CLASSES OF SUBSTANCES 189 



219. Carbohydrates. A very important group of com- 

 pounds can be made from the elements carbon, hydrogen, 

 and oxygen. They are made in nature, chiefly by the 

 activity of plants. Because of their composition (G and 

 H 2 O) they are called carbohydrates. Starch, sugars, and 

 cellulose are common carbohydrates ; they occur in seeds, 

 all parts of living plants, and fruits. The carbohydrates 

 form a very important part of the food of most animals. 



220. Proteids. Another group of substances that 

 are necessary to the life of higher animals is called 

 proteids. These contain the elements carbon, hydrogen, 

 oxygen, and nitrogen; sometimes sulphur or phosphorus 

 also. Proteids occur in the white of eggs, in lean meat, 

 cheese, wheat flour (gluten), gelatin, etc. 



221. Solutions. When a substance is dissolved in a 

 liquid it is said to be in solution. The liquid in which 

 a substance is dissolved is called a solvent. Certain 

 solids, liquids, or gases may be thus put in solution; 

 their molecules are separated and they mix with those 

 of the solvent. This mixture of a substance in a solvent 

 is called a solution. There is, of course, a limit to the 

 amount of any given substance that a liquid can dis- 

 solve. When the solvent holds in solution all that it 

 can dissolve of any substance, the solution is said to be 

 saturated. In the case of solids, dissolving is hastened 

 if the solvent be heated. It is well known that some 

 substances dissolve better in hot water than in cold. 

 Stirring or shaking assists solution by mixing the parti- 

 cles more rapidly. A substance that can be dissolved 

 in a liquid is said to be soluble. 



