without- fresh Supplies of Water and Air. 497 



we may, perhaps, be excused for going a little farther into de- 

 tail on this point than we should otherwise have done. 



The experiments of various chemists, from Scheele down to 

 De Saussure, have shown that seeds do not germinate without 

 receiving continual supplies of fresh air ; and that, in the pro- 

 gress of their evolution, they convert the oxygen gas of such air 

 into an equal volume of carbonic acid gas. As plants spring 

 from seeds, it was natural to expect that, to carry on their pro- 

 gressive developement, they would also require fresh air, and 

 would, in like manner, convert its oxygen into carbonic acid gas. 

 Both these facts were proved by Dr. Ingenhousz in his Experi- 

 ences sur les Vegetanx, t. ii. p. 35. 37. ; by M. Senebier in his 

 Physiologie Vegetale, t. iii. p. 113.; and by Theodore de Saus- 

 sure in the Annates de Chimie, t. xxiv. p. 139. M. Senebier 

 farther maintained that the air, thus employed in vegetation, 

 lost precisely the quantity of oxygen gas necessary to the forma- 

 tion of the carbonic acid gas produced, a result confirmed by the 

 experiments of De Saussure and by those of other writers ; so 

 that, in the progressive stages of developement and growth, 

 plants, like the seeds from which they sprang, not only require 

 a pure air, but convert a portion of its oxygen into an equal 

 volume of carbonic acid gas. 



In opposition to these results, Dr. Priestley, from certain 

 experiments, was led to believe that plants, instead of vitiating 

 the air by their vegetation, reverse the effects produced in it by 

 combustion and the respiration of animals, and thus become the 

 chief means by which the purity of the atmosphere is maintained. 

 {Observations on Air, abridged, vol. iii. p. 251.) He caused plants 

 to vegetate in vessels of air which had been previously vitiated 

 by combustion or respiration, and, in some instances, this foul air 

 was restored by the plants to a condition capable of again sup- 

 porting those processes ; but he did not ascertain the mode in 

 which the air itself was vitiated, although he believed that light 

 contributed to effect its subsequent purification. His great 

 contemporary, Scheele, repeated these experiments, but could 

 never find the foul air which he employed to be purified by 

 growing plants, either when the vessels were placed in sunshine 

 or in shade. For this difference in their results a sufficient 

 reason may be found in the fact, that the foul air used by 

 Priestley consisted, in part, of the carbonic acid gas previously 

 produced in it by respiration or combustion ; whilst, in all the 

 experiments made on foul air by Scheele, he carefully removed 

 this acid gas by washing the air in milk of lime before placing 

 the plants in it {On Air and Fire, p. 37. 163.), a procedure, 

 as will presently be shown, quite sufficient to defeat the object 

 of purifying the air. 



In other experiments, made on the purification of air by the 

 Vol. XV. — No. 114. l l 



