TESTING OF THE DIALYSIXG TUBES 157 



this case also the dialysis is carried on in an incubator, 

 in order to expose all the tubes to approximately equal 

 conditions, 



After some sixteen hours the ninhydrin re- 

 action is applied. As this reaction depends so 

 much upon the degree of concentration, it is 

 advisable to carefully guard against the following 

 sources of error. First of all, the dialysate must 

 not be allowed to evaporate unevenly. To avoid this, 

 an excess of toluol is added, and the Erlenmeyer 

 tube is preferably covered with a watch glass. It is 

 clear that, should the different dialysates evaporate 

 unevenly, the ninhydrin reactions would be of vary- 

 ing intensity. The second source of error lies in the 

 boiling of the separate test-tubes, which is applied 

 in order to produce the formation of the colouring 

 substances. We shall return to this presently. 



In the application of the ninhydrin reaction we must 

 never forget the fact, that ninhydrin is a most delicate 

 reacting agent for albuminous substances, peptones, 

 polypeptides, and amino-acids. Perspiration reacts 

 very readily with ninhydrin, as do also the epidermic 

 scales, &c. It is most important to avoid any contact 

 of the dialysing tube with the hand ; only sterilized 

 forceps should be employed for holding them, and 

 all the apparatus in use should be absolutely clean 

 and dry. One must never rely upon any rapid 

 drying methods. In the first place, it will not do 10 



