298 PUBLICATION OF ATMOSPHEBE. [BOOK n. 



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. 



But, although the experiments of phytochemists have led 

 to these general conclusions, it is not at all probable that the 

 digestion of plants is limited to the decomposition and recom- 

 position of carbonic acid. It has already (page 261) been 

 stated that Messrs. Edwards and Colin have proved that 

 water is decomposed in the act of germination, the hydrogen 

 being fixed and the oxygen set free; and there can be little 

 doubt that this phenomenon occurs in plants during other 

 periods, perhaps all periods, of their vigorous growth. 



Theodore de Saussure found that germinating seeds absorb 

 nitrogen. It has been shown by M. Boussingault, that plants 

 abstract from the air a quantity of this gas, which they fix in 

 their tissue. But under what circumstances, or in what state, 

 this element is fixed in plants is unknown at present. 

 Nitrogen may enter directly into plants if their green parts 

 are fit to fix it ; it may pass into plants with the aerated 

 water absorbed by the roots ; and it may be possible, says 

 M. Boussingault, that, as some suppose, there exist in the 

 atmosphere very small quantities of ammoniacal vapour, 



