CHAP. X. 
DIGESTION. 
317 
on the atmosphere. In a letter I have recently received from 
him 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 inclosed 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 we breathe." 
Thus, the vegetable kingdom may be considered as a special 
provision of nature, to consume that which would render the 
world 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. 
The result of the foregoing phenomena is the formation 
of numerous principles peculiar to the vegetable kingdom, 
and the deposition of others which are foreign to plants, but 
which have been introduced into their system in the current 
of the sap. Thus are produced the silex of the Grass tribe ; 
the sugar of the Cane, and of various fruits; the starch of Corn, 
Potatoes, and other farinaceous plants; the gum of the Cherry; 
the tannin of the Oak ; and all those multitudes of alkaline, 
oily, resinous, and other principles of which the modern 
chemist has ascertained the existence. These, belonging to the 
