Part 29

A considerable amount of work with gases had been done by Priestley, Scheele, Cavendish, Lavoisier, Dalton, Gay-Lussac, and others before our hundred-year period began. Cavendish, about 1780, had analyzed atmospheric air with remarkable accuracy, and had even separated the argon from it and wondered what it was, and later Gay-Lussac had shown great skill in the study of gas reactions. During our period gas analysis has been further developed by many chemists. Bunsen, in particular, brought the art to a high degree of perfection in the course of a long period beginning about 1838, the last edition of his “Methods of Gas Analysis” having been published in 1877.

Important devices for the simplification of gas analysis in order that it might be used more conveniently for technical purposes have been introduced by Orsat in France and by Winkler, Hempel and Bunte in Germany.

It appears that our countryman Morley has surpassed all others in accurate work with gases in connection with his determinations of the combining weights and volumes of hydrogen and oxygen about the year 1891. Some of his publications have appeared in the Journal (=30=, 140, 1885; =41=, 220, 1891; and others).

Electrolytic analysis, involving the deposition of metals, or sometimes of oxides, usually upon a platinum electrode, was brought into use in 1865 by Wolcott Gibbs through an article published in the Journal (=39=, 58, 1865). He there described the electrolytic precipitation of copper and of nickel by the methods still in use. The application of the process has been extended to a number of other metals, and it has been largely employed, particularly in technical analyses. Important investigations and excellent books on this subject have been the contributions of Edgar F. Smith of the University of Pennsylvania, and the useful improvement, the rotating cathode, was devised by Gooch and described in the Journal (=15=, 320, 1903).

_General Inorganic Chemistry._

_The Chemical Symbols._—It is to Berzelius that we owe our symbols for the atoms, derived usually from their Latin names, such as C for carbon, Na for sodium, Cl for chlorine, Fe for iron, Ag for silver, and Au for gold. We owe to him also the use of small figures to show the number of atoms in a formula, as in N_{2}O_{5}. This was a marked improvement over the hieroglyphic symbols proposed by Dalton, which were set down as many times as the atoms were supposed to occur in formulas, forming groups of curious appearance, but in some respects not unlike some of our modern developed formulas. The advantages of Berzelius’s symbols were their simplicity, legibility, and the fact that they could be printed without the need of special type. It is true that at a later period Berzelius used certain symbols with horizontal lines crossing them to represent double atoms, and that these made some difficulty in printing. It should be mentioned also that Berzelius at one time made an effort to simplify formulas by placing dots over other symbols to represent oxygen, and commas to represent sulphur atoms. Examples of these are:

ĊaS⃛, calcium sulphate; F̋e, iron disulphide

This form of notation was quite extensively employed for a time, especially by mineralogists, but it was entirely abandoned later.

It is interesting to notice that Dalton, who lived until 1844, to reach the age of 78, differed from other chemists in refusing to accept the letter-symbols of Berzelius. In a letter written to Graham in 1837 he said: “Berzelius’s symbols are horrifying. A young student in chemistry might as soon learn Hebrew as to make himself acquainted with them. They appear like a chaos of atoms ... and to equally perplex the adepts of science, to discourage the learner, as well as to cloud the beauty and simplicity of the atomic theory.”

This forcibly expressed opinion was apparently tinged with self-esteem, but there is no doubt that Dalton was sincere in believing that the atoms were best represented by his circular symbols, because, as is well known, he thought that all the atoms were spherical in form, and it is evident that circles give the proper picture of spherical objects. At the present time some insight as to the structure of atoms is being gained, and it appears possible that the time may come when pictures of their external appearance that are not wholly imaginary may be made.

_Changes in Formulas._—Even before the year 1826, Berzelius displayed great skill in arriving at many formulas that agree with our present ones, for example, H_{2}O for water, ZnCl_{2} for zinc chloride, N_{2}O_{5} for nitric acid (anhydride), CaO for calcium oxide, CO and CO_{2} for the oxides of carbon, and many others. But at the same period other authorities, especially Gay-Lussac in France and Gmelin in Germany, on account of a lack of appreciation for Avogadro’s principle and for other reasons, such as the use of symbols to represent combining weights rather than atoms, were using different formulas for some of these compounds, such as HO, ZnCl and NO_{5}, so that their formulas for many of the compounds of hydrogen, chlorine, nitrogen and several other elements differed from those of Berzelius. The employment of different formulas involved the use of different atomic or combining weights. For example, with the formula H_{2}O for water the composition by weight requires the ratio 1 to 16 for the weights of the hydrogen and oxygen atoms, while with HO the ratio is 1 to 8.

Berzelius attempted to bring about greater uniformity in formulas and atomic weights by making changes in his table of atomic weights published in 1826. He practically doubled the relative atomic weights of hydrogen, chlorine, nitrogen, and of the other elements that gave twice as many atoms in his formulas as in those of others, and at the same time he wrote the symbols of these elements with a bar across them to indicate that they represented double atoms. For example, he wrote:

H̶O ZnC̶l N̶O_{5},

instead of

H_{2}O, ZnCl_{2} N_{2}O_{5}

This appears to have been an unfortunate concession to the views of others on the part of Berzelius, for the barred symbols were not generally adopted, partly on account of difficulties in printing, and the great achievement in theory made by him was lost sight of for a long period of time.

_The Law of Atomic Heats._—In 1819, Dulong and Petit of France, from experiments upon the specific heats of a number of solid elementary substances, came to the conclusion that the atoms of simple substances have equal capacities for heat, or in other words, that the specific heats of elements multiplied by their atomic weights give a constant called the atomic heat. For instance, the specific heats of sulphur, iron, and gold have been given as 0·2026, 0·110, and 0·0324, while their atomic weights are about 32, 56, and 197, respectively; hence the atomic heats obtained by multiplication are 6·483, 6·116, and 6·383.

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 │ │ └─────────┴────────────────┴───────────────────┴───────────────────┘