114 ON THE FOOD OF PLANTS. 



Now admitting that carbonic acid is a neces- 

 sary part in composing the food of plants, it 

 appears to me that Mr. Kirwan's theory is by 

 far the most probable. He says, " That car- 

 *' bonic acid soon precipitates, by superiority of 

 " its specific gravity, and is then condensed in, 

 " or mechanically absorbed by, soils, or con- 

 " tained in dew.'* He also further remarks : 

 " That plants do not thrive, but most frequently 

 '* perish, when surrounded by an atmosphere of 

 ** fixed air, has long been observed by that 

 ** great explorer of the most hidden processes 

 " of nature, Dr. Priestley : but that fixed air, 

 " imbibed by the roots, is favourable to their 

 " growth, seems well established by the expe- 

 *' riments of Dr. Perceval of Manchester, and 

 *' fully confirmed by those of M. Ruskert. 

 " This last mentioned philosopher planted two 

 " beans in pots of equal dimensions, filled with 

 " garden mould ; the one was watered almost 

 '* daily with distilled water, the other with water 

 " impregnated with fixed air, in the proportion 

 " of half a cubic inch to an ounce of water ; 

 " both were exposed to all the influence of the 

 " atmosphere, except rain ; the bean treated 

 *' with aerated water, appeared overground nine 

 " days sooner than that moistened with distil- 

 " led water, and produced twenty-five beans ; 



