ELEMENTS OF THE BODY 29 



completely changed, and so forms an example of chemical 

 action. Some substances are very prone to mix to form 

 solutions. Thus, impure salt has such an attraction for 

 water that it takes it from the air and becomes damp. So 

 salt is said to have an affinity for water. In the same 

 way some substances are very prone to mix so as to be- 

 come changed by chemical action. Thus, there is always 

 chemical action between soda and vinegar when they are 

 brought together, so soda is said to have a chemical 

 affinity for vinegar. In the same way, air has a great 

 " chemical affinity " for wood in a fire. The chief value of 

 gold comes from the fact that it has no chemical affinity at 

 all except for one or two uncommon substances. So it 

 will remain unaltered in the midst of substances which 

 would destroy other metals. 



35. Use of alkalies. If a fluid contains an acid, it is 

 said to be acid'm reaction ; if an alkali it is alkaline ; and if 

 it contains neither it is neutral in reaction. Now, the blood 

 is always alkaline from the presence of a small amount of 

 soda and potash. Acid products are being formed in the 

 body continually, and the duty of the alkalies is to unite 

 with them at once and change them to harmless sub- 

 stances, which may be handled by the blood in safety. 

 The alkalies are found in nearly all foods. 



36. Chemical actions in the body. Everything which makes 

 up the cells and fluids of the body is composed of some or all the sub- 

 stances water, albumin, fat, sugar, or starch, with the minerals 

 salt, lime, and soda and potash. These must be eaten to sustain life, 

 and so they are foods. Other kinds of substances are harmful or 

 poisonous. All food substances are eaten three times a day, and yet 

 only water and the minerals leave the body in anything like the form 

 in which they entered. The rest are entirely changed by chemical 

 actions and leave the body as gases or liquids or as solids dissolved in 

 water. The digestion of food from the time it is taken into the mouth 



