THE BALANCE OF NUTRITION 223 



By means of the water calorimeter, it has been. determined 

 that the conversion of one gram of ice at o C. into liquid water 

 at the same temperature requires 79.24 gram calories of heat. 

 By the use of this factor the indications of the ice calorimeter 

 can be converted into calories. 



310. Heats of Combustion. As related to nutrition in- 

 vestigations, the chemical energy of an organic substance is 

 most commonly measured by converting it into heat by complete 

 combustion with oxygen and measuring the heat by one of the 

 methods just indicated, the result being called the heat of com- 

 bustion of the substance. In the method most commonly used 

 in nutrition investigations, the substance is burned in highly 

 compressed oxygen (about 25 atmospheres) in a lined steel 

 bomb, the heat being taken up by water. The method was 

 first devised by Berthelot and subsequently modified by Mahler, 

 Hempel and Atwater. One form of this calorimeter is shown 

 in section in Fig. 32. 



311. Heats of combustion do not measure total energy. 

 It should be clearly understood that the heat of combustion of 

 an organic compound does not, as is sometimes erroneously 

 stated, measure its total energy but simply the amount mani- 

 fested in a particular chemical change. Thus, in the complete 

 oxidation of one gram of starch to gaseous carbon dioxid and 

 liquid water 4183 gram calories of energy are transformed into 

 heat. How much additional energy is still contained in the 

 resulting carbon dioxid and water we do not know, nor is it 

 necessary that we should. In using the heat of combustion as 

 a measure of the chemical energy of starch the possible energy 

 content of the carbon dioxid and water is simply assumed as 

 an arbitrary zero, much as the engineer may assume a datum 

 plane for his levels without regard to its height above sea level. 

 In other words, the heat of combustion of starch or of any other 

 substance shows how much chemical energy can be secured 

 from it for conversion into other forms by processes of oxidation 

 such as occur in the body. 



312. Law of initial and final states. In view of the very 

 complicated nature of the metabolic processes, the question 

 naturally arises whether the amount of chemical energy which 

 a feed ingredient such as starch really puts at the disposal of 

 the organism is the same as the amount of chemical energy which 



