In the preceding chapters we have become acquainted with the most important properties of the four elements, hydrogen, oxygen, nitrogen, and carbon. They are sometimes termed the organogens, because they enter into the composition of organic substances. Their mutual combinations may serve as types for all other chemical compounds—that is, they present the same atomic relations (types, forms, or grades of combinations) as those in which the other elements also combine together.
| Hydrogen, | HH, | or, | in | general, | HR. |
|---|---|---|---|---|---|
| Water, | H 2 O, | „ | „ | H 2 R. | |
| Ammonia, | H 3 N, | „ | „ | H 3 R. | |
| Marsh gas, | H 4 C, | „ | „ | H 4 R. |
One, two, three, and four atoms of hydrogen enter into these molecules for one atom of another element. No compounds of one atom of oxygen with three or four atoms of hydrogen are known; hence the atom of oxygen does not possess certain properties which are found in the atoms of carbon and nitrogen.
The faculty of an element to form a compound of definite composition with hydrogen (or an element analogous to it) gives the possibility of foretelling the composition of many other of its compounds. Thus, if we know that an element, M, combines with hydrogen, forming, by preference, a gaseous substance such as HM, but not forming H 2 M, H 3 M, H n M m , then we must conclude, on the basis of the law of substitution, that this element will give compounds M 2 O, M 3 N, MHO, MH 3 C, &c. Chlorine is an example of this kind. If we know that another element, R, like oxygen, gives with hydrogen a molecule H 2 R, then we may expect that it will form compounds similar to hydrogen peroxide, the metallic oxides, carbonic anhydride, or carbonic oxide, and others. Sulphur is an instance of this kind. Hence the elements may be classified according to their resemblance to hydrogen, oxygen, nitrogen, and carbon, and in conformity with this analogy it is possible to foretell, if not the properties (for example, the acidity or basicity), at any rate the composition, of some of their compounds. This forms the substance of the conception of the valency or atomicity of the elements . Hydrogen is taken as the representative of the univalent elements, giving compounds, RH, R(OH), R 2 O, RCl, R 3 N, R 4 C, &c. Oxygen, in that form in which it gives water, is the representative of the bivalent elements, forming RH 2 , RO, RCl 2 , RHCl, R(OH)Cl, R(OH) 2 , R 2 C, RCN, &c. Nitrogen in ammonia is the representative of the trivalent elements, giving compounds RH 3 , R 2 O 3 , R(OH) 3 , RCl 3 , RN, RHC, &c. In carbon are exemplified the properties of the quadrivalent elements, forming RH 4 , RO 2 , RO(OH) 2 , R(OH) 4 , RHN, RCl 4 , RHCl 3 , &c. We meet with these forms of combination , or degrees of union of atoms, in all other elements, some being analogous to hydrogen, others to oxygen, and others to nitrogen or to carbon. But besides these quantitative analogies or resemblances, which are foretold by the law of substitution (Chapter VI.), there exist among the elements qualitative analogies and relations which are not fully seen in the compounds of the elements which have been considered, but are most distinctly exhibited in the formation of bases, acids, and salts of different types and