632 



THE CONSERVATION OF ENERGY 



[CH. XLTT. 



heat is produced, whether the element is free or in a chemical com- 

 pound. The following figures show the approximate number of 

 heat-units produced by the combustion of 1 gramme of the following 

 substances : 



Hydrogen 

 Carbon 

 Urea. 

 Albumin 



34662 

 8100 

 2530 

 5600 



Fat . 

 Cane sugar 

 Starch 



9400 

 3950 

 4160 



It is, however, most important to remember that the " physiologi- 

 cal heat- value " of a food may be different from the " physical heat- 

 value," i.e., the amount of heat produced by combustion in the body 

 may be different from that produced when the same amount of the 

 same food is burnt in a calorimeter. This is the case with the pro- 

 teins, because they do not undergo complete combustion in the body, 

 for each gramme of protein yields a third of a gramme of urea, which 

 has a considerable heat-value of its own. Thus albumin, which, by 

 complete combustion, yields 5600 heat-units, has a physiological 

 heat-value = 5600 minus one-third of the heat-value of urea (2530) 

 = 5600 846 = 4754. Kubner has shown that this figure must be 

 reduced to nearly 4000, as some of the imperfectly burnt products 

 of decomposition of proteins escape as uric acid, creatinine, etc., in 

 the urine, and there is a small quantity of similar substances in the 

 faeces. No difference between the physical and physiological heat- 

 values of fats and carbohydrates exists, provided, of course, that all 

 the fat and carbohydrate in the food is absorbed. 



Having obtained in this way the energy value of the food taken 

 in, expressed as units of heat, th.e next step is to arrive at the heat 

 produced in the animal body. Other manifestations of energy in the 

 body, such as kinetic energy, must also be taken into account, and it 

 is usual to express these also in terms of heat, one calorie being 

 equivalent to 425-5 gramme-metres (see p. 129). 



This is also accomplished by calorimetry. From time to time 

 numerous calorimeters designed for this purpose have been intro- 

 duced, but by far the best is the Atwater-Benedict instrument, and 

 its special value consists in the circumstance that it can be used for 

 making observations on human beings. The method employed will 

 be seen to be based precisely on the same principles as those of the 

 bomb calorimeter. The apparatus is represented diagrammatically 

 in the accompanying drawing (fig. 394). 



The Atwater-Benedict Calorimeter consists of a room with non- 

 ducting walls. Through this run coils of water-pipes, fitted with 

 metal ^ discs. Only one of these tubes is shown in the figure (A). 

 Any rise of the temperature of the room is at once taken up by the 

 discs and communicated to the water. The whole of the heat 

 production of the individual in the calorimeter is therefore spent 



