30 The Living Plant 



CO2), and water, with the formula of the grape sugar they form, 

 thus, — 



In photosynthesis CO2 and H2O form CgHigOe 



carbon dioxide water grape sugar 



Obviously now the proportions of the two former must be in- 

 creased in order to yield the latter, thus, — - 



6 CO2 + 6 H2O are needed to form CgHiaOe 



But a chemical equation must balance exactly on the two sides, 

 and this in the present case can occur only thus, — 



6 CO2 + 6 H2O = C^HioOo + 6 O2 



But such a balance of the equation miplies that, in the making 

 of sugar from carbon dioxide and water, oxygen is set free, and 

 not only so, but in a volume exactly equal to that of the carbon 

 dioxide absorbed. So striking a conclusion based upon purely 

 theoretical evidence demands rigid test through observation or 

 experiment. That a gas of some kind is released from green 

 plants in the light is easily seen in submerged water plants which, 

 if kept in an aquarium, give off tiny bubbles when lighted, though 

 not in the dark; and everybody has seen those large gas bubbles 

 which are caught in the felted green scum-plants floating on ponds. 

 Analysis shows that the bubbles, in both cases, consist mainly of 

 oxygen. But the matter can be tested much better by experiments. 

 In a word, it is only necessary to place a green plant or a leaf in 

 a suitable tight glass chamber, give it a known quantity of carbon 

 dioxide (it has plenty of water), expose it for some time to the 

 light, and then make a chemical analysis of the air in the chamber. 

 The experiment yields an invariable result. A certain amount 

 of the carbon dioxide has disappeared, and in its place there is 

 present an exactly equivalent amount of pure oxygen. As to the 

 significance thereof, it seems plain that the oxygen is a by- 

 product formed incidentally in the chemical transformations, and 

 useless in the main process. 



