might be written as containing different compound radicles under different conditions.
The fault of the old chemistry was that more attention was paid to symmetrical formulæ than to reactions; the merit of the new consisted in bringing the student once more back to nature.
And the appeal to nature was answered, and answered abundantly. The new conception of compound radicles was rich in results; from it there was developed—first, the theory of types, and, subsequently, the wider theory of valency, which has led to that of atom-linking, and these in their turn have reacted on the older and more fundamental notions of the science, and have given a new meaning to such terms as "chemical" and "mechanical actions," "compounds" and "mixtures," etc., while, at the same time, they point the way to the chemistry of the future when we shall have gained a definite conception of the inner mechanism of the molecule, and of the laws which regulate the combinations of molecules in groups, and the decompositions of molecules with subsequent formations of new atomic systems.
Let us shortly examine these ideas. If sodium be thrown on to water, caustic soda is produced, a substance made up of hydrogen, oxygen, and the simple radicle sodium; by another reaction a substance can be obtained consisting of hydrogen, oxygen, and the compound radicle nitryl (NO2). These two bodies have analogous formulæ, Na OH and (NO2) OH, they may both be regarded as derived from water, H HO, by the replacement of one half of the hydrogen by a radicle; in one case by Na, in the other by NO2. Again, the whole of the hydrogen in water may be replaced by sodium, with production of the compound sodium oxide, Na2O.; but in many of its reactions this compound is the analogue of common ether, which is also a compound of oxygen with a (compound) radicle ethyl, and has the formula (C2H5)2O. Now, these substances, Na OH, (NO2) OH, Na2 O, and (C2H5)2O, both on account of the methods by which they are produced, and because of their general reactions, may be classed together as derivatives of water, or may be said to belong to the water-type. Similarly, other types have been instituted, and large groups of compounds have been brought into the same class as being all referable to one parent type. This step in advance is evidently an outcome of the theory of compound radicles; without that conception a system of classification by types would have been impossible.
But it was found that while such compound radicles as C2H5 or NO2 were capable of replacing but one part by weight of hydrogen in water, other compound radicles, such as CO or C2H4, were capable of taking the place of two parts by weight of hydrogen. Comparing together these two sets of radicles, it might be said that CO 2 NO2 or C2H4 2 C2H5, so far as the power of combining with hydrogen was concerned. This conception of binding power being extended to the elements, and being deepened and widened by laborious experimental
