130 BIO-CHEMICAL JOURNAL 



filtrate after precipitating the albumen with trichloracetic acid (added 

 when the mixture is boiling). As the experiments have throughout 

 been comparative, and carried out under as nearly as possible like 

 conditions, the method has been satisfactory enough. 



In the determination of the residual nitrogen in tissues, it was not 

 always possible to carry out the experiments under perfectly identical 

 conditions ; it was necessary, furthermore, to determine the absolute 

 quantities of residual nitrogen in each case. Preliminary experiments 

 were therefore undertaken to test the reliability of older methods, and 

 to determine how much the results varied under different conditions. 

 For this purpose a mixture of a solution of Witte's peptone and serum 

 was made, and this mixture was diluted with varying quantities of 

 water ; it was found that the amount of residual nitrogen obtained 

 after precipitating with tannic acid (in the cold) or with trichloracetic 

 acid (boiling), varied quite appreciably with the strength of the solution 

 in which precipitation took place ; it was found that more residual 

 nitrogen was obtained in the filtrate when the precipitants were added 

 to a dilute than when added to a concentrated solution. Tannic acid 

 precipitates of course a certain part of the peptone ; trichloracetic acid, 

 on the other hand, precipitates only a minute quantity if added to a 

 sufficiently dilute boiling solution. On the other hand, trichloracetic 

 acid, when added to the solution of the peptone-serum mixture above 

 a certain dilution, produces a precipitate in such a finely divided state 

 that filtration through filter paper is almost impossible. As a general 

 result it was found that the trichloracetic acid precipitation method 

 gives fairly accurate results when the strength of the solution was 

 maintained within certain definite limits of concentration. 



It was important, therefore, to have a method available which 

 would allow the coagulation to be carried out under perfectly constant 

 conditions, and at the same time give the residual nitrogen in absolute 

 numbers. 



For such a method I am indebted to the suggestion of S. N. Pinkus. 

 He has shown that if anhydrous sulphate of soda be added to solutions 

 of albumens or to tissues, the water is abstracted from the same with a 

 very slightly increase of temperature, and without any appreciable 



