842 CALORIMETERS. [BOOK n. 



and for supplying the animal with air for breathing purposes. With 

 the air-jacket, or space between the inner and middle cylinders, are 

 connected a manometer and a thermometer. When an animal (or 

 other source of heat) is placed in the inner cylinder, the temperature 

 and the pressure of the air in the air-jacket are increased ; and from 

 the amounts of increase measured by the thermometer and the mano- 

 meter the amount of heat given out from the animal is calculated. 



The calorimeters of D'Arsonval and Rubner are constructed on 

 very similar principles. 



Various attempts have been made to ascertain the amount of 

 heat given out by the body in an indirect manner, as for instance 

 by calculating the heat given out by the oxidation of the food. 

 As trustworthy as any is the plan of simply subtracting the 

 normal daily mechanical expenditure from the normal daily 

 income. Thus, 150,000 k.-m. subtracted from one million k.-m. 

 gives 850,000 k.-m. as the daily expenditure in the form of heat ; 

 i. e. between one-fifth and one-sixth of the total income is expended 

 as mechanical labour, the remaining four-fifths or five-sixths leaving 

 the body in the form of heat. If we take the higher estimate of a 

 million and a quarter k.-m., and suppose the work not to be in- 

 creased, the proportion in the form of heat will of course be 

 greater. 



529. The Energy of Mechanical Work. We have already 

 in treating of muscle and elsewhere partly discussed this subject, 

 but may here say the rest that has to be said. 



The older writers, even after it had been proved that the 

 animal body was constructive so far as the formation of fat was 

 concerned, still held to the distinction between nitrogenous or 

 plastic and non-nitrogenous or respiratory food. Put broadly, this 

 view was that all the nitrogenous food went to build up the 

 proteid tissues, the muscular flesh and the like,- and that the 

 nitrogenous egesta arose solely from the functional metabolism of 

 these tissues, while the non-nitrogenous food was used with equal 

 exclusiveness for respiratory or calorific purposes, being either 

 directly oxidized in the blood or, if present in excess, stored up as 

 fatty tissue. According to this view the two classes of income 

 corresponded exactly to the two forms of expenditure. We have 

 already urged several objections against this view. We have seen 

 that in the blood itself very little oxidation takes place, that it is 

 the active tissue, and not the passive blood-plasma, which is the 

 seat of oxidation. We have further seen that proteid food may 

 undoubtedly be, in the above sense, respiratory and incidentally 

 give rise to the storing-up of fat. One division of the view is 

 thereby overthrown. We have now to inquire whether the other 

 division holds good, whether muscle and the other proteid tissues 

 are fed exclusively on the proteid material of food, and whether 

 muscular energy comes exclusively from the metabolism of the 

 proteid constituents of muscle. We have already seen ( 63) 



