MKMOIR X.] THE DECOMPOSITION OF CARBONIC- ACID GAS. 1(3 9 



gen gases; from a given quantity of water a definite 

 quantity of air is produced. When they are exposed 

 in water which has been boiled, and then impregnated 

 with carbonic acid, the decomposition goes on with ra- 

 pidity, and large quantities of gas are evolved. 



The obvious inference seeming to arise from these 

 facts is that all the oxygen collected is derived from 

 the direct decomposition of carbonic acid. 



Having by long boiling and subsequent cooling ob- 

 tained water free from dissolved air, I saturated it with 

 carbonic-acid gas. Some grass leaves, the surfaces of 

 which were carefully freed from adhering bubbles or 

 films of air by having been kept in carbonated water 

 for three or four days, were provided. Seven glass 

 tubes, each half an inch in diameter and six inches 

 long, were filled with carbonated water, and into the 

 upper part of each the same number of blades of grass 

 were placed, care being taken to have all as near as could 

 be alike. The tubes were placed side by side in a small 

 pneumatic trough. It is to be particularly remarked that 

 the leaves were of a pure green aspect as seen in the wa- 

 ter; no glistening air-film such as is always on freshly 

 gathered leaves nor any air -bubbles were attached to 

 them. Great care was taken to secure this perfect free- 

 dom from air at the outset of the experiments. 



The little trough was now placed in such a position 

 that a solar spectrum, kept motionless by a heliostat, 

 and dispersed in a horizontal direction by a flint-glass 

 prism, fell upon the tubes. By bringing the trough 

 nearer to the prism or moving it farther off", the different 

 colored spaces could be made to fall at pleasure on the 

 inverted tubes. The beam of light was about three 

 fourths of an inch in width. 



In a few minutes after the beginning of the experi- 

 ment, the tubes on which the orange, yellow, and green 



