414 COLLECTED STUDIES IN IMMUNITY 



Thus by allowing sulphuric acid to act on anilin, which, as is well known, 

 is highly toxic, the toxicity is completely destroyed, for the result- 

 ing sulfanilic acid can be taken in large doses without injury. In 

 like manner the amidobenzoic acids are non-toxic; so also the meta- 

 and para-oxybenzoic acids derived from phenol, while the ortho 

 isomer (salicylic acid) still exhibits the familiar toxic effects, although 

 they are far less intense than those of phenol. These surprising 

 results cannot be ascribed to purely chemical effects, as, for example, 

 by assuming that the acid derivatives are more difficult to oxidize than 

 the original substance and that they therefore do not abstract oxygen 

 from the tissues. Certain observations, however, which I had made 

 many years previously in connection with vital staining furnish a 

 very simple explanation. I found that the power to stain gray nerve 

 tissue is possessed by only a small number of dyes, and especially 

 by certain basic dyes (chrysoidin, Bismarck brown, neutral red, 

 phosphin, flavanilin, methylene blue), whereas of the acid dyes, in 

 which OH constitutes the auxochrome group, only one, alizarin, 

 possesses this property. All dyes which contained a sulphuric acid 

 radical were absolutely negative, and I examined a very large number. 

 What is especially significant is that even neurotropic stains lost this 

 property entirely if sulfonic acids were introduced, a fact demonstrated 

 in the flavanilin sulfonic acids, the alizarin sulfonic acids, and the 

 sulfonic acids derived from methylene blue. From this it follows that 

 the introduction of the above-mentioned acid group changes the dis- 

 tribution in the organism and causes especially a complete destruc- 

 tion of neurotropic properties. The central action of a poison is to 

 be explained logically by an accumulation of the toxic substance in 

 the central nervous system. Since, therefore, the central part of the 

 toxic action has been completely destroyed by the introduction of a 

 sulfonic acid radical we find that the reduction in toxicity is readily 

 explained. It is obvious that under these conditions other toxic 

 properties, which do not depend on the central nervous system may 

 be preserved intact. Thus according to my observations the blood 

 destructive properties of phenylhydrazin and benzidin are still present 

 in their monosulfonic acids. 1 



1 The action of these combinations is not as strong as the original sub- 

 stance, but this is probably due to the fact that the sulfonic acid radical (and 

 even t'he neutral sulfonic radical) by itself reduces the toxic power of the amido 

 group. This mitigating action explains why sulfanilic acid which is derived 

 from anilin is no blood poison; this power of the sulfonic acid group, however, 



