26 Biological Stains 



colored. We know that color is caused by selective absorption 

 of certain wave lengths of light, so that the light transmitted by 

 or reflected from a given substance lacks these particular wave 

 lengths of the visible spectrum and accordingly appears colored. 

 Such a substance, in other words, acts as if it were a prism pro- 

 vided with a screen to block off certain parts of the spectrum in 

 which the refracted light is normally transmitted. Conceivably 

 the chromophores confer upon the molecule properties of just this 

 sort, acting possibly as a system of resonators tuned to vibrate at 

 the same rate as the waves to be absorbed. The theory cannot be 

 discussed here in any detail; but it should be pointed out that 

 valence electrons under certain circumstances confer resonance 

 on a given compound. The unsaturated compounds, such as 

 those containing the double bonds of the chromophores, show a 

 special tendency toward resonance; and it is a fact that as the 

 general degree of unsaturation and complexity of a compound in- 

 creases, the spectral absorption tends to pass from ultraviolet 

 toward infra-red, that is, the color transmitted (or reflected) 

 changes from yellow through reds and blues. 



The benzene compounds containing chromophore radicals are 

 known as chromogens. A chromogen, however, although it is 

 colored, is not a dye, in that it possesses no aflSnity for fibers or 

 tissues. It may coat them, but only mechanically, and it will be 

 easily removed by mechanical processes. That is, it will not 

 "take". (See, however, the discussion of fat stains, p. 69). In 

 order for a substance to be a dye, it must contain in addition to the 

 chromophore group, a group which imparts to the compound the 

 property of electrolytic dissociation. Such auxiliary groups are 

 known as auxochromes. They may slightly alter the shade of the 

 dye, but are not the cause of the color. Their function is to fur- 

 nish salt-forming properties to the compound. Certain chromo- 

 phoric groups have also slight auxochromatic properties. 



To illustrate these different types of groups, let us take a typical 

 example. The nitro group (-NO2) is a chromophore. When three 

 of these groups displace three hydrogen atoms in a benzene mole- 

 cule, we have the compound trinitrobenzene, 



O2N NO, 



NO2 



which is yellow. It is not a dye, however, but is a chromogen. It 

 is insoluble in water, and is neither an acid nor a base; that is, it 

 does not dissociate electrolytically and consequently cannot form 

 salts with either alkalies or acids. If, however, one more hydrogen 

 atom is replaced, this time with the hydroxyl group (-0H), which 

 is an auxochrome, the resulting compound, 



