GENERAL METABOLISM AND DIETETICS 327 



Calorie), or gramme-calorie, is employed; while for the finest 

 work the micro calorie, or milligramme-calorie, is used. 



In order to measure the amount of heat retained or given out by 

 a man, it is necessary to know the specific heat of his tissues. This 

 has been determined as 0-8. A hibernating dormouse weighing 

 10 grammes, and with a body temperature of, say, 4 C., when placed 

 in 100 c.c. of water at C. until cooled to 3 C., does not yield 10, 

 but only 8, calories to the water. Consequently, if a 70 kgm. man 

 has his temperature raised 1 C., he stores up 70x0-8=56 calories 

 of heat. 



The Intake of Energy. The combustion of each of the foodstuffs 

 liberates a definite amount of heat, be it burnt inside or outside the 

 animal body. Exact measurements of the heat of combustion of the 

 different constituents of the body are obtained by means of an instru- 

 ment known as the bomb calorimeter. The substance to be burnt is 

 dried, weighed and placed in a strong steel bomb, the inside wall of 

 which is protected either by thick enamel or platinum. The bomb 

 is then filled with oxygen under pressure (20 to 25 atmospheres), and 

 placed in a vessel (Fig. 185) containing a weighed quantity of water. 

 This water vessel contains a delicate thermometer and a stirrer, D, 

 which can be driven by a motor. It is also carefully protected by 

 a jacket (A) a vacuum jacket is best from alterations in tem- 

 perature of the surrounding atmosphere. The temperature of the 

 water is carefully noted, and then the substance is burnt in the 

 oxygen by causing a wire with which it is in contact to glow. This 

 is effected by passing an electric current through wires led into the 

 bomb. Combustion is rapidly completed ; the temperature of the 

 water is raised thereby, and the rise noted. From this by means of 

 appropriate calculations allowing for the caloric capacity of the 

 apparatus, etc., the heat liberated by the combusted substance is 

 ascertained. 



In the calorimeter all the carbon combined in the molecule of 

 foodstuff is converted to carbon dioxide, the hydrogen to water, and 

 the nitrogen is freed as nitrogen gas. In the body the carbon mostly 

 is excreted as C0 2 , and the hydrogen as water; but the whole of the 

 nitrogen and a small part of the carbon and hydrogen are excreted 

 combined in urea, uric acid, creatinin, etc. These bodies have a 

 considerable specific heat of combustion; therefore, in estimating the 

 heat value of the protein in the diet, it is necessary to subtract the 

 heat value of the nitrogenous excreta from the total value, obtained 

 on burning the protein in the bomb calorimeter. 



The table on p. 328 gives the combustion value of some of the 

 chief substances met with in the body, expressed in several 

 terms. 



The Indirect Method. The food given is carefully weighed, and its 

 content in protein, fat, and carbohydrate, calculated from tables 

 giving the analytical composition of the various foodstuffs. 



In more accurate work the amount of protein in the diet may be 

 estimated by determining, when the diet contains no other nitro- 



