252 PRINCIPLES OF VENTILATION. 



2. Much discussion has taken place upon the question 

 whether or not vegetation is, upon the whole, serviceable in puri- 

 fying the atmosphere ; that is, whether plants give out most car- 

 bonic acid or most oxygen. Priestley 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. Subsequent experiments seem to 

 show, however, that the carbonic acid given out during the 

 night, equals or even exceeds in amount the oxygen given out 

 by day. But this might be owing to the employment of plants 

 which had become weak and unhealthy, by being kept in an 

 impure atmosphere, previous to being experimented on, and 

 which had not been exposed to a fair degree of light. Dr. Dau- 

 beny, of Oxford, has recently shown 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 pro- 

 portion 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 pro- 

 portion to the degree in which the contrary effect occurs during 

 the day.^ 



Thus we see that the two great organized kingdoms of 

 nature are made to cooperate 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 

 to the welfare and activity of every order of beings, and which 

 is quickly destroyed when the operations of any of them become 



* Plants decompose carbonic acid during the day, and form it again 

 during the night, — the oxygen they inhale at that time entering again 

 into combination with their carbon, — and, during the healthy state of a 

 plant, the decomposition by day, and recomposition by night, of this 

 gaseous matter, are perpetually going on. The quantity of carbonic acid 

 decomposed is in proportion to the intensity of the light which strikes a 

 leaf, the smallest amount being in shady places ; and the healthiness of 

 a plant is ccBteris paribus in proportion to the quantity of carbonic acid 

 decomposed. Therefore, the healthiness of a plant should be in propor- 

 tion to the quantity of light it receives by day. — [Lind. The. of Hort.] 



