Part 53

Further investigations showed that the atomic heats display a considerable variation. Those of carbon, boron, beryllium, and silicon are very low at ordinary temperatures, although they increase and approach the usual values at higher temperatures. More recent work has shown, however, that the specific heats of other elements vary greatly with the temperature, almost disappearing at the temperature of liquid hydrogen, and hence possibly disappearing entirely at the absolute zero, where the electrical resistance of the metals appears to vanish likewise.

It has been found that most of the solid elements near ordinary temperatures give atomic heats that are approximately 6·4. Berzelius applied the law in fixing a number of atomic weights, and its importance for this purpose is still recognized.

It may be mentioned here that two well-known Yale men, W. O. Mixter and E. S. Dana, while students in Bunsen’s laboratory at Heidelberg in 1873, made determinations of the specific heats of boron, silicon, and zirconium. This was the first determination of this constant for zirconium, and it was consequently important in establishing the atomic weight of that element.

Isomorphism and Polymorphism.—Mitscherlich observed in 1818 that certain phosphates and arsenates have the same crystalline form, and afterwards he reached the conclusion that identity in form indicates similarity in composition in connection with the number of atoms and their arrangement. This law of isomorphism was of much assistance in the establishment of correct formulas and consequently of atomic weights. For instance, since the carbonates of barium, strontium, and lead crystallize in the same form, the oxides of these metals must have analogous formulas. From such considerations Berzelius was able to make several improvements in his atomic weight table of 1826.

Mitscherlich was the first to observe two forms of sulphur crystals, and from this and other cases of dimorphism or of polymorphism it became evident that analogous compounds were not necessarily always isomorphous, a circumstance which has restricted the application of the law to some extent.

Besides its application in fixing analogous formulas, the law of isomorphism has come to be of much practical use in the understanding and simplification of the formulas for minerals, for these natural crystals very often contain several isomorphous compounds in varying proportions, and an understanding of this “isomorphous replacement,” as it is called, makes it possible to deduce simple general formulas for them.

In some cases isomorphism takes place to a greater or less extent between substances which are not chemically similar, and this brings about a variation in composition which at times has caused confusion. For instance, the mineral pyrrhotite has a composition which usually varies between Fe{7}S{8} and Fe{11}S{12}, and both these formulas have been assigned to it. It was recently shown by Allen, Crenshaw and Johnston in the Journal (=33=, 169, 1912) that this is a case where the compound FeS is capable of taking up various amounts of sulphur isomorphously.

The idea of solid solution was advanced by van’t Hoff to explain the crystallization of mixtures, including cases of evident isomorphism. This view has been widely accepted, and it has been particularly useful in cases where isomorphism is not evident. Solid solution between metals has been found to be exceedingly common, many alloys being of this character. A case of this kind was observed by Cooke and described in the Journal (=20=, 222, 1855). He prepared two well-crystallized compounds of zinc and antimony to which he gave the formulas Zn{3}Sb and Zn{2}Sb, but he observed that excellent crystals of each could be obtained which varied largely in composition from these formulas. As the two compounds were dissimilar in their formulas and crystalline forms, Cooke assumed that isomorphism was impossible and concluded “that it is due to an actual perturbation of the law of definite proportions, produced by the influence of mass.” We should now regard this as a case of solid solution.

A Lack of Confidence in Avogadro’s Principle.—One reason why chemists were so slow in arriving at the correct atomic weights and formulas was a partial loss of confidence in Avogadro’s principle. About 1826 the young French chemist Dumas devised an excellent method for the determination of vapor densities at high temperatures, and his results and those of others showed some discrepancies in the expected densities. For example, the vapor density of sulphur was found to be about three times too great, that of phosphorus twice too great, that of mercury vapor and that of ammonium chloride only about half large enough to correspond to the values expected from analogy and other considerations. Thus, one volume of oxygen with two volumes of hydrogen make two volumes of steam, but only one third of a volume of sulphur vapor was found to unite with two volumes of hydrogen to make two volumes of hydrogen sulphide. Berzelius saw clearly that the results pointed to the existence of such molecules as S{6}, P{4}, and Hg_{1}, but it was not generally realized in those days that Avogadro’s rule is fundamentally reliable, and Berzelius himself appears to have lost confidence in it on account of these complications, for he did not apply Avogadro’s principle to decisions about atomic weights, except in the cases of substances gaseous at ordinary temperatures.

Electro-chemical Theories.—The observation was made by Nicholson and Carlisle in 1800 that water was decomposed into its constituent gases by the electric current. Then in 1803 Berzelius and Hisinger found that salts were decomposed into their bases and acids by the same agency, and in 1807 Davy isolated potassium, sodium, and other metals afterwards, by a similar decomposition. Since those early times a vast amount of attention has been paid to the relation of electricity to chemical changes, a relation that is evidently of great importance from the fact that while electric currents decompose chemical compounds, these currents, on the other hand, are produced by chemical reactions.

Berzelius was particularly prominent in this direction, and in 1819 he published an elaborate electro-chemical theory. He believed that atoms were electrically polarized, and that this was the cause of their combination with one another. He extended this idea to groups of atoms, particularly to oxides, and regarded these groups as positive or negative, according to the excess of positive or negative electricity derived from their constituent atoms and remaining free. He thus arrived at his dualistic theory of chemical compounds, which attained great prominence and prevailed for a long time in chemical theory. According to this idea, each compound was supposed to be made up of a positive and a negative atom or group of atoms. For example, the formulas for potassium nitrate, calcium carbonate, and sulphuric acid corresponded to K{2}O.N{2}O{5}, CaO.CO{2} and H{2}O.SO{3} where we now write KNO{3}, CaCO{3} and H{2}SO{4}, and the theory was extended to embrace organic compounds also.

The eminent English chemist and physicist Faraday announced the important law of electro-chemical equivalents in 1834. This law shows that the quantities of elements set free by the passage of a given quantity of electricity through their solutions correspond to the chemical equivalents of those elements. Faraday made a table of the equivalents of a number of elements, regarding them important in connection with atomic weights, but at that time no sharp distinction was usually made between equivalents and atomic weights, and it was not fully realized that one atom of a given element may be the electrical equivalent of several atoms of another.

Faraday’s law, which is still regarded as fundamentally exact, has been of much practical use in the measurement of electric currents and in calculations connected with electro-chemical processes. In discussing his experiments, Faraday made use of several new terms, such as “electrolyte” for a substance which conducts electricity when in solution, and is thus “electrolyzed,” “electrode,” “anode,” and “cathode,” terms that have come into general use, and finally “ions” for the particles that were supposed to “wander” towards the electrodes to be set free there.

This term “ion” remained in comparative obscurity for more than half a century, when it was brought into great prominence among chemists by Arrhenius in connection with the ionic theory.

Cannizzaro’s Ideas.—Up to about 1869 chaos reigned among the formulas used by different chemists. Various compound radicals and numerous type-formulas were employed, dualistic and unitary formulas of several kinds were in use, but the worst feature of the situation was the fact that more than one system of atomic weights was in vogue, so that water might be written

HO, H̶O, or H_{2}0

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.