THE DIALYSATION PROCESS 155 



The biuret reaction is unfortunately rather difficult 

 to detect when it is a case of demonstrating slight 

 traces of the reddish-violet coloration. This is due 

 to the fact that the eye is but slightly sensitive to these 

 tints. Again there are great individual differences. 

 If the observer is unable to detect a light biuret 

 reaction then he has to rely on standardized tubes ; or 



j 



else he must make use of the ninhydrin reaction and 

 try, by means of lengthy dialysation, to. increase the 

 quantity of albumen in the dialysate, so far as 

 the tubes are permeable to albumen. Seeing that 

 white of egg, as well as serum, always contains 

 substances which diffuse and react with ninhy- 

 drin, we are bound to find out, by means of a 

 standardized tube, what quantity of a given albumen 

 solution we may use without running the risk 'of the 

 dialysate showing a ninhydrin reaction. How to 

 perform the ninhydrin test we shall describe later, 

 when we give the test for equal permeability to decom- 

 posites of albumen. 



The biuret reaction is performed as follows : To 

 the mixture of the dialysate with caustic soda about 

 i c.c. of a very much diluted copper sulphate solution 

 -e.g., i in 500 c.c. is added. This solution is run 

 down by means of a pipette along the inside of the 

 test-tube, so as to obtain a surface layer. Then we 

 observe by transmitted light the dividing line 

 between the blue layer, which often, however, appears 



