EVOLUTION OF CARBONIC ACID DURING THE NIGHT. 21 



Sir H. Davy made an elegant experiment in illustration of the 

 facts just stated. He placed a turf, four inches square, in a 

 porcelain dish which swam on the surface of water impregnated 

 with carbonic acid gas. A glass vessel of the capacity of 230 

 cubic inches was made to cover the grass, to which water was 

 occasionally supplied by a funnel furnished with a stopcock. 

 The water upon which the porcelain dish swam was daily sup- 

 plied with new water saturated with carbonic acid, so that a 

 small quantity of that gas must always have been present in the 

 receiver. The volume of air in the receiver was found to in- 

 crease by exposure to daylight, so much so, that after the lapse 

 of eight days, an increase of thirty cubic inches was observed. 

 The air inside the receiver on being analysed was found to con- 

 tain 4 per cent, more oxygen than the air of the exterior 

 atmosphere. (Davy's Agricultural Chemistry, Lecture V.) In 

 confirmation of the same facts we may also refer to the excellent 

 experiments of Dr. Daubeny.* 



In the preceding part of the work, we have furnished proofs 

 that the carbon of plants is derived from the atmosphere. We 

 have yet to consider the action of humus and of certain mineral 

 matters upon the development of vegetation, and also the source 

 whence plants receive their nitrogen. 



* On the Action of Light upon Plants, : nd of Plants upon the Atmo*» 

 phere, PhiL Trans., Part I., 1836 



