378 
PHYSIOLOGY. 
BOOK II. 
atmosphere. In a letter to myself he expresses himself 
thus : — 
“ As the observations of Ellis left it in some doubt whether 
the balance was in favour of the purifying or the deteriorating 
influence upon the air which is exercised by plants during dif- 
ferent portions of the day and night, I conducted my experi- 
ments in such a manner that a plant might be enclosed in a 
jar for several successive days and nights, whilst the quality of 
the air was examined at least two or three times a day, and fresh 
carbonic acid admitted as required. A register being kept of 
the proportion of oxygen each time the air was examined, as 
well as of the quantity of carbonic acid introduced, it was 
invariably found that, so long as the plant continued healthy, 
the oxygen went on increasing^ the diminution by night being 
more than counterbalanced by the gain during the day. This 
continued until signs of unhealthiness appeared in the confined 
plant, when, of course, the oxygen began to decrease. 
“ In a perfectly healthy and natural state, it is probable that 
the purifying influence of a plant is much greater ; for when I 
introduced successively different plants into the same air, at 
intervals of only a few hours, the amount of oxygen was much 
more rapidly increased, — in one instance to more than 40 per 
cent of the whole, instead of 20 as in the air w'e breathe.” 
Thus, the vegetable kingdom may be considered as a special 
provision of nature, to consume that which would render the 
w'orld uninhabitable by man, and to have been so beautifully 
contrived that its existence depends upon its perpetual 
abstraction of that, without the removal of which our own 
existence could not be maintained. 
But although this is true of green plants, it does not appear 
to be so of Fungi. Marcet has shown from carefully conducted 
experiments, “ that Mushrooms, vegetating in atmospheric 
air, produce on that air very different modifications from 
those of green plants in analogous situations ; in fact, that 
they vitiate the air promptly, either by absorbing its oxygen 
to form carbonic acid at the expense of the carbon of the 
vegetable, or by disengaging carbonic acid formed in various 
ways. That the modifications which the atmosphere expe- 
riences when in contact with growing Mushrooms are the 
