the same atomic volume, the atomic volume of sulphate of copper is 280; whilst the atomic volume of sulphate of zinc is 293; again, the atomic volume of carbonate of cadmium is 232; whilst that of carbonate of iron, or carbonate of manganese, is 195; we might adduce many more examples, but these are sufficient to show that the atomic volumes of isomorphous bodies are not always identical. Again, some bodies have the same atomic volume, but not the same form; for example, carbonate of iron, oxide of silver, sulphate of cadmium, and peroxide of iron, none of which bodies have any isomorphous connection, have yet the same atomic volume.

265. Attempts have been made, when the crystalline form corresponds, but the chemical constitution differs, to harmonize them by altering the atomic weights of some of the bodies. "The following is one of the most remarkable instances of this kind:-Ca O, CO2, as arragonite, is isomorphous with nitre (KO, NO,) in its usual form; Ca O, CO2, as calc-spar, with KO, NO, as it is sometimes obtained, and with Na O, NO, as it always crystallizes. Hence, Ca O, CO, in its two conditions, is isomorphous with KO, NO, in its two conditions. For this reason, Count Schaffgotsch halves the atomic weights of potassium and nitrogen; nitre then becomes = KNO3. This agrees with Ca O, CO, Ca CO,; in both compounds, three atoms of oxygen are combined with one atom of metal and one atom of either carbon or nitrogen. This halving of the atomic weight of potassium involves the halving of those of ammonium (NH), sodium, silver, and gold, because potash is isomorphous with oxide of ammonium (NH, O) and soda, and the last of these with oxide of silver; and because silver, in combination with the most various quantities of gold, always crystallizes in the same form, a circumstance which indicates the isomorphism of these two metals. The halving of the atomic weight of silver had before been proposed by H. Rose, because in grey copper ore one atom of silver takes the place of two atoms of copper, and the crystalline form of Ag S, as well as that of Cu, S, belongs to the regular system. According to this view, the atomic weights of Ň, H, K, Na, Ag, and Au, would be reduced to one-half of the values now assigned to them; potash would be

M

K, O, soda Na, O, oxide of silver Ag, O, and suboxide of silver Ag, O. By halving the atomic weights of potassium and chlorine the similar forms of KO, CI O, and Ba O, SO, would also be explained, for the composition would then be K CI O, and Ba SO.."* Numerous facts are opposed to Schaffgotsch's hypothesis, and therefore no value is attached to it.

266. Before directing the attention of the student to the value of isomorphism in determining equivalent numbers, we will briefly bring before his notice the points at which we have arrived.

1. We have seen that some bodies are similar in crystalline form, and similar in chemical constitution and properties; bodies which are thus alike in form and constitution can replace each other in indefinite proportions. These isomorphous bodies, as we have seen, are not always made up of the same number of elementary atoms.

2. We have seen that some bodies are similar in crystalline form, but unlike in chemical constitution and and properties; bodies which are alike in form but unlike in chemical constitution cannot, as far as our knowledge at present extends, replace one another in indefinite proportions.

3. We find that many bodies which are not generally considered isomorphous, but which are analogous in chemical constitution, possess the property of replacing each other in compounds (258).

267. It is probable that, just as the allotropic state of elementary bodies, according to Brodie's experiments (193), is due to those substances possessing a different atomic number in this state to what they possess in their ordinary state, so the dimorphism of bodies may be due to the same cause; and possibly many of the substances which are similar in form, but which at present are not considered to have a similar chemical constitution, may be made to harmonize in the latter respect in the same way. This idea is supported, I think, by Scheerer's discovery (253), that two atoms, and sometimes three atoms, of water are isomorphous with one atom of the oxides forming Group X.

268. The aid derived from isomorphism in determining

"Gmelin's Handbook of Chemistry," vol. i.

equivalents will be best understood by the following examples. Not long ago, only two oxides of chromium were known; they have the following composition:

In the first oxide, every 187 parts of chromium are united with 8 of oxygen.

In the second oxide, every 18-7 parts of chromium are united with 16 of oxygen. Now the atomic constitution of the first was determined to be not the protoxide, but the sesquioxide, because it was found to be isomorphous with sesquioxide of iron; the second oxide (chromic acid) became, therefore, the teroxide; and this view of the composition of this oxide was beautifully confirmed by the discovery that chromic acid was isomorphous with manganic and sulphuric acids. Since this view was taken of the constitution of these two oxides, another oxide of chrome has been discovered, which contains two-thirds less oxygen than chromic acid; therefore, if in the latter compound three equivalents of oxygen are united with one of chromium, this new oxide must be the protoxide of chromium, which is proved to be the case by its yielding salts isomorphous with the salts of protoxide of iron, &c. In like manner, alumina is regarded as the sesquioxide of aluminum, because it is isomorphous with sesquioxide of iron.*

The student, after he has finished this chapter, is recommended to study the chapters on Isomorphism, Dimorphism, Allotropism, and Isomerism, in Kane's "Elements of Chemistry," 2nd edit.

164

CHAPTER VI.

ATOMIC WEIGHTS OF THE ELEMENTS. THE CONSTITUTION OF COMPOUND BODIES. THE MEANING OF CHEMICAL

FORMULE.

Classification of the elements, 269. Equivalents, 271. Atomic weights, 276. Arguments for doubling the atomic weight of oxygen, &c., 282. Arguments against doubling the atomic weight of oxygen, &c., 330. The different views which have been and are held on the constitution of compounds, 338. The unitary system, 358. Williamson's reasons for considering that many bodies are formed on the type of water, 373. The different views which have been and are held on the constitution of the alcohols and ethers, and the different theories of etherification, 374. The different views which have been and are held on the constitution of ammonia, 396. Double decomposition regarded as the type of chemical action in general, 419. The meaning of formulæ, 420. The same body can have more than one rational formula, 422. Molecular types, 426. Equivalent values of radicals, 438. Equivalent notation, 446. Conjugate radicals, 447. Gerhardt's system of classification, Table of the classification of bodies according to their chemical functions.

451.

269. Classification of the elements.-The elements are usually divided into two classes, metals and metalloids; this division is the one which was adopted in the infancy of the science, and is still generally continued, more on account of its simplicity and convenience than its correctness; in the following table they are distributed into the most natural families or groups. In many instances, these natural relations between the individual elements thus grouped together are very striking, in others they are more obscurely marked, and in the case of the metals of the earths proper, as well as of the noble metals, the natural affinities of these elements with the others are as yet very incompletely known.