PERFORMANCE OF THE EXPERIMENT 



easily be recognized by diluting a truly violet solution 

 with water until the intensity of the colour corre- 

 sponds with that of the sample, when one can see at 

 once that, though the solution has been very much 

 weakened, the colour still appears violet. A reddish, 

 or, rather, yellowish-brown tint means that either the 

 work has not been properly carried out, or else that 

 the blood contained acids or alkalies in excess. The 

 experiment must be repeated, otherwise it may 

 happen that the existing conditions conceal a positive 

 reaction. We shall deal with this point in fuller 

 detail, when we return to the question of the sources 

 of faulty observations. 



Under certain conditions a special control test may 

 be needed. Such would be the case in dealing with 

 micro-organisms cultivated on a medium which 

 could not be readily separated by centrifuging. 

 In this case the germ-free medium must be boiled by 

 itself, until the filtered boiled water gives no traces 

 of coloration with ninhydrin. Then the cultures are 

 prepared in exactly the same way, and the following 

 tests are performed: (i) Serum alone; (2) serum + 

 medium ; and (3) serum + culture. Should the experi- 

 ment (2) produce decomposition, then a positive 

 reaction in experiment (3) would certainly not prove 

 that the micro-organisms had been decomposed. 



A very important control test, for proving the suit- 

 ability of the organ or the substrate used, is the 



