EXCHANGE OF MATERIAL. 441 



of the proteids and the sulphuric acid, H 2 S0 produced by their 

 burning, is in the ratio 5.2:1, or about the same as in the urine 

 (see page 382). The determination of the quantity of sulphuric 

 acid eliminated with the urine gives us an important means of con- 

 trolling the extent of the transformation of proteids, and such a 

 control is especially important in cases in which we wish to study 

 the action of certain nitrogenized non-albuminous bodies on the 

 metabolism of proteids. A determination of the nitrogen alone is 

 not in such cases sufficient. 



If it is found, on comparing the nitrogen of the food with that of 

 the urine and faeces, that there is an excess of the first, this means 

 that the body has increased its stock of nitrogenized substances 

 proteids. If, on the contrary, the urine and faeces contain more 

 nitrogen than the food taken at the same time, this denotes that 

 the body is giving up part of its nitrogen that is, a part of its own 

 proteids. We can from the quantity of nitrogen, as above stated, 

 calculate the corresponding quantity of proteids by multiplying by 

 6.25. Usually, according to VOIT'S proposition, the nitrogen of 

 the urine is not calculated as decomposed proteids, but as decom- 

 posed muscle-substance. Flesh contains on an average about 3.4$ 

 nitrogen, and each gramme of nitrogen of the urine corresponds in 

 round numbers to about 30 grms. flesh. 



A disproportionally large part of the carbon leaves the body as 

 carbon dioxide, which escapes chiefly through the lungs and skin. 

 The remainder of the carbon is eliminated under the form of 

 organic combinations by the urine and faeces, in which the quantity 

 of carbon can be determined by elementary analysis. The amount 

 of gaseous carbon dioxide eliminated is determined by means of 

 PETTENKOFER'S respiration apparatus, which is described in special 

 works. By multiplying the quantity of carbon dioxide found by 

 0.273 we obtain the quantity of carbon eliminated as C0 2 . If we 

 compare the total quantity of carbon. eliminated in various ways 

 with the carbon contained in the food, we obtain some idea as to 

 the transformation of the carbon compounds. If the quantity of 

 carbon in the food is greater than in the excretion, then the excess 

 is deposited; while if the reverse be the case, it shows a correspond- 

 ing loss of bodily substance. 



The nature of the substances here deposited or lost, whether 



