CHAr. 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 Avas 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 pecviliar 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 ail those multitudes of alkaline, 

 oily, resinous, and other principles of which the modern 

 chemist has ascertained the existence. These, belonging to the 



