44 STUDIES IN PHYSIOLOGY 



mutton, and beans nearly 20% is seen to be proteids, while 

 butter and potatoes contain less than 2%. The percentage 

 of fat is high in butter (about 80 %) and in many other 

 animal foods ; it is hardly present at all, however, in the 

 foods derived from plants. On the other hand the carbohy- 

 drates (starches and sugars) are usually wanting altogether 

 in animal foods, but they constitute a large percentage of 

 the foods of vegetable origin. 



2. TESTS FOR THE NUTRIENTS 



With the help of a few chemicals and simple apparatus 

 it is easy to determine the presence or absence of each kind 

 of nutrient in a given food. These tests are as follows : 



Tests for Proteids. 1 (1) Many proteid substances, like 

 white of egg and lean meat, when heated, are coagulated or 

 hardened into a solid mass. (2) If the temperature is 

 raised still higher and these foods are scorched, a peculiarly 

 unpleasant odor -is noticed, which may be compared to that 

 of burning feathers. (3) One of the best methods of de- 

 monstrating the presence of proteids is by the use of nitric 

 acid and ammonia. Some of the food to be tested is placed 

 in a test tube, concentrated nitric acid is added, and the 

 mixture is warmed. If the food changes to a yellow color, 

 we may be sure of the presence of proteids. After wash- 

 ing the egg with water and adding concentrated ammonia, 

 we find that the yellow color changes to a deep orange. 



Tests for Fats. 2 (1) A simple method of testing a given 

 food for fats is by heating a small quantity, and then 

 placing it on a piece of paper. If fat is present, it will 

 make a translucent grease spot on the paper. (2) Ether or 

 benzine, 3 when poured upon foods, dissolves the fats, and 



1 See " Laboratory Exercises," No. 11. 



2 See " Laboratory Exercises," No. 12. 



8 Caution ! Ether or benzine must never be used near a flame or a 

 hot stove, since the vapor of these substances is very inflammable. 



