THE PRODUCTION AND DISSIPATION OF HEAT 1103 



The Total Quantity of Heat.— The total quantity of heat liberated 

 by an animal is ascertained (a) by determining the heat values of the 

 different foodstuffs ingested by the method of direct oxidation, and 

 (6) by measuring the heat evolved by it with the help of the water or 

 air-calorimeter. But whether reduced into its constituents in a 

 bomb-calorimeter or more slowly burned in the body, the food yields 

 the same amount of heat, provided it is fully consumed and is not 

 allowed to discharge its energy as work. A plant exposed to sunlight, 

 combines carbon dioxid and water into sugar, while oxygen is given 

 off and heat is absorbed. Now, if 1 grm. of sugar is placed in a 

 steel receptacle (Berthelot) into which oxygen is passed under a pres- 

 sure of 450 lbs. to the square inch, the combustion of this substance 

 may be incited by an electric spark. ^ It will then be found to have 

 yielded carbon dioxid and water and an amount of heat equal to that 

 absorbed. The latter is determined by immersing the steel receptacle 

 in a liter of water. On determining the temperature of this water by 

 means of a thermometer, it will be noted that it has risen 3.755° C. 

 during this combustion. It may then be said that 1 grm. of sugar 

 furnishes 3.755 calories of heat, because 1 calorie is the quantity of 

 heat required to raise 1 kilogram (1 liter) of water 1° C. This method 

 of absolute reduction in the bomb-calorimeter has also been applied to 

 a large number of other food substances with the following results: 



Animal fat 9 . 500 calories Casein 5 . 867 calories 



Butter 9.231 calories Egg albumin 5.735 calories 



Olive oil 9.489 calories Beef 5.640 calories 



Glycerin 4.317 calories Veal 5.662 calories 



Elastin 5 . 961 calories Albumins 5 .711 calories 



The different carbohydrates have yielded the following heat values : 



Dextrose 3.742 Maltose > 3.949 



Levulose 3.755 Starch 4. 182 



Galactose 3.721 Dextrin 4. 112 



Cane sugar 3.955 Cellulose. 4.185 



Milk sugar 3.951 



From these figures Rubner^ has deduced the following "standard" values: 



1 gram of protein 4.1 calories 



1 gram of carbohydrate 4.1 calories 



1 gram of fat 9.3 calories 



These values, however, are physical values and represent the heat 

 evolved by them when completely oxidized to carbon dioxid and 

 water. In the animal body these substances are not always thoroughly 

 utilized and hence, their nutritive value may not correspond precisely 

 with these figures. This is true in particular of the proteins, because 

 in the bomb-calorimeter the nitrogen of these substances is converted 

 into nitric acid, while in the body they are oxidized to urea. Conse- 



^ Schlossmann, Zeitschr. fiir phys. Chemie, xxxvii, 1903, 324. 

 2 Zeitschr. fiir Biol., xlii, 1901, 261; also: Atwater, Am. Jour, of Physiol., x, 

 1904, 30. 



