72 LESSONS IN CHEMISTRY. 



inhale, would soon prove fatal to animal life. We know also, 

 that by the combustion of the fuel in our cities, and during 

 the decay of animal and vegetable matters, enormous quan- 

 tities of carbonic acid are produced at the expense of the 

 life-supporting oxygen. Yet the atmosphere maintains its 

 composition unchanged (26), and analysis has proved that the 

 wonderful mixture of gases that we draw into our lungs does 

 not contain less oxygen or more carbonic acid than that 

 which was inhaled by our ancestors many thousand years 

 ago. 



106. The experiments above described will at once suggest 

 to you the means by which nature prevents an undue accu- 

 mulation of carbonic acid in the atmosphere. By a beautiful 

 arrangement the existence of the vegetable tribes has been 

 made to depend upon the decomposition of that gas, noxious 

 to man hut essential to their growth, and every green leaf 

 which plants hang out into the au' is provided with an appa- 

 ratus by which it is decomposed, and an equal bulk (103) 

 of pure oxygen liberated to serve for the respu-atioa of 

 animals. 



107. The diffused light of day is sufficient to enable plants 

 to decompose carbonic acid and appropriate the carbon which 

 it contains, but in the bright sunshine their growth proceeds 

 with greater vigour, and the more plants are exposed to the 

 sun the richer are they found to become in all those com- 

 pounds in which that element abounds. It is only in the 

 presence of the light that the leaf acquires its beautiful green 

 colour, or fonns woody fibre (61). In the deep mine and 

 dark cellar, plants grow, but exhibit a sickly blanched appear- 

 ance, and produce compounds of a different kind from those 

 which we discover in them in their healthy condition.* 



From what has been stated respecting the composition of 

 starch, cellular fibre, and the other compounds of which the 

 bulk of our crops consists, you can readily understand how by 

 the decomposition of the carbonic acid and water which the 

 sap contains, aU the materials which their formation requires 

 may be procured by the growing plant. The changes which 

 the curious principle diastase is capable of effecting in the 

 hands of the chemist, enables us to conceive how in one part 



* Thus, in the blanched shoots of the potato a poisonous principle 

 called eolanin, is discovered which, when the plant becomes green by 

 exposure to the light, disappears. 



