THE FOLIAGE 45 



treme rapidity with which the absorption of carbonic acid from 

 the air, and the retention of its carbon, actually take place. 



Ordinary atmospheric air consists substantially of two 

 different kinds of gases, one nitrogen and the other oxygen. 

 Carbonic acid gas is a compound of the solid substance carbon 

 with the gaseous substance oxygen. Carbonic acid is present 

 in the atmosphere, but the proportion is but small. If one were 

 to build up a column out of two thousand five hundred penny- 

 pieces it would be about 15 feet in height. If one of the pennies 

 were removed from the column that would represent the pro- 

 portion of carbonic acid in the atmosphere. Overlying an acre 

 of land there are about 20,000 Ibs. of carbon in the form of 

 carbonic acid gas. 



To those who find difficulty in understanding how an in- 

 visible, intangible thing like carbonic acid gas can be converted 

 by the foliage of a tree into solid carbon compounds, the follow- 

 ing explanation may be useful. There are three states of matter: 

 the solid, the liquid and the gaseous. Solids possess volume 

 and shape, liquids possess volume but not shape, while gases 

 possess neither fixed volume nor shape. A little reflection will 

 make all this clear. In the case of a solid object, such as a 

 watch, the size and shape are easily ascertainable. In the case 

 of a liquid, such as half-a-pint of beer, the volume is easily 

 ascertainable, but there is no shape. The half-pint can flow 

 away into a plate as readily as fill a tumbler. Gases, having 

 neither shape nor fixed volume, disperse throughout any 

 vessel, however large it may be. A solid object may pass into 

 a liquid or gaseous state, and gases can be made to assume solid 

 forms. In each form there exist molecules of matter, but in a 

 gaseous form they are less united and more distant from each 

 other than in the case of a liquid, or still more of a solid. A 

 familiar instance of the relations between solids, liquids and 

 gases is as follows : When one volume of oxygen combines with 

 two volumes of hydrogen the result of the combination of the 

 two gases is the liquid water. Given a temperature low 

 enough the liquid water is converted into the solid ice, and the 

 processes can be readily reversed. 



The leaf (including the blade of grass, which is a form of 

 leaf) is the fundamental food-factory of the world, upon which 

 all animal life necessarily depends. The skins of the leaves are 



