and similar discrepancies might appear in nearly all formulas containing
elements of different valencies. In 1858, however, an article by the
Italian chemist Cannizzaro appeared in which the outlines of a course in
chemical philosophy were presented. This acquired wide circulation in
the form of a pamphlet at a chemical convention somewhat later, and it
dealt so clearly and ably with Avogadro’s principle, Dulong and Petit’s
law, and other points in connection with formulas that it led to a rapid
and almost universal reform among those who were using unsatisfactory
formulas.
At about this time also the dualistic formulas of Berzelius were
generally abandoned, and hydrogen came to be regarded as the
characteristic element of all acids. For instance, CaO.SO_{3}, called
“sulphate of lime,” came to be written CaSO_{4} and was called “calcium
sulphate,” and while it had been shown as early as 1815 by Davy that
“iodic acid,” I_{2}O_{5}, showed no acid reaction until it was combined
with water, the accumulation of similar facts led to the formulation of
sulphuric acid as H_{2}SO_{4} instead of SO_{3} or H_{2}O.SO_{3}, and
that of other “oxygen acids” in a similar way. As a necessary
consequence of this view of acids, the bases came to be regarded as
compounds of the “hydroxyl” group, OH. Therefore the formula for caustic
soda came to be written NaOH instead of Na_{2}O.H_{2}O, and so on.
_The Periodic System of the Elements._—The periodicity of the elements
in connection with their atomic weights was roughly grasped by Newlands
in England, who announced his “law of octaves” in 1863. This was at the
time when the atomic weights were being modified and their numerical
relations properly shown. The subject was worked out more fully by L.
Meyer in Germany a little later, but it was most clearly and elaborately
presented by the Russian chemist Mendeléeff in 1869.
In order that this subject may be explained to some extent Mendeléeff’s
table is given here, with the addition of the recently discovered
elements and some other modifications.
┌─────────┬────────────────┬───────────────────┬───────────────────┐
│ Groups │ I │ II │ III │
│ „ │ A B │ A B │ A B │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ Typical │ R_{2}O │ RO │ R_{2}O_{3} │
│Compounds│ │ │ │
│ „ │ RCl │ RCl_{2} │ RCl_{3} │
│ „ │ RH — │(RH_{2}) — │ — (RH_{3}) │
╞═════════╪════════════════╪═══════════════════╪═══════════════════╡
│Series 1 │ │ │ │
│ │ │ │ │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 2 │ Lithium │ Beryllium│ _Boron_ │
│ │ 6·94 │ 9·1 │ 11·0 │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 3 │ Sodium │ Magnesium│ Aluminium│
│ │ 23·00 │ 24·32 │ 27·1 │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 4 │Potassium │ Calcium │Scandium │
│ │ 39·10 │ 40·07 │ 44·1 │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 5 │ Copper│ Zinc │ Gallium │
│ │ 53·57 │ 65·37 │ 69·9 │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 6 │Rubidium │Strontium │ Yttrium │
│ │ 85·43 │ 87·63 │ 89·0 │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 7 │ Silver│ Cadmium │ Indium │
│ │ 107·88│ 112·40 │ 114·8 │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ │ │ │Lanthanum │
│ 8 │ Cæsium │ Barium │139·0 to* │
│ │ 132·81 │ 137·87 │Lutecium │
│ │ │ │ 174.0 │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
│ „ │ „ │ „ │ „ │
│ │ │ │ │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 9 │ Gold │ Mercury │ Thallium │
│ │ 197·2 │ 200·6 │ 204·0 │
├─────────┼────────────────┼───────────────────┼───────────────────┤
│ 10 │ —— │ Radium │ —— │
│ │ │ 226·4 │ │
└─────────┴────────────────┴───────────────────┴───────────────────┘
┌─────────┬────────────────────┬─────────────────────┐
│ Groups │ IV │ V │
│ „ │ A B │ A B │
├─────────┼────────────────────┼─────────────────────┤
│ Typical │ RO_{2} │ R_{2}O_{5} │
│Compounds│ │ │
│ „ │ RCl_{4} │ RCl_{3} │
│ „ │ — (RH_{4}) │ — RH_{3} │
╞═════════╪════════════════════╪═════════════════════╡
│Series 1 │ │ │
│ │ │ │
├─────────┼────────────────────┼─────────────────────┤
│ 2 │ _Carbon_ │ NITROGEN │
│ │ 12·00 │ 14·01 │
├─────────┼────────────────────┼─────────────────────┤
│ 3 │ _Silicon_│ _Phosphorus_│
│ │ 28·3 │ 31·04 │
├─────────┼────────────────────┼─────────────────────┤
│ 4 │ Titanium │Vanadium │
│ │ 48·1 │ 51·0 │
│ „ │ „ │ „ │
│ │ │ │
│ „ │ „ │ „ │
│ │ │ │
├─────────┼────────────────────┼─────────────────────┤
│ 5 │ Germanium│ Arsenic │
│ │ 72·5 │ 74·96 │
├─────────┼────────────────────┼─────────────────────┤
│ 6 │Zirconium │Niobium │
│ │ 90·6 │ 93·5 │
│ „ │ „ │ „ │
│ │ │ │
│ „ │ „ │ „ │
│ │ │ │
├─────────┼────────────────────┼─────────────────────┤
│ 7 │ Tin 119·0│ Antimony │
│ │ │ 120·2 │
├─────────┼────────────────────┼─────────────────────┤
│ │ (Cerium) │ │
│ 8 │ 140·25 │Tantalum │
│ │(Lutecium) │ 181·5 │
│ │ 174.0 │ │
│ „ │ „ │ „ │
│ │ │ │
│ „ │ „ │ „ │
│ │ │ │
├─────────┼────────────────────┼─────────────────────┤
│ 9 │ Lead │ Bismuth │
│ │ 207·10 │ 208·0 │
├─────────┼────────────────────┼─────────────────────┤
│ 10 │ Thorium │ —— │
│ │ 292·4 │ │
└─────────┴────────────────────┴─────────────────────┘
┌─────────┬─────────────────────┬───────────────────┬─────────────────┐
│ Groups │ VI │ VII │ VIII │
│ „ │ A B │ A B │ A B │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ Typical │ RO_{3} │ R_{2}O_{7} │(RO_{4}) — │
│Compounds│ │ │ │
│ „ │ RCl_{2} │ — RCl │ — R │
│ „ │ — RH_{2} │ — RH │ — R │
╞═════════╪═════════════════════╪═══════════════════╪═════════════════╡
│Series 1 │ │ HYDROGEN │ HELIUM │
│ │ │ 1·008 │ 3·99 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 2 │ OXYGEN │ FLUORINE │ NEON │
│ │ 16·00 │ 19·0 │ 20·2 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 3 │ _Sulphur_ │ CHLORINE │ ARGON │
│ │ 32·07 │ 35·46 │ 39·88 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 4 │ Chromium │Manganese │ Iron │
│ │ 52·0 │ 54·93 │ 55·84 │
│ „ │ „ │ „ │ Cobalt │
│ │ │ │ 58·97 │
│ „ │ „ │ „ │ Nickel │
│ │ │ │ 58·68 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 5 │ _Selenium_│ _Bromine_│ KRYPTON│
│ │ 79·2 │ 79·92 │ 82·92 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 6 │Molybdenum │ —— 100 │Ruthenium │
│ │ 96·0 │ │ 101·7 │
│ „ │ „ │ „ │ Rhodium │
│ │ │ │ 102·9 │
│ „ │ „ │ „ │Palladium │
│ │ │ │ 106·7 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 7 │ Tellurium │ _Iodine_ │ XENON │
│ │ 127·5 │ 126·92 │ 130·2 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ │ │ │ │
│ 8 │ Tungsten │ —— 188 │ Osmium │
│ │ 184·0 │ │ 190·9 │
│ │ │ │ │
│ „ │ „ │ „ │ Iridium │
│ │ │ │ 193·1 │
│ „ │ „ │ „ │Platinum │
│ │ │ │ 195·2 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 9 │ —— │ —— │ NITON │
│ │ │ │ 222·4 │
├─────────┼─────────────────────┼───────────────────┼─────────────────┤
│ 10 │ Uranium │ │ │
│ │ 238·5 │ │ │
└─────────┴─────────────────────┴───────────────────┴─────────────────┘
┌────────────────────────────────────────────────────────────────┐
│ * Lanthanum, Cerium, Praseodymium, Neodymium, ——, │
│Rare·Earth 139·0 140·25 140·6 144·3 │
│ Metals: │
│ „ Gadolinium, Terbium, Dysprosium, Holmium, Erbium,│
│ 157·3 159·2 162·5 163·5 167·7 │
└────────────────────────────────────────────────────────────────┘
┌─────────────────────────────────────────┐
│ * Samarium, Europeum, ——, │
│Rare·Earth 150·4 152·0 │
│ Metals: │
│ „ Thulium, Ytterbium, Lutecium,│
│ 168·5 172·0 174·0 │
└─────────────────────────────────────────┘
NOTE.—Distinctions in printing: GASEOUS ELEMENTS. _Other non-metallic
elements_, metallic elements. The heavy line encloses approximately
the acid-forming elements.
In this table the elements arranged in the order of their atomic weights
fall into eight groups where the known oxides progress regularly, with
the exception of two or three elements, from R_{2}O in Group I to
R_{2}O_{7} in Group VII, while in Group VIII two oxides (of ruthenium
and osmium) are known which carry the progression to RO_{4}.
It was pointed out by Mendeléeff that, with the exception of series 1
and 2 at the top of the table, the alternate members of the groups show
particularly close relationships. These subordinate groups, marked A and
B, in most cases show remarkable analogies and gradations in their
properties, for example, in the alkali-metals from lithium to cæsium,
and in the halogens from fluorine to iodine. The two divisions of a
group do not usually show very close relations to each other, except in
their valency, and they even display, in several instances, opposite
gradations in chemical activity in the order of their atomic weights.
For instance, cæsium stands at the electro-positive end, while gold
stands at the electro-negative end of its subordinate group. The
difference between the two divisions is very great in Groups VI and VII,
but it is extreme in Group VIII, where heavy metals are on one side and
inactive gases on the other. Many authorities separate these gases into
a “Group O” by themselves at the left-hand side of the table, but this
does not change their relative positions, and the plan may be objected
to on the ground that many vacant places are thus left in the groups
VIII and O.
The periodic law has been useful in rectifying certain atomic weights.
At the outset Mendeléeff was obliged to change beryllium from 14·5
(assuming Be_{2}O_{3}) to 9 (assuming BeO), and later the atomic weights
of indium and uranium were changed to make them fit the system. All of
these changes have been confirmed by physical means.
Mendeléeff found a number of vacant places in his table, and was thus
able to render further service to chemical science by predicting the
properties of undiscovered elements, and his predictions were very
closely confirmed by the later discovery of scandium, gallium, and
germanium. The table indicates that there are still two undiscovered
elements below manganese and probably two more among the rare-earth
metals. The interesting observation has just recently been made by Soddy
that the products of radioactive disintegration appear to pass in a
symmetrical way through positions in the periodic system, giving off a
helium molecule at alternate transformations until the place of lead is
reached. It appears, therefore, that the five vacant places in the table
above bismuth are probably occupied by these evanescent elements, and it
is to be noticed that all of the elements that have been placed in this
region of high atomic weights are radioactive.
There are some inconsistencies in the periodic system. The increments in
the atomic weights are irregular, and there are three cases, argon and
potassium, cobalt and nickel, and tellurium and iodine, where a higher
atomic weight is placed before a lower one in order to bring these
elements into their undoubtedly proper places. There is a peculiarity
also in the heavy-metal division of Group VIII, where three similar
elements occur in each of three places, and where the usual periodicity
appears to be suspended, or nearly so, in comparison with most of the
other elements. However, there seems to be a still more remarkable case
of this kind in Group III, where fourteen metals of the rare earths have
been placed. They are astonishingly similar in their chemical
properties, hence it seems necessary to assume that periodicity is
suspended here throughout the wide range of atomic weights from 139 to
174, where no elements save these have been found.
Several other interesting features of the table may be pointed out. The
chlorides and hydrides, as indicated by the “typical compounds,” show a
regular progression in both directions towards Group IV. (Where the
type-formulas do not apply, as far as is known, to more than one or two
elements, they have been placed in parentheses in the table given here.)
It is a striking fact that the acid-forming elements occur together in a
definite part of the table, and that the gases and other non-metallic
elements, except the inactive gases of Group VIII, occur in the same
region.
_Atomic Numbers._—As the result of a spectroscopic study of the wave
lengths or frequencies of the X-rays produced when cathode rays strike
upon anticathodes composed of different elements, Moseley in 1914
discovered that whole numbers in a simple series can be attributed to
the atoms. These atomic numbers are: 1 for hydrogen, 2 for helium, 3 for
lithium, 4 for beryllium, and so on, in the order in which the elements
occur in Mendeléeff’s periodic table, and in the cases of argon and
potassium, cobalt and nickel, and tellurium and iodine, they follow the
correct chemical order, while the atomic weights do not. They appear to
indicate, therefore, an even more fundamental relation between the atoms
than that shown by the atomic weights.
These numbers are now available for every element up to lead, and they
are particularly interesting in indicating, on account of missing
numbers, the existence of two undiscovered elements in the manganese
group, and two more among the rare-earth metals, in confirmation of the
vacant places below lead in Mendeléeff’s table.
_The Isolation of Elements._—In the year 1818 about 53 elements were
recognized, and since that time about 30 more have been discovered, but
the elements already known comprised the more common ones, and nearly
all of those which have been commercially important. A few of them,
including beryllium, aluminium, silicon, magnesium, and fluorine, were
then known only in their compounds, as they had not yet been isolated in
the free condition.
Berzelius in 1823 prepared silicon, a non-metallic element resembling
carbon in many respects. This element has recently been prepared on a
rather large scale in electric furnaces at Niagara Falls, and has been
used for certain purposes in the form of castings.
Wöhler created much sensation in 1827 by isolating aluminium and finding
it to be a very light, strong and malleable metal, stable in the air,
and of a silver-white color. For a long time this metal was a
comparative rarity, being prepared by the reduction of aluminium
chloride with metallic sodium; but about 25 years ago Hall, an American,
devised a method of preparing it by electrolyzing aluminium oxide
dissolved in fused cryolite. This process reduced the cost of aluminium
to such an extent that it has now come into common use.
Wöhler and Bussy prepared beryllium in 1828, and Liebig and Bussy did
the same service for magnesium in 1830. The latter metal has come to be
of much practical importance, both as a very powerful reducing agent in
chemical operations, and as an ingredient of flash-light powders and of
mixtures used for fireworks. It is also used in making certain light
alloys.
After almost innumerable attempts to isolate fluorine, during a period
of nearly a century, this was finally accomplished in 1886 by Moissan in
France by the electrolysis of anhydrous hydrogen fluoride. The free
fluorine proved to be a gas of extraordinary chemical activity,
decomposing water at once with the formation of hydrogen fluoride and
ozonized oxygen. This fact explains the failure of many previous
attempts to prepare it in the presence of water.
_Early Discoveries of New Elements._—The remarkable activity of chemical
research at the beginning of our period is illustrated by the fact that
three new elements were discovered in 1817. In that year Berzelius had
discovered selenium, Arfvedson, working in Berzelius’s laboratory had
discovered the important alkali-metal lithium, and Stromeyer had
discovered cadmium.
In 1826 Ballard in France discovered bromine in the mother-liquor from
the crystallization of common salt from sea water. Bromine proved to be
an unusually interesting element, being the only non-metallic one that
is liquid at ordinary temperatures, and being strikingly intermediate in
its properties between chlorine and iodine. It has been obtained in
large quantities from brines, and is produced extensively in the United
States. The elementary substance and its compounds have found important
applications in chemical operations, while the bromides have been found
valuable in medicine and silver bromide is very extensively used in
photography.
In 1828 Berzelius discovered thorium. The oxide of this metal has
recently been employed extensively as the principal constituent of
incandescent gas-mantles, and the element has acquired particular
importance from the fact that, like uranium, it is radioactive,
decomposing spontaneously into other elements.
Vanadium had been encountered as early as 1801 by Del Rio, who named it
“erythronium,” but a little later it was thought to be identical with
chromium and was lost sight of for a while. In 1830, however, it was
re-discovered by, and received its present name from Sefström in Sweden.
Berzelius immediately made an extensive study of vanadium compounds, but
he gave them incorrect formulas and derived an incorrect atomic weight
for the element, because he mistook a lower oxide for the element
itself. Roscoe in England in 1867 isolated vanadium for the first time,
found the right atomic weight, and gave correct formulas to its
compounds. Vanadium is particularly interesting from the fact that it
displays several valencies in its compounds, many of which are highly
colored. It has found important use as an ingredient in very small
proportions in certain “special steels” to which it imparts a high
degree of resistance to rupture by repeated shocks.
Columbium was discovered early in the nineteenth century in the mineral
columbite from Connecticut by Hatchett, an Englishman, who did not,
however, obtain the pure oxide. It was afterwards obtained by Rose who
named it niobium. Both names for the element are in use, but the former
has priority. Attention was called to this fact by an article in the
Journal by Connell, an Englishman (=18=, 392, 1854).
_The Platinum Group of Metals._—In 1854 a new member of the platinum
group of metals, ruthenium, was discovered by Claus. Platinum had been
discovered about the middle of the eighteenth century, while its other
rarer associates, iridium, osmium, palladium, and rhodium, had been
recognized in the very early years of the nineteenth century. It was
during the latter period that platinum ware began to be employed to a
considerable extent in chemical operations, and this use was greatly
extended as time went on. The discovery was made by Phillips in 1831
that finely divided platinum by contact would bring about the
combination of sulphur dioxide with atmospheric oxygen, and this
application during the past 20 years has become enormously important in
the sulphuric acid industry, while other important applications of
platinum as a “catalytic agent” have also been made. Wolcott Gibbs and
Carey Lea have contributed perhaps more than any other recent chemists
to a knowledge of the platinum metals. Carey Lea (=38=, 81, 248, 1864)
dealt chiefly with the separation of the metals from each other, while
Gibbs’s work (=31=, 63, 1861; =34=, 341, 1862) included investigations
of many of the compounds.
It may be mentioned that while platinum and its associates were formerly
known only in the uncombined condition in nature, the arsenide
sperrylite, PtAs_{2}, was described by the late S. L. Penfield, and the
senior writer of this chapter, in articles published in the Journal
(=37=, 67, 71, 1889).
_Applications of the Spectroscope._—The discovery in certain mineral
waters of the rare alkali-metals rubidium and cæsium by Bunsen and
Kirchoff in 1861 was in consequence of the application of spectroscopy
by these same scientists a short time previously to the identification
of elements imparting colors to the flame. Since that time the
employment of the spectroscope for chemical purposes has been much
extended, as it has been used in the examination of light from electric
sparks and arcs, as well as from Geissler tube discharges and from
colored solutions.
The metals rubidium and cæsium are interesting in being closely
analogous to potassium and in standing at the extreme electro-positive
end of the series of known metals. It should be noticed here that
Johnson and Allen of our Sheffield Laboratory, having obtained a good
supply of rubidium and cæsium material from the lepidolite of Hebron,
Maine, made some important researches upon these elements, accounts of
which were published in the Journal (=34=, 367, 1862; =35=, 94, 1863).
They established the atomic weight of cæsium, thus correcting Bunsen’s
determination which was unsatisfactory on account of the small quantity
and impurity of his material. Pollucite, a mineral rich in cæsium, which
had been found in very small amount on the Island of Elba, has more
recently been obtained in large quantities—hundreds of pounds—at Paris,
Maine, and its vicinity. This American pollucite was first analyzed and
identified by the senior writer of this article (=41=, 213, 1891), and
later (=43=, 17, 1892 _et seq._) the results of many investigations on
cæsium and rubidium compounds, in which the junior writer played an
important part, carried out in Sheffield Laboratory, were published in
the Journal.
The application of the spectroscope led to the discovery of thallium in
1861 by Crookes of England, and to that of indium in 1863 by Reich and
Richter in Germany. Both of these metals are extremely rare, but they
are of considerable theoretical interest. Thallium is particularly
remarkable in showing resemblances in its different compounds to several
groups of metals.
The spectroscope was employed again in connection with the discovery of
gallium in 1875 by Boisbaudran. It is in the same periodic group as
thallium and indium, and it has a remarkably low melting point, just
above ordinary room-temperature. It has been among the rarest of the
rare elements, but within two or three years a source of it has been
found in the United States in certain residues from the refining of
commercial zinc. The recent issues of the Journal (=41=, 351, 1916;
=42=, 389, 1916) show that Browning and Uhler of Yale have availed
themselves of this new material in order to make important chemical and
physical researches upon this metal.
_Germanium._—The discovery of germanium in the mineral argyrodite in
1886 by Winkler revealed a curious metal which gives a white sulphide
that may be easily mistaken for sulphur and which is volatilized
completely when its hydrochloric acid solution is evaporated, so that it
is evasive in analytical operations. This element had been predicted
with much accuracy by Mendeléeff, and it is rather closely related to
tin.
A few years after the discovery of germanium, Penfield published in the
Journal (=46=, 107, 1893; =47=, 451, 1894) some analyses of argyrodite,
correcting the formula given by Winkler to the mineral; also he
described canfieldite, an analogous mineral from Bolivia, in which a
large part of the germanium was replaced by tin.
_The Rare Earths._—Before the year 1818 two rare earths, the oxides of
yttrium and cerium, were known in an impure condition. Since that time
about fourteen others have been discovered as associates of the first
two. The rare earths are peculiar from the fact that many of them are
always found mixed together in the minerals containing them, and also
from the circumstance that most of them are remarkably similar in their
chemical reactions and consequently exceedingly difficult to separate
from each other. In many cases multitudes of fractional precipitations
or crystallizations are needed to obtain pure salts of a number of these
metals. The solutions of the salts of several of these elements give
characteristic absorption bands when examined spectroscopically by the
use of transmitted light.
No important practical application has been found for any of these
earthy oxides, except that about one per cent of cerium oxide is mixed
with thorium oxide in incandescent gas-mantles in order to obtain
greatly increased luminosity.
_The Inactive Gases._—As long ago as 1785, Cavendish, that remarkable
Englishman who first weighed the world and first discovered the
composition of water, actually obtained a little argon in a pure
condition by sparking atmospheric nitrogen with oxygen converting it
into nitric acid (another discovery of his) and absorbing the excess of
oxygen. The volume of this residual gas as estimated by him corresponds
very closely to the volume of argon in the atmosphere, as now known.
It was more than a century later, in 1894, that Rayleigh and Ramsay
discovered argon in the air. Lord Rayleigh had found that atmospheric
nitrogen was about one-half per cent heavier than chemical nitrogen, a
fact which led to the investigation. It was only necessary to repeat
Cavendish’s experiment on a large scale, or to absorb oxygen with hot
copper and nitrogen with hot magnesium, in order to obtain argon. The
gas attracted much attention, both on account of having but a single
atom in its molecule, and particularly because it failed to enter into
chemical combination of any kind. This gas has been used of late for
filling the bulbs of incandescent electric lamps in cases where a gas
pressure without chemical action is desired.
In 1890 and 1891, Hillebrand published in the Journal (=40=, 384, 1890:
=42=, 390, 1891) a series of analyses of the mineral uraninite and
reported in some samples of the mineral as much as 2·5 per cent of an
inactive gas. Hillebrand examined the gas spectroscopically but, just
missing an important discovery, he detected only the spectrum lines of
nitrogen. Ramsay, in searching for argon in some sort of natural
combination, and doubtless remembering Hillebrand’s work, heated some
cleveite, a variety of uraninite, and obtained, not argon, but a new
gas. This gave a yellow spectrum-line corresponding to a line previously
observed in the light of the sun’s corona and attributed to an element
in the sun called helium. Helium, therefore, in 1895 had been found on
the earth. This gas is a constant constituent of uranium minerals, as it
is produced by the breaking down of radioactive elements. It has been
found in very small quantity in the atmosphere, and is the most
difficult of all known gases to liquefy, as its boiling point, as shown
by Onnes in 1908, is only 4° above the absolute zero. It has not yet
been solidified.
In 1898 Ramsay and Travers, by the use of ingenious methods of
fractional distillation and absorption by charcoal, obtained three other
much rarer inactive gases from the atmosphere which they called neon,
krypton and xenon.
The inactive gases are all colorless, and as they form no chemical
compounds they are characterized by their densities, which give their
atomic weights, by their boiling points, and by their characteristic
Geissler-tube spectra.
The gaseous radium emanation, or niton, belongs also to the inactive
group, and it was also collected and studied by Ramsay who was compelled
to work with only 0·0001 cc. of it, as the volume obtained by heating
radium salts is very small. It is an evanescent element, disappearing
within a few days on account of radioactive disintegration. Meanwhile it
glows brilliantly when liquefied and cooled to the temperature of liquid
air. It has an atomic weight of 222, four units below that of radium,
and the difference is considered as due to the loss by radium of an atom
of helium in passing into the emanation.
_The Radioactive Elements._—The discovery of radium in 1898 by Madame
Curie, and the study of that and other radioactive elements has produced
a profound effect upon chemical theory. It was found that the two
elements of the highest atomic weights, uranium and thorium, are always
spontaneously decomposing into other elements at a fixed rate of speed
which can be controlled by no artificial means, and that the elements
resulting from these decompositions likewise undergo spontaneous changes
into still other elements at greatly varying rates of speed, forming in
each case a remarkable series of temporary elements. These
transformations are accompanied by the emission at enormous velocities
of three kinds of rays, one variety of which has been shown to consist
of helium atoms. The greater number of the elements formed in these
transformations have not as yet been obtained in a pure condition, and
they are known only in connection with their radioactivity, volatility,
etc.; but radium and niton, two of these products, have been obtained in
a pure condition, so that their atomic weights and their places in the
periodic system have been fixed.
We owe much of our knowledge of the radioactive transformations to the
researches of Rutherford and of Soddy, and of their co-workers, but one
of the important products of the transformation of uranium, an element
which he called ionium, was characterized by Boltwood of Yale (=25=,
365, 1908).
Radium and niton, apart from their radioactive properties, resemble
barium and the inert gases of the atmosphere, respectively. The rates at
which their progenitors produce them, and the rates at which they
themselves decompose, bring about a state of equilibrium after a time.
Therefore a given amount of uranium, which decomposes exceedingly
slowly, can yield even after thousands of years only a very small
proportional quantity of undecomposed radium, one-half of which
disappears in about 2500 years, because the amount decomposed must
eventually be equal to the amount produced. The first conclusive
evidence that radium is a product of the decomposition of uranium was
given by Boltwood in the Journal (=18=, 97, 1904). He found that all
uranium minerals contain radium; and the amount of radium present is
always proportional to the amount of uranium, which shows the genetic
relation between the two.
In the case of niton, which is produced by radium, and is called also
the radium emanation, the rate of decay is rapid, so that if the gas is
expelled from radium by heating, equilibrium is reached after a few
days, with the accumulation of the largest possible amount of niton.
The conclusion has been reached by Rutherford and others that the final
product besides helium, in the radioactive transformations, is lead, or
at least an element or elements resembling lead to such a degree that no
separation of them by chemical means is possible. Atomic weight
determinations by Richards and others have shown that specimens of lead
found in radioactive minerals give distinctly different atomic weights
from that of ordinary lead. This fact has led to the view that possibly
the atoms of the elements are not all of the same weight, but vary
within certain limits—a view that is contrary to previous conclusions
derived from the uniformity in atomic weights obtained with material
from many different sources.
The results of the investigations upon radioactivity have led to
modified views in regard to the stability of the elements in general.
There has been little or no proof obtained that any artificial
transmutation of the elements is possible, but the spontaneous
transformation of the radioactive elements brings forward the
possibility that other elements are changing imperceptibly, and that a
state of evolution exists among them. All of the radioactive changes
that we know proceed from higher to lower atomic weights, and we are
entirely ignorant of the process by which uranium and thorium must have
been produced originally.
Since radioactive changes have been found to be accompanied by the
release of vast amounts of energy, compared with which the energy of
chemical reactions is trivial, a new aspect in regard to the structure
of atoms has arisen,—they must be complex in structure, the seats of
enormous energy.
The determination of the amount of radium in the earth’s crust has
indicated that the heat produced by it is amply sufficient to supply the
loss of heat due to radiation, and this source of heat is regarded by
many as the cause of volcanic action. The sun’s radiant heat also has
been supposed to be supplied by radioactive action, so that the older
views regarding the limitation of the age of the earth and the solar
system on account of loss of heat have been considerably modified by our
knowledge of radioactivity.
_Physical Chemistry._
The application of physical methods as aids to chemical science began in
early times, and some of these, such as the determinations of gas and
vapor densities, specific heats, and crystalline forms have been
mentioned already in this article. Within recent times physical
chemistry has greatly developed and a few of its important achievements
will now be described.
_Molecular Weight Determinations._—Gas and vapor densities in connection
with Avogadro’s principle, formed the only basis for molecular weight
determinations until comparatively recent times. The early methods of
Gay-Lussac and Dumas for vapor density were supplemented in 1868 by the
method of Hofmann, whereby vapors were measured under diminished
pressure over mercury. In 1878 Victor Meyer introduced a simpler method
depending upon the displacement of air or other gas by the vapor in a
heated tube. As refractory tubes, such as those of porcelain or even
iridium, could be used in this method, molecular weights at extremely
high temperatures were determined with interesting results. For
instance, it was found that iodine vapor, which shows the molecule I_{2}
at lower temperatures, gradually becomes monatomic with rise in
temperature, that sulphur vapor dissociates from S_{8} to S_{2} under
similar conditions, and that most of the metals, including silver, have
monatomic vapors.
In 1883 and later it was pointed out by Raoult that the molecular
weights of substances could be found from the freezing points of their
solutions, but this method was complicated from the fact that salts,
strong acids and strong bases behaved quite differently from other
substances in this respect, and allowances had to be made for the types
of substances used. The complication was afterwards explained by the
ionization theory of Arrhenius. Better apparatus for this method was
soon devised by Beckmann, who introduced also a method depending upon
the boiling points of solutions, and these two methods are still the
standard ones for determining molecular weights in solution. They are
very extensively employed by organic chemists.
It has been found that the majority of substances when dissolved have
the same molecular weight as in the gaseous condition, provided that
they can be volatilized at comparable temperatures. For instance,
sulphur in solution has the formula S_{8}, iodine is I_{2} and the
metals are monatomic.
_Van’t Hoff’s Law and Arrhenius’s Theory of Ions._—Modern views on
solutions date largely from 1886, when van’t Hoff called attention to
the relations existing between the osmotic pressure exerted by dissolved
substances and gas pressure.
Pfeffer, a botanist, was the first to measure osmotic pressure (1877).
Basing his conclusions chiefly upon Pfeffer’s determinations, van’t Hoff
formulated a new and highly important law, which may be stated as
follows: The osmotic pressure exerted by a substance in solution is
equal to the gas pressure that the substance would exert if it were a
gas at the same temperature and the same volume. Further investigations
have fully established the fact that molecules in dilute solution obey
the simple laws of gases.
It was pointed out by van’t Hoff that salts, strong acids and strong
bases showed marked exceptions to his law in exerting much greater
osmotic pressures than those calculated for them.
The next year in 1887, Arrhenius explained this abnormal behavior of
salts, strong acids and strong bases by assuming that they dissociate
spontaneously into ions when they dissolve, and that these more numerous
particles act like molecules in producing osmotic pressure. He showed
that these exceptional substances all conduct electricity in solution,
while those conforming with van’t Hoff’s law do not, and according to
his theory the ions become positively or negatively charged when they
are formed, and these charged ions conduct the current. For example a
molecule of sodium chloride was supposed to give the two ions Na^+ and
Cl^-, thus exerting twice as much osmotic pressure as a single molecule.
Determinations of osmotic pressure or related values, such as depression
of the freezing point and of electric conductivity, indicated that
ionization could not be regarded as complete in any case except in
exceedingly dilute solutions, and that the extent of ionization varied
with different substances. The fact that osmotic pressures and electric
conductivities gave closely agreeing results in regard to the extent of
ionization in various cases, is the strongest evidence in support of the
theory.
It was difficult at first for many chemists to believe that atoms, such
as those of sodium and chlorine, and groups such as NH_{4} and SO_{4}
could exist independently in solution, even though electrically charged.
However, the theory rapidly gained ground and is now accepted by nearly
every chemist as a satisfactory explanation of many facts.
During recent years, many investigations relating to osmotic pressure
and ionization have been carried out in the United States, but only the
work of Morse, A. A. Noyes, and the late H. C. Jones can be merely
alluded to here. It should be mentioned that the eminent author of the
ionic hypothesis gave the Silliman Memorial course of lectures at Yale
in 1911 on Theories of Solution.
_Colloidal Solutions._—Graham, an English chemist, in 1861 was the first
to make a distinction between substances forming true solutions, which
he called crystalloids, and those of a gummy nature resembling glue,
which in solution do not diffuse readily through parchment membranes, as
crystalloids do, and which he called colloids. The separation of
colloids by means of parchment was called dialysis, and this process has
come into extensive use in preparing pure colloidal solutions. Slow
diffusion is now regarded as characteristic of colloids rather than
their gummy condition.
Colloidal solutions occupy an intermediate position between true
solutions and suspensions, resembling one or the other according to the
kind of colloid and the fineness of division. By preparing filters with
pores of varying degrees of fineness, Bechold has been able to separate
colloids from each other in accordance with the size of their particles.
It has also been possible to prepare different solutions of a colloid
varying gradually from one in which the particles were undoubtedly in
suspension to one which had many of the properties of a true solution.
Beginning in 1889, Carey Lea described in the Journal (=37=, 476, 1889
_et seq._) a variety of methods for preparing colloidal solutions of the
metals, consisting in general of treating solutions of metallic salts
with mild reducing agents. His work on colloidal silver was particularly
extensive and interesting. Solutions of this kind have recently yielded
some extremely interesting results by means of the ultra-microscope, an
apparatus devised by Zsigmondy and Siedentopf. A very intense beam of
light is passed through the solution and observed at right angles with a
powerful microscope. Under these conditions, particles much too small to
be seen by other means, reveal their presence by reflected light. It has
been possible in a very dilute solution of known strength to count the
particles and thus to calculate their size. The smallest colloidal
particles measured in this way were of gold and were shown to have
approximately ten times the diameter, or 1000 times the volume,
attributed to ordinary molecules. It is of interest that the particles
appear in rapid motion corresponding to the well-known Brownian
movement.
The chemistry of colloids has now assumed such importance that it may be
considered as a separate branch of the science. It has its own technical
journal and deals largely with the chemistry of organic products. All
living matter is built up of colloids, and hæmoglobin, starch, proteins,
rubber and milk are examples of colloidal substances or solutions. Among
inorganic substances, many sulphides, silicic acid, and the amorphous
hydroxides, like ferric hydroxide, frequently act as colloids.
_Law of Mass Action._—Berthollet about the beginning of the last century
was the first chemist to study the effect of mass, or more correctly,
the concentration of substances on chemical action. His views summarized
by himself are as follows: “The chemical activity of a substance depends
upon the force of its affinity and upon the mass which is present in a
given volume.” The development of this idea, which is fundamentally
correct, was greatly hindered by the fact that Berthollet drew the
incorrect conclusion that the composition of chemical compounds depended
upon the masses of the substances combining to produce them, a
conclusion in direct contradiction to the law of definite proportions,
and since this view was soon disproved by Proust and others,
Berthollet’s law in its other applications received no immediate
attention. Mitchell, however, pointed out in the Journal (=16=, 234,
1829) the importance of Berthollet’s work, and Heinrich Rose in 1842
again called attention to the effect of mass, mentioning as one
illustration the effect of water and carbonic acid in decomposing the
very stable natural silicates. Somewhat later several other chemists
made important contributions to the question of the influence of
concentration upon chemical action, but it was the Norwegians, Guldberg
and Waage, who first formulated the law of mass action in 1867.
This law has been of enormous importance in chemical theory, since it
explains a great many facts upon a mathematical basis. It applies
particularly to equilibrium in reversible reactions, where it states
that the product of the concentrations on the one side of a simple
reversible equation bears a constant relation to the products of the
concentrations on the other side, provided that the temperature remains
constant. In cases of this kind where two gases or vapors react with two
solids, the latter if always in excess may be regarded as constant in
concentration, and the law takes on a simpler aspect in applying only to
the concentrations of the gaseous substances. For example, in the
reversible reaction
3Fe + 4H_{2}O ⇄ Fe_{3}O_{4} + 4H_{2},
which takes place at rather high temperatures, a definite mixture of
steam and hydrogen at a definite temperature will cause the reaction to
proceed with equal rapidity in both directions, thus maintaining a state
of equilibrium, provided that both iron and the oxide are present in
excess. If, however, the relative concentrations of the hydrogen and
steam are changed, or even if the temperature is changed, the reaction
will proceed faster in one direction than in the other until equilibrium
is again attained.
The principle of mass action also explains why it is sometimes possible
for a reversible reaction to become complete in either direction. For
instance, in connection with the reaction that has just been considered,
if steam is passed over heated iron and if hydrogen is passed over the
heated oxide, the gaseous product in each case is gradually carried
away, and the reaction continually proceeds faster in one direction than
in the other until it is complete, according to the equations