i 3 4 POPULAR SCIENCE MONTHLY 



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 sup- 

 posed 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 (N0 2 ), 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 ap- 

 pear 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 Arm- 

 strong 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 rela- 

 tive value of one arrangement over that of another, as to their re- 

 spective powers of light absorption and consequently of color produc- 

 tion, 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 pre- 

 cisely those that will produce vibrations in the ether corresponding in 

 wave-length to portions of the visible spectrum, consequently the ap- 

 pearance 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 com- 

 pounds 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 



