114 SCIENCE PRIMERS. [§ vil. 



wraps round the lean muscular flesh. Now, proteids 

 are, when cooked, insoluble in water (see p. 49) ; and 

 fat, you know, will not mingle with water. Both these 

 parts of meat, both these food-stuffs, must be acted 

 upon before they can pass fr-^m the inside of the 

 alimentary canal, through the epithelium of the mucous 

 membrane, into the blood capillaries. > . ^ 



Besides meat we eat bread. Bread is chiefly com- 

 posed of starch ; but besides starch we find in it a 

 substance containing nitrogen, exceedingly like the 

 proteid matter of muscle or of blood. 



Potatoes contain a very great deal of starch with a 

 very small quantity of proteid matter ; and nearly all 

 the vegetables we eat contain starch, with more or less 

 proteid matter. 



Then we generally eat more or less sugar, either as 

 such or in the form of sweet fruits. We also take salt 

 with our meals, and in almost everything we eat, 

 animal or vegetable, meat, bread, potatoes or fruit, we 

 swallow a quantity of mineral substances, that is, 

 various kinds of salts, such as potash, lim6, magnesia, 

 iron, with sulphuric, hydrochloric, phosphoric, and other 

 acids. 



In everything on which we live we find 

 one or more of the following food-stuffs : — 

 Proteid matter, starch or sugar, and fat, 

 together with certain minerals and water. 

 It is on these we live: any article which 

 contains either proteid matter, or starch, or 

 fat, is useful for food. Any article which 

 contains none of them is useless for food, 

 unless it be for the sake of the minerals or 

 water it holds. 



