CONTRARY EFFECTS OF ANIMAL AND VEGETABLE LIFE. 189 



destroyed. Several years ago, during alterations in Hyde Park, 

 a considerable depth of soil was added to a part of it, in which 

 grew some fine elm-trees ; the trunks of these were protected 

 from pressure by circular walls, but at a little distance from 

 them ; nevertheless, the trees languished and died. Now the 

 reason of this was simply, that the roots being covered with too 

 great a depth of earth, could not exercise their usual function of 

 respiration ; to perform which, they seem generally to direct 

 their course as near the surface of the ground, as is consistent 

 with the support they have to afford to the plant. 



Influence of Vegetation on the Atmosphere. 

 292. Much discussion has taken place upon the question, 

 whether or not vegetation is upon the whole serviceable in purify- 

 ing the air ; that is, whether plants do altogether give out most 

 carbonic acid, or most oxygen, to the atmosphere. By Priestley it 

 was maintained, that the latter was the only effect of vegetation ; 

 and that Plants and Animals are thus constantly effecting changes 

 in the atmosphere, which counterbalance one another. Subse- 

 quent experiments seemed to show, however, that the carbonic 

 acid given out during the night, might equal or even exceed in 

 amount the oxygen given out by day ; but this was probably 

 due to the employment of plants, which had become unhealthy 

 by being kept in a limited quantity of air, and which had not 

 been exposed to a fair degree of light. For it has been recently 

 shown by Dr. Daubeny of Oxford, that in fine weather, a plant 

 consisting chiefly of leaves and stems, if confined in a capacious 

 vessel, and duly supplied with carbonic acid during sunshine, as 

 fast as it removes it, will go on adding to the proportion of 

 oxygen present, so long as it continues healthy ; the slight 

 diminution of oxygen and increase of carbonic acid, which take 

 place during the night, bearing no considerable proportion to the 

 degree, in which the contrary effect occurs by day. 



293. Thus we see that " the two great organised kingdoms 

 of nature are made to co-operate in the execution of the same 

 design ; each ministering to the other, and preserving that due 

 balance in the constitution of the atmosphere, which adapts it 



