THE WOOD. 63 



when a part mixes with the blood, which it ren- 

 ders fit for circulation, and the remainder uniting 

 with carbon, a substance which we swallow in 

 most kinds of food, becomes carbonic acid gas, a 

 kind of air which is destructive of animal life.* 

 Carbonic acid gas, then, which is composed of two 

 ingredients, oxygen gas and carbon, is as ne- 

 cessary to support the life of plants, as oxygen is 

 of animals. It is received through the pores into 

 the leaves of plants, where it is separated into its 

 two constituent parts; the carbon is retained in 

 the leaf, and is deposited in the form of wood; 

 the oxygen is given out, and, mixing with the air, 

 purifies and fits it for the respiration of animals. 

 In this wonderful way do the animal and vegetable 

 worlds prepare sustenance for each other. Man and 

 the inferior animals never cease exhaling a gas which 

 goes to form the substance of vegetables ; and the 

 latter, while they are in an active state of existence, 

 that is, while there is light, are constantly emit- 

 ting a gas, of which if a due supply were to fail, 

 the whole animal world would cease to exist. 



Pick up one of the withered leaves, many 

 kinds of which lie at your feet; it is to all ap- 

 pearance dead ; it has fulfilled the office for 

 which it was created, and is now utterly useless : 



* Carbon enters largely into the composition of most in- 

 flammable substances. Charcoal is pure carbon ; coal is 

 carbon mixed with sulphur and another gas called hydrogen. 

 When carbon mixes with oxygen chemically, heat is the 

 result ; sometimes this heat is intense, when combustion takes 

 place, as when a fire is lighted ; at other times, heat is evolved 

 in a less degree, as when we breathe, and, in the latter case, 

 animal heat is produced. The heat observed in the Arum 

 (see page 44) and some other plants, proceeds probably from 

 a combination of oxygen and carbon. 



