ASSIMILATION 53 



gen. but there is more than enough hydrogen to form 

 water. The excess of hydrogen is responsible for the 

 name hydrocarbon. 



Proteids. — Proteids are practically identical with 

 dead protoplasm. We eat them whenever we eat the 

 tissues of plants or animals. In animals there is a 

 larger proportion of proteids and fats and a smaller pro- 

 portion of starches than in plants. Chemical analysis 

 shows that proteids are composed principally of carbon, 

 hydrogen, oxygen and nitrogen. In addition they may 

 contain traces of sulphur and phosphorus. They are 

 especially important because of the nitrogen that they 

 contain. Nitrogen is absolutely necessary to the manu- 

 facture of protoplasm, and as animals can not use free 

 nitrogen from the air they can obtain it only from the 

 proteids that they eat. 



Necessary Proportions of Food Substances. — With- 

 out proteids the body would starve very quickly, but it 

 could subsist for a considerable period without starch or 

 fats. These substances are especially useful because 

 they are completely oxidized in the body and conse- 

 quently set free an enormous amount of heat. In gen- 

 eral, the amount and character of the food required by 

 man depends on the amount of work that he does. We 

 find that although the diet differs in different countries, 

 men who do the same amount of hard work eat approxi- 

 mately the same quantity of food made up approximately 

 of the same proportions of carbohydrates, hydrocarbons 

 and proteids. 



Nutriment in Food Stuffs. — Cereals furnish more 

 nutriment in proportion to weight and cost than other 



