754 PHYSIOLOGY. 



In a like manner, plants purify the water in which they grow, 

 and render it habitable by animals. We all know by early 

 experience, that if fish or other aquatic animals be placed in 

 water in which no plants arc grown, they will soon perish. This is 

 the case, because as there is then nothing present in the water to 

 destroy the noxious matters which are given off by them in their 

 respiration and other processes, — they are destroyed by their own 

 action upon the medium in which they are placed. In Nature, 

 we always find plants existing with animal life in the water, so 

 that the injurious influence communicated by the latter to that 

 medium, is counteracted by the respiration of the former : this 

 compensating influence of plants and animals is beautifully 

 illustrated in our aquaria. We are taught by these facts that it 

 is absolutely necessary, if we desire to maintain a large town in 

 a healthy state, to set apart large areas and plant them freely. 



How far our views regarding the purifying influence of plants 

 in Nature may require modification by the discovery of Boussin- 

 gault of the evolution of a certain proportion of such a poisonous 

 gas as carbonic oxide is known to be, at the same time with 

 oxygen, it is at present impossible to say ; but the subject is 

 one of the very greatest importance, and cannot but repay a 

 careful investigation. Boussingault has thrown out a sugges- 

 tion, that in some cases, so far from plants purifying the air, 

 on the contrary, the atmosphere of marshy districts, where 

 they are in excess, owes to them its insalubrity. Probably, also, 

 one cause of the unhealthiness of densely wooded districts may 

 be due to the evolution of carbonic oxide gas. With reference 

 to the above conclusions of Boussingault, it maybe remarked, that 

 his experiments were solely made by putting plants or the green 

 parts of plants in water prei-iously impregnated with carbonic 

 acid. The conditions under which carbonic oxide was formed 

 by plants were therefore not altogether natural ones ; and hence 

 it is desirable that future experimenters should test plants grow- 

 ing in the air as well as in water, in as nearly as possible their 

 natural states of existence. 



There exists a widely-spread notion, that plants, when grown 

 in rooms where there is but little ventilation, and, therefore, 

 especially in our sleeping apartments, have an injurious influence 

 upon the air contained in them. This idea has arisen from a 

 knowledge of the fact that plants, when not exposed to solar 

 light, have a contrary effect upon the atmosphere to that which 

 they have when submitted to its influence ; that is to say, that 

 they then absorb oxygen and give off carbonic acid gas, instead 

 of absorbing carbonic acid gas and giving off oxygen. The 

 amount of carbonic acid gas, however, which is then given off 

 by plants is so extremely small, that it can have no sensible 

 effect upon the atmosphere in which they are placed. It might 

 he readily shown that it would require some thoiisands of plants, 



