as the base of various derivatives, we have a right to look for this same influence between the unsaturated groups, or that condition which gives rise to isorropesis and hence to color. A great portion of the coloring products known have just this sort or structure and the origin of their colors, therefore, may receive the interpretation indicated.
For many years the color theory proposed by Witt has been the basis of all chemical investigations in this domain. Here it is supposed that the color of an organic compound depends upon the presence of an atomic group known as a chromophore, such, for example, as the nitro group (NO2), etc., and the introduction of more and more of these groups into a compound produces a gradual increase in depth of color. The various radicals with their respective color-giving groups are known as chromogens; upon union of these with other radicals of an acidic or basic character, we arrive at the conditions for coloring products or dye-stuffs. Now the carbonyl group alone does not appear as a pronounced chromosphere, but when two carbonyl groups, as in the ortho-or para-position in the benzol ring, are present one of the best of chromophores is developed. From such results as these Armstrong was led to believe that the particular linkings present in the benzol ring when two carbonyl groups were para to each other might account for the pronounced color reaction shown by these compounds. He characterized this type of structure, wherein the para carbon atoms have double linkings with the oxygen atoms as "quinonoid" (quinoid), in contradistinction to that of the alternate double linkings in a benzol ring or "benzenoid" (benzoid) . Eventually, he came to the conclusion that color in an organic compound depends upon the presence of this quinoid arrangement. From the chemical standpoint Armstrong had advanced upon solid ground. The real insight, however, into the relative value of one arrangement over that of another, as to their respective powers of light absorption and consequently of color production, must rest upon spectroscopic evidence. For example, we have seen that double linkings in themselves do not possess any power for light absorption. Their mere presence, therefore, can not account for color in a chemical compound, but if by their presence some form of oscillation is produced, we may expect the establishment of definite vibrations in the ether, which will be possible of detection in the spectrum. In compounds of the quinoid type the conditions are precisely those that will produce vibrations in the ether corresponding in wave-length to portions of the visible spectrum, consequently the appearance of color. In compounds of the benzoid type alone the oscillations correspond to vibrations of such frequencies that they fall in the ultra-violet region of the spectrum, and hence such compounds will be free from color. The oscillations which exist whenever the quinoid type of compounds is concerned, and which distinguish this type from that of the benzoid, must be due to the oxygen atoms in
