going various pulsations, such as may be illustrated by the accompanying figures:
The clotted lines show the linkings developed from the free affinities when the ring is pulsating between the two forms (a) and (b). The centric formula for benzol (Baeyer's), as shown in (c), may be, therefore, an intermediate form for all the possible forms. The free or residual affinity possessed by each carbon atom asserts itself under the various conditions which can be brought into existence by these pulsations, with the effect that the several linkings produced must involve always a pair of carbon atoms and then in turn during the second stage of the pulsation must suffer a break and consequently give rise to some particular one of the seven possible phases, with its characteristic absorption band of course depending upon the carbon atoms in question. Altogether, when the entire ring is free to pulsate in every direction, there will arise seven absorption bands which represent the seven possible combinations of linking-change.
The derivatives of benzol may be expected to show some variation in type and manner of pulsation from that of the parent ring, but whatever changes occur the effect upon the characteristic absorption spectrum of the original molecule will always indicate the exact nature of each change. In this connection it will be well to consider a few of the more important derivatives, which, as is generally known, are primarily formed by the replacement of one or more of the hydrogen atoms by an equivalent atom or group of atoms—a process called substitution. The alkyl radicals (methyl, ethyl, etc.) stand as a type of the neutral groups and consequently, when they are present, little change in the spectrum of the original substance should be observed. The spectrum of toluol, C6H5 • CH3, ethyl benzol, C6H5C2H5, etc., are almost identical, but only the first two absorption bands of the original benzol spectrum are well marked, the remaining bands having fused more or less into one broad band. With aniline, C6H5NH2, where the basic unsaturated amido-group (NH2) has replaced the hydrogen atom, we get only a broad absorption band caused, no doubt, by the residual affinity of the nitrogen atom which binds or holds all the free affinities of the benzol ring. Upon the addition of an excess of hydrochloric acid to aniline, we obtain the saturated compound known as aniline hydrochlorate, C6H5 • NH3CI, the nitrogen having passed from the trivalent to the quinquivalent state. This compound, as