CHEMICAL CONSTITUTION AND PHARMACOLOGICAL ACTION 435 



with rhodamin exhibits a beautiful fluorescence. Rhodamin itself, 

 however, shows fluorescence only when in solution; when in the dry 

 state, even in the finest possible form, it merely shows a pure red 

 color. Because of this fluorescence Witt assumes that the dye forms 

 a homogeneous mixture with the fibres of the silk, i.e., it is in the 

 form of a solution. Since the fibre, however, is a solid substance 

 this solution must be what Yan't HofT terms a "solid solution." We 

 know that the same dye often produces different tints in various kinds 

 of fibres. This is analogous to the fact that the same substance 

 often dissolves in different solvents in entirely different tints, as is 

 the case, for example, with iodine. Witt therefore believes that 

 the process of staining proceeds exactly the same as the distribution 

 of a substance in two different solvents. Thus, if we dissolve anilin 

 in water, we find that we can shake all the anilin out tvith ether y 

 because the solvent power of the ether is greater than that of water. In 

 the staining process such a vast difference in solvent power shows 

 itself by the fact that the materials introduced entirely exhaust the 

 staining-bath. If, however, the difference in solvent powers is less 

 than this, e.g. in the combination water, ether and resorcin, we shall 

 find that the resorcin is distributed between both fluids in accordance 

 with a law of distribution which can be figured out mathematically 

 for every case. In dyeing this type corresponds to the dyes which are 

 said to "take" poorly. In these the staining-bath does not become 

 exhausted under ordinary conditions. Exhaustion can be effected 

 only through the addition of certain substances which limit solution 

 (salt dyes, etc.). 



In the introductory chapter I have already mentioned that all 

 neurotropic and lipotropic substances lose the property to stain brain 

 substance and fat by the introduction of the sulfonic acid radical. 

 If these substances are examined in a test-tube it is found that this 

 substitution has caused them to lose also the solubility in ether or 

 in fats. Thus, although flavanilin is easily taken up by ether from 

 an alkaline solution, not a trace of flavanilinsulfonic acid is taken up. 

 Another interesting case may be mentioned, one which concerns 

 staining with neutral red. This has the following formula: 



NH 2 N N(CH 3 ) 2 



