244 



PEACTICAL PHYSIOLOGY 



DEMONSTRATION. 1st Stage Incineration. A measured quantity of 

 urine 5 c.c. is placed in a combustion flask, 1 and to it about twice 

 that amount of concentrated pure sulphuric acid is added. The mixture 

 is then gradually heated till just below the boiling point of the acid 

 (the flask being placed in a slanting position), and is kept at this 

 temperature till it becomes clear. The chemical reaction which takes 

 place is that the H 2 S0 4 decomposes the organic matter, the carbon 

 being oxidised to carbon dioxide, and the nitrogen changed into 

 ammonia, which immediately on its formation combines with the 



FIG. 150. Distilling portion of Kjeldahl's apparatus. 



excess of sulphuric acid present to form ammonium sulphate. 

 The first effect of adding the acid is to produce charring (i.e. 

 the mixture becomes black), and the reaction is complete when- 

 ever all this liberated carbon has been oxidised and the mixture 

 has become colourless. This latter transformation can be con- 

 siderably hastened by adding a few crystals of potassium permanga- 

 nate which oxidise the carbon. (For other methods see Advanced 

 Course.) 



2nd Stage Distillation. The contents of the flask are allowed to cool, 

 and are then dissolved in distilled water, the resulting solution being 

 carefully transferred to a large Erlenmeyer's flask (a) i.e. a conical 

 flask made of combustion glass and holding 600 c.c. To this solution 

 of ammonium sulphate (containing an excess of sulphuric acid) a few 



1 This flask should have a capacity of 300 c.c. ; it is made of hardened glass, has 

 a long neck, and a round bottom. 



