MATERIALS WHICH PLANTS DERIVE FROM THE AIR. 33 



carbonic acid is expelled, and the water being incapable of 

 retaining the lime in solution, deposits it as a crust on the 

 sides of the vessel, and becomes *' soft." You mil find that 

 an acquaintance with the properties of this gas will assist us 

 in explaining many interesting matters connected with the 

 soil. 



28. The atmosphere is the never-failing reservoir from which 

 innumerable tribes of plants receive the carbon necessary for 

 their support. From the same source, also, the countless 

 tons of carbon which the ancient forests required for their 

 growth were derived. These, by a wise provision of nature, 

 now supply us in our beds of coal with valuable deposits of 

 fuel, which, consumed in our fires, unite once more with the 

 oxygen of the air, and thus, after a rest of many thousand 

 years, the carbon again takes its place in the atmosphere, to 

 serve as food for plants, to cover the suiface of the earth 

 with shady forests and waving grain, to give strength to 

 the tree and perfume to the flower, and to produce food for 

 the support of man and animals. How Avell calculated is 

 such information as that which I am now endeavouring to 

 communicate to excite our desire for knowledge — to enlarge 

 our ideas of that wisdom by which such aiTangemonts have 

 been planned! 



29. Besides the gases which I have described as compos- 

 ing the atmosphere, there are also diffused through it minute 

 quantities of various gaseous compounds produced in the in- 

 immerable operations going on everywhere around us; but 

 these form so trifling an amount of its vast volume, and so 

 little affect its general qualities, that it is not necessary for 

 our purpose to notice them. 



