COMBUSTION OF FAT. 523 



tain."* Thus the oxygen taken into the lungs in respiration 

 has not only to convert carbon into carbonic acid, but also to 

 oxidise such elements as sulphur and phosphorus, which appear 

 in the urine in the state of sulphates and phosphates ; whence 

 is explained, in part, why there is uniformly more oxygen con- 

 sumed in respiration than is contained in the amount of car- 

 bonic acid thrown off in that function. It may be asked 

 whether the hydrogen contained in the proximate constituent 

 principles of the solids and of the blood does not also require 

 oxygen when these proximate principles are decomposed. It 

 is often said at present that part of the oxygen taken in during 

 respiration must form water with the hydrogen of the sub- 

 stances decomposed during the actions of the body. In so far 

 as any of these are pure hydrocarburets, or components merely 

 of carbon and hydrogen, this must be the case ; but there are, 

 strictly speaking, no hydrocarburets in the living body. Even 

 fat itself contains about 5 per cent of oxygen to 46 per cent of 

 hydrogen, and nearly 50 per cent of carbon. If, then, as is 

 taught, fat, sugar, and starch, or something equivalent to 

 starch, are the proximate elements in venous blood, which, 

 principally by slow combustion with the oxygen received in 

 respiration, maintain animal heat, the process must be as fol- 

 lows : the oxygen takes the carbon from the starch, or its 

 equivalent, and from the sugar, and in both cases there remains 

 water the hydrogen and oxygen left being just sufficient for 

 that purpose ; but in the case of fat, when the oxygen of re- 

 spiration takes its carbon, the oxygen left is not sufficient to 

 convert the hydrogen left into water, so that an additional por- 

 tion of the oxygen received during respiration is required to 

 convert the whole residue into water. 



When, however, we compare the chemical constitution of the 

 solids, such as the muscular tissue, with the chemical constitu- 



* Miller, ' Elements of Chemistiy,' vol. ii. p. 812. 



