8 EXPERIMENTAL RESEARCHES 



the protein molecule chemical atom-groups, e.g., 

 the nitro-, the amido-, etc., or, on the other 

 hand, the reactive power of any such groups as 

 may be present can be inhibited, as by the 

 removal of the amid. I would here remind you 

 of the researches of Obermayer and Pick, which 

 show that by purely chemical substitution of 

 this kind one may also profoundly alter the 

 antigenic character of albuminous substances, 

 so that in place of specificity for a kind 

 (" Artspecifitat ") we have a specificity of 

 chemical constitution. I would remind you 

 also of the behaviour of certain colloid dyes 

 for example, chromazon-red, which possesses all 

 the properties of the azo-dyes, but differs from 

 them in possessing an aldehyde group ; it is 

 thus able to react with hydrazin, and by union 

 therewith to be transformed into a hydrazon of 

 blue colour. Besides, I do not believe that for 

 the explanation of such reactions, especially of 

 their specific nature, a few analogies drawn from 

 certain phenomena of the hitherto obscure chem- 

 istry of colloids are sufficient ; and as a matter 

 of fact it seems to me that the ever-increasing 

 endeavour to build causal relationships on the 

 ground of purely formal analogies, is but little 

 fitted to advance our knowledge. The condi- 



