198 SOURCES OF ERROR (IX THE PROCESS) 



\\cll as the tubes, in this medium. The rinsing ot 

 the dial \\sino' lubes must also oe done with distilled 

 water. 



Finally, we must bear in mind another source of 

 error, which we have not yet specially referred to. It 

 may sometimes happen, that the substrate added to 

 the serum absorbs some Constituents of the latter, and 

 retains them. Such a case would manifest itself in 

 the fact, that the serum alone would react positively, 

 while the dialysate of the experiment, organ + sub- 

 strate, would give a negative reaction. Further, a 

 reaction might give a negative result, although 

 decomposition had actually taken place. The optical 

 method would easily detect such sources of error. 



There is no single point in the rules which lacks a 

 definite foundation. Researches have generally 

 been wrecked owing to trifling details. A glance 

 at the literature, however, shows that at present the 

 method is properly used in many places, and leads to 

 surprisingly beautiful results. 



Further sources of error are : The use of vessels 

 that are not dry, and of boiling-sticks that have been 

 touched by the hands, soiling the pipettes with 

 saliva, inaccurate measurement of the ninhydrin 

 solution, the use of infected water, the cultivation of 

 bacteria in the same incubator as is used for experi- 

 ments on the action of ferments, covering the con- 

 tents of the tubes, and the outside fluid, with an 



