298 THE ANIMALS AND MAN 



the mixture or allow it to remain over night in a warm room. If grape 

 sugar is present in any quantity, the contents of the tube will turn first 

 greenish, then yellow and finally brick red. 



Fats (butter, oils, oil of nuts, etc.). — Like carbohydrates, 

 fats contain carbon, oxygen, and hydrogen but in different 

 proportions. They are insoluble in and lighter than water. 

 They are easily oxidized as we know from the rapidity with 

 which they burn. Like carbohydrates, they are heat and 

 energy producers and proteid savers. 



Tests for Fats. — Rub the substance to be tested upon a sheet of 

 white paper, or heat it in an oven upon paper. If oil is present, it will 

 show as a grease spot. 



Water. — Water undergoes no chemical change in the body. 

 Its importance as a food is due to the fact that it forms a 

 large percentage of the composition of the body, nearly 59% 

 of the total weight. A large amount of water is thrown off 

 daily as waste, through the skin and kidneys. This must 

 be restored daily to keep the body well. Our food provides 

 only about one-third of the water we need and we must 

 drink three or four pints each day to supply the deficiency. 

 • Water promotes digestion, by aidmg in softening and 

 dissolving the food and stimulating contractions of the 

 muscle. A large amount of muscle and a much larger 

 amount of blood is water. 



Inorganic salts (chlorides, sulphates, carbonates, etc.). — 

 Inorganic salts are found in the cells and fluids of the body, 

 and particularly in the bones, of which they form an im- 

 portant part. They are non-oxidizable and hence have no 

 importance as sources of energy. They are supposed to 

 function in controlling the flow of water to and from the 

 tissues. This is by virtue of the principle of osmotic pres- 

 sure. These salts are supplied to the body largely through 

 vegetable food. 



Diffusion and Osmosis. — When one liquid is poured 

 into another the resulting solution will be a blend of the two 



