Chap, hi.] THE TEAtfSFORMATION OF FORCES IN BIOLOGY. 463 



The figures obtained by the process we have just described are 

 evidently only approximative. The oxygen exhaled by the lungs 

 under the form of carbonic acid, even when the quantity of oxygen 

 employed in the formation of water is added, according to the 

 observations of M. Regnault, does not represent all the oxygen 

 absorbed. The cutaneous surface also respires and exhales car- 

 bonic acid. Finally, a very important part of oxygen serves to form, 

 at the expense of the albuminoidal substances, on the one hand, 

 the immediate azotised, regressive crystallisable principles, such 

 as urea ; and, on the other hand, sulphates, phosphates, &xs. ; all 

 bodies eliminated by the kidneys, perspiration, (fee. 



To approach the truth more nearly, we must add to this direct 

 study of pulmonary exhalation the comparative study of the 

 aliments and of the excrementitious matters ; this is what M. 

 Boussingault has called the indirect method. 



We can, for example, ascertain the comparative quantities of 

 the ingesta and excreta of a bird inclosed in a receiver, by placing 

 the receiver in a calorimeter. We thus prove that the heat 

 developed is notably greater than that corresponding to the 

 combustion of the carbon and of the hydrogen. The excess is 

 due, without doubt, to the oxydation of the albuminoids, the 

 true combustible power of which is still imperfectly understood. 



Nevertheless, Frankland has given a table of the calorific and 

 mechanical energy developed by the combustion of diverse ali- 

 mentary substances. According to these statements, fat alimen- 

 tary matters furnish more disposable heat and force than 

 saccharine and amylaceous matters, and the latter produce more 

 than food composed of beef, veal, pork, or fish. 



According to Frankland, 1 kilogramme of dried muscular 

 substance, purified ia ether, and transformed into urea by com- 

 bustion, develops 4368 units of heat.i 



There are then no respiratory aliments exclusively destined to 

 be burned, thus furnishing heat, and plastic aliments exclusively 

 destined to be assimilated and to furnish useful labour, as Liebig 

 1 Eevue de Caurs Scieniifiques, 1866-1867, p. 81. 



