ABSORPTION AND EXHALATION OF GASES BY LEAVES. 803 
experience, that if fish or other aquatic animals be placed in 
water in which no plants are grown, they will soon perish. This 
is partly because, as there is then nothing present in the water 
to destroy the noxious matters which are given off by the 
animals in their respiration and other processes, they are de- 
stroyed 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 assimilation 
ofthe former: this compensating influence cf plants and animals 
is beautifully illustrated in our aquaria. We are taught by 
these facts that it is absolutely necessary, if we desire to main- 
tain 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 
may require modification bythe discovery by Boussingault of the 
evolution of a certain proportion of such a poisonous gas as car- 
bon oxide, together with oxygen, it is at present impossible 
to say ; but the subject is one of the very greatest importance, 
and cannot but repay further careful investigation. Boussingault 
has even thrown out a suggestion, that in some cases, so far 
from plants purifying the air, they may, on the contrary, cause 
the atmosphere of marshy districts, where they are in excess, to 
be unhealthy. It is also probable that one cause of the un- 
healthiness of densely wooded districts may be due to the 
evolution of carbon oxide. With reference to the above con- 
clusions of Boussingault, it may be remarked, that his experi- 
ments were solely made by putting plants or the green parts 
of plants in water previously impregnated with carbon dioxide. 
The conditions, therefore, under which carbon oxide was formed 
were not altogether natural ones ; and hence it is desirable that 
future experimenters should test plants growing in the air as 
well as in water, and in every respect in as nearly as possible 
their ordinary states of existence. 
There exists a widely spread notion, that plants, when grown 
in rooms where there is but little ventilation, and hence, espe- 
cially in our sleeping apartments, have an injurious influence 
upon the contained air. This idea has arisen from a knowledge 
of the fact that plants, as already noticed, 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 carbon dioxide, in- 
stead of absorbing carbon dioxide and giving off oxygen. But 
the amount of carbon dioxide 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 be readily 
shown that it would require some thousands of plants, in this 
way, to vitiate the air of a room to anything like the extent 
of that of a single animal, and that, therefore, the idea of a 
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