280 Biological Chemistry. 



can be estimated by precipitating it as the ammonium salt 

 by the addition of ammonium chloride or ammonium 

 sulphate. Different methods for precipitation, and estima- 

 tion of the uric acid in the precipitate, have been proposed. 

 The principle of the method usually employed for the 

 estimation is that of dissolving the precipitated urate in 

 sulphuric acid, and titration with standard potassium per- 

 manganate solution. Creatinine gives a brown coloration 

 with potassium hydroxide and picric acid (Jaffe's reagent). 

 The amount present in urine can be ascertained colori- 

 metrically by comparing the tint produced with Jaffe's 

 reagent with a standard solution of potassium bichromate, 

 or with the depth of colour produced with known amounts 

 of creatinine. A colorimeter is generally used for the 

 purpose. The total nitrogen is also usually determined in 

 the urine by Rjeldahl's method. The difference between 

 this amount and the amounts of nitrogen in the urea, uric 

 acid, creatinine, and ammonia, gives the amount of nitrogen 

 excreted in other forms, such as in the amino-acids and 

 the other nitrogenous constituents which are only present 

 in small quantities. 



In the faeces the substances most usually estimated 

 are the nitrogen and the fat the former by Kjeldahl's 

 method, and the latter by extraction in a Soxhlet ap- 

 paratus. 



THE RESULTS TO BE OBTAINED IN THE INVESTIGATIONS 

 OF THE TOTAL METABOLISM. 



Many important results have been obtained of both 

 physiological and pathological interest in investigations 

 of the character outlined above. Only a few examples 

 can be given in this place to indicate their utility. 



Some instances of their employment in researches on 

 the physiology of nutrition may be given first. Of the 





