CALORIMETRY 149 



complete as that which occurs without. In the case of 

 carbohydrates and fats there is no doubt that this is 

 so, for these substances, provided that they are really 

 being metabohsed and not stored, are completely oxidised 

 in the body to carbonic acid and water. But in the case 

 of proteins, the excretory products, urea, uric acid, etc., 

 are not completely oxidised. The whole of the available 

 energy of the proteins is not used. For this reason the 

 energy hberated in the body by proteins will fall shghtly 

 short of the energy hberated by the same proteins in vitro. 

 The energy available by complete oxidation of a sub- 

 stance is determined by means of the Bomb Calorimeter, 

 which consists of a steel case containing a known amount 

 of the substance in an atmosphere of oxygen. This is 

 immersed in a known volume of water. Combustion is 

 effected electrically, and when completed the amount of 

 heat evolved is measured. With this apparatus the 

 following values have been determined— 



1 gm. carbohydrate on combustion gives off 4-1 C. 

 Igm. fat „ „ „ 9-3 C. 



1 gm. protem „ „ „ 5-0 C. 



We now have all the data for constructing the energy 

 balance. To take an example — 



Heat given out by the subject . . 4833 C. 



Work done, calculated as heat . . 602 C. 



Total energy hberated, calculated as heat 5435 C. 



The total energy obtainable from the amount of food 

 absorbed during that period, less the energy present in 

 the excreta, v/as 5459 C. The two figures thus agree to 

 within 0-5 per cent. 



Indirect Calorimetry 



Since the energy hberated on oxidation within the body 

 is practically identical with that hberated on oxidation 

 outside the body, it follows that if we know the amount 



