AT OUR HOMES 227 



given back into the air. It is in this way that the large 

 amounts of oxygen gas withdrawn from the air by respiration, 

 by decay of vegetation, and by the combustion of wood 

 and coal, are returned to it, and the per cent of oxygen in the 

 air maintained practically unchanged. 



Neither botanists nor chemists undertake to say definitely 

 just how the changes occur in the chloroplasts, nor the exact 

 nature of the chemical products at successive stages of these 

 changes. It is, however, an aid to an understanding of 

 the chemical relationship between the products of chloro- 

 phyll activity by assuming that it can be expressed in this 

 way: 



CO 2 + H 2 O = O 2 + CH 2 O 



The CH 2 may be considered a compound of carbon and 

 water, and is type of the compounds known as carbo- 

 hydrates, of which starch, sugar, and cellulose are familiar* 

 examples. 



Assuming this change to have taken place, the chemical 

 relationship of these carbohydrates becomes more apparent 

 by a comparison of their chemical formulae, rewritten with 

 reference to the CH 2 O group of atoms : 



Starch ................. (CeH^)* 1 = (6CH 2 O - H,O} 



Grape sugar ............ C 6 H :2 O 6 = 6CH 2 O 



Cane sugar ............. Ci 2 H 22 Oii = i2CH 2 O - H 2 O 



Cellulose ............... (CeHioOs)/ = (6CH 2 O - 



These hypotheses conform quite closely to what is known 

 about these substances. 



1 The subscripts x and y are some unknown but definite numbers of these 

 groups. By conceiving the molecules of starch and of cellulose to be made up 

 of different numbers of groups, and these groups arranged in different com- 

 binations (even as the same kind of bricks may be built together to form 

 entirely different structures), it is possible to account for such different sub- 

 stances as starch and cellulose even when made up of the same chemical 

 elements united in the same proportions by weight. 



