aromatic bodies, was proposed. He insisted that
the time had come when chemists must ask what those ultimate particles,
or atoms, of the elements themselves were doing in these compounds
of various types. The answer was a grand one, and the result, our
magnificent store of information concerning the _constitution_ of
organic compounds, or the way in which the atoms are connected with
each other. It is not to be inferred that our knowledge on this
subject, in any one case, is complete. Far from it! Much that is most
interesting and important is apparently as remote from our grasp as
ever. But we do know something about the general relations of the atoms
in the molecule, and our knowledge, so far as it goes, is definite and
precise.
Somewhat later, Van’t Hoff and Lebel, at the same time but
independently, introduced the study of the space relations of organic
compounds by suggesting the simplest possible space formula (the
tetrahedron) for marsh gas or methane, of which all other organic
compounds may, theoretically at least, be regarded as derivatives.
Many inexplicable relations, especially among isomers, now became
clear. The theory was at first bitterly assailed, especially by Kolbe.
It found an able champion in Wislicenus (1838–), however, and has so
thoroughly established itself, that it may be safely said that at the
present day it is the controlling idea in the large majority of organic
investigations.
The carbon atom is characterized by a wonderful facility in uniting
not only with other elements, but with itself. It would even appear as
though its influence in this regard extended to other elements united
with it, as nitrogen, for instance, shows an unexpected ability to
unite with nitrogen in organic compounds.
Further, the carbon atom is characterized by an unusually constant
valency, namely, four. These two characteristics account for homology,
that is, for a series of similar compounds differing in composition
one from the other by—CH2, and enables us to trace back all organic
compounds to one mother substance—marsh gas or methane.
These ideas have also been more or less successfully applied to the
study of the composition of inorganic compounds. The assistance organic
chemistry has given to the general subject is incalculable. Finally,
it may be said, that while in the nature of the case our ideas of
structure in organic compounds cannot be regarded as proved, or as
not subject to possible future modifications, we have, at least, a
consistent theory and good working hypothesis. A homely illustration of
our present ideas may be drawn from the modern high city building. The
skeleton of this building is made of iron, about which are grouped the
brick, stone, wood, and other materials to form a complete building.
So the organic body is built on a chain or frame-work or skeleton of
carbon atoms, about which are grouped the atoms of hydrogen, oxygen,
and nitrogen, or radicle compounds thereof.
It is not possible here to even name some of the more eminent workers
who for a quarter of a century have contributed to our knowledge of
organic chemistry. This branch of chemistry has been the vogue, and has
been pushed almost to the limit of possibility since 1875. Many almost
unexplored fields still remain, but chemists recognize the fact that
in theory and practice organic chemistry has reached a high degree of
perfection, and they are returning to continue the researches in other
fields which have for so long been almost neglected.
IV. ANALYTICAL CHEMISTRY.
No branch of chemical science has a more general interest for the
public than that which relates to the determination of the materials of
which bodies are composed, and the proportions in which they exist.
At the beginning of the century considerable progress had been made
in this branch of knowledge by the researches of Boyle (1626–1691),
Hoffmann, Margraff (1709–1780), Scheele and Bergmann (1735–1784).
Berzelius, as has already been mentioned, had added a new and valuable
factor to chemical analysis by the development of the blowpipe, and
in the early part of the century mineral analysis was still further
advanced by Klaproth (1743–1817), Rose (1798–1873), and many others.
No one man did so much to advance this branch of chemical science as
Fresenius (1818–1897). He collated and proved all the proposed methods
of analysis, both qualitative and quantitative, and out of a confused
mass of material formed a logical system of procedure, which has proved
invaluable to the progress of chemical science in all its branches.
The volumetric methods of analysis, which save so much time and labor
without sacrificing accuracy, were developed by Gay-Lussac, Vauquelin
(1763–1879), Mohr (1806–1879), Volhard, Sutton, Fehling, and Liebig.
The methods of gas analysis have been worked out chiefly by Bunsen,
ably assisted by Winkler and Hempel.
The methods of determining the elementary bodies in organic compounds
have been developed by Dumas, Liebig, Will, Varrentrap, and Kjeldahl,
to the last of whom chemical analysis owes a debt of gratitude for the
invention of a speedy and accurate method of determining nitrogen.
Not much less is the debt due to Gooch for the invention of the
perforated platinum crucible, carrying an asbestos felt for securing
precipitates by filtration, in a form suitable to ignition without
further preparation.
[Illustration: WILLIAM CROOKES, F. R. S.]
Through the classic researches of Arago (1786–1853) and Biot
(1774–1862), polarized light has been made a most valuable adjunct
to chemical research, serving as it does to measure the quantity of
various alkaloids, essential oils, and sugars.
Based on these researches of Biot and Arago, Ventzke, Soleil,
Scheibler, Duboscq, Landolt, and Lippich have constructed apparatus,
which have made an exact science of optical saccharimetry. Optical
analysis is not without its relation to theoretical chemistry, for by
it has been proved the assumption that optically active bodies contain
an asymmetrical carbon atom,—that is, one which combines with four
different atoms or radicles.
Electricity has become also one of the most useful factors in chemical
analysis. Many metals are easily deposited by electrolytic action, and
their separation and determination rendered easy and certain.
Chemical analysis has not only given us accurate knowledge of the
constituents of matter, but by revealing the deportment of molecules
and groups of molecules in inorganic and organic compounds, has opened
up a path for organic and synthetic chemistry which otherwise must have
remained forever closed.
The discovery and development of spectrum analysis is one of the great
achievements of the nineteenth century in chemical science.
Wollaston, in 1802, first noticed that the spectrum of the sun’s
light, when greatly magnified, was not composed of colors gradually
changing from one to the other, but that the continuity of the colors
was interrupted by dark bands. Fraunhofer, in 1814, had made a map
of the solar spectrum, showing 576 of these dark lines. Fraunhofer
was entirely ignorant of the cause of these dark lines, but when he
had found them, not only in the light from the sun, but also from the
moon and the fixed stars, he properly concluded that they were due to
something entirely independent of the earth.
It remained for Bunsen and Kirchhoff, in 1860, to point out the
fact that these dark lines were characteristic of certain chemical
elements existing in the sun and its photosphere, and this fact is the
foundation of spectrum analysis. The broad black band in the sun’s
spectrum, called by Fraunhofer D, corresponded exactly in position
and in width with the yellow band produced by a flame containing
incandescent sodium. There was no doubt whatever, therefore, that
the two phenomena were due to the same cause; but why in the one case
should the band be black and in the other yellow? This question was
answered by the discovery of the fact that a ray of light colored by
incandescent sodium, passing through a luminous atmosphere of the same
metal, would lose by absorption all of its yellow color, and would
display a black band where before the yellow color existed.
Based upon this observation, the development of spectrum analysis went
forward with amazing rapidity. The hundreds of lines in the sun’s
spectrum were found to occupy exactly the position of luminous lines
in the spectra of various metals, and thus it was possible for the
chemist to extend his investigations beyond the limits of the earth,
and distinguish the chemical elements in the sun and in the fixed stars
billions of miles farther away from us than the sun itself. Celestial
chemistry has thus become a fixed and definite science.
But the value of spectral examinations has extended still farther. Many
luminous lines were observed in the spectrum which were not found in
the spectra of any known element. The inference then logically arose
that there were elements yet undiscovered to which these lines were
due. From this starting point investigations proceeded which have led
to the discovery of a large number of elementary bodies. Among the
important elements that have been discovered by means of spectrum
analysis may be mentioned: cæsium, rubidium, thallium, indium, gallium,
ytterbium, and scandium.
Spectrum analysis is also extremely useful in proving the verity of
supposed new elements; for if a supposed new element should be found to
give a series of spectral lines coincident with those already known, it
would be a positive proof of the fact that the supposed new element was
but a mixture of bodies already known to exist.
V. SYNTHETICAL CHEMISTRY.
This branch of chemical science has for its object the building up of
the more complex from the simpler forms of matter. In the early part
of the century, Chevreul and Wöhler laid the foundation of the science
by the synthesis of fatty-like bodies and urea. Berthellot and Friedel
(1832–) in France, and Williamson and Frankland in England, added much
to our knowledge. Kolbe, in Germany, made salicylic acid so abundantly
as to banish the natural article from the market. The synthesis of
coloring matters resembling indigo was also a great blow to that
industry. From the products of the distillation of coal, chemists were
able to make thousands of valuable bodies of the greatest utility. Many
medicinal substances and nearly all the common dyes trace their origin
to coal.