Il6 AGRICULTURAL CHEMISTRY 



hard wood, but a pound of the different kinds of wood 

 gives nearly the same amount of heat. The amount of 

 heat which a material produces when burned is measured 

 in the calorimeter, and is given in terms of calories or 

 heat units. The presence of water in fuels lowers their 

 caloric value, because it requires a definite amount of 

 heat to evaporate and expel as steam the moisture before 

 combustion can take place. 



143. Caloric Value of Fuels (Comparison made on 

 basis of equivalent weights). 



Calories. Calories. 



Perfectly With Per cent, of 



dry. water. water. 



Hard coal 9160 8690 2.63 



Soft coal 7664 7128 5.02 



Pine (red) 5997 5155 13.83 



Cedar 5974 5031 14.09 



Maple 5978 5117 12.49 



Birch 5978 4758 21.70 



144. Foods. The materials used as human and animal 

 foods are mechanical mixtures of various organic com- 

 pounds, as starch, sugar, fat, albumin, etc., together 

 with various mineral salts. The composition of the or- 

 ganic compounds of foods forms a part of the study of 

 organic chemistry, while their economic value and the uses 

 made of them by the body, are studied in physiological 

 chemistry. Knowledge in regard to the composition and 

 uses of foods, particularly of human foods, is somewhat 

 limited, although along this line, many facts and laws of 

 economic and sanitary importance have been discovered. 

 The subject of foods is treated more fully in the chap- 

 ters relating to the chemistry of foods. 



