54 FOOD OF THE VEGETATING PLANT. CHAP. I. 



placed over wort in a state of fermentation, gene- 

 rally became quite dead in the space of a day, and 

 did not even recover when put into an atmosphere 

 of common air.* 



But Dr. Percival, of Manchester, observed that 

 a plant of Mint, immersed in water by the root and 

 exposed to a current of atmospheric air mixed with 

 carbonic acid gas, was more vigorous and luxuriant 

 than a plant of the same species similarly situated 

 and exposed to a current of pure atmospheric air.-j" 

 Improving upon this hint, Saussure made some 

 experiments with a view to determine the dose of 

 carbonic acid gas which, being mixed with atmos- 

 pheric air, is the most favourable to vegetation. 

 Having made some Peas to germinate in water till 

 they acquired the height of four inches and weighed 

 about twenty grains, he then placed a number of 

 them in glasses filled with water by threes, so as 

 that the roots only were immersed, and introduced 

 them into receivers filled with different mixtures 

 of common air and carbonic acid gas. They were 

 situated so as to receive the direct rays of the sun, 

 moderated when too intense. The mean augmen- 

 tation in weight of such as were placed in pure 

 atmospheric air, and exposed during ten days to 

 the sun, was eight grains to each plant. Such as 

 *vere exposed to the sun, in an atmosphere of pure 

 carbonic acid gas, faded and withered away without 

 any further developement. In an atmosphere con- 

 * Priestley on Air, vol. i. p. 36. i Manch. Trans, vol. ii, 



