PURIFICATION OF THE AIR. 375 



scale, however, vegetation is one of the indis- 

 pensable provisions for the continuance of 

 animal life on earth ; and we find in the atmo- 

 sphere the link which connects these kingdoms 

 in a more intimate bond of mutual dependence 

 than might at first sight appear. 



There appears to be a beautiful provision, 

 even in the succession of day and night, by 

 which the temporary loss caused by the cessa- 

 tion of plants from their labours in our hemi- 

 sphere is counterbalanced. For while in this 

 hemisphere, at night, all nature is asleep, both 

 animal and vegetable; in the other the sun 

 shines, and the wide domain of plants pour 

 out their daily streams of oxygen into the air. 

 Thus, the purifying influence of vegetation 

 upon the entire mass of air is never arrested 

 all over the world, the time of rest in one region 

 being that of activity in another. It has been 

 suggested by Liebig, that those vasts movements 

 of air,- the Trade Winds, are also instrumental 

 in preserving the general balance of purity in 

 the entire atmosphere, by conveying from the 

 tropics to the poles the pure air, rich in 

 oxygen, ascending from the profuse vege- 

 tation of these burning regions, to the frozen 

 north, where vegetation has scarcely a repre- 

 sentative. Mingling with the ascending flow of 

 warm air, this pure and highly oxygenated 



