and of Carbonates by the Light of the Sun. 168 



it is the agent that brings about the decomposition of car- 

 bonic acid. The faculty of producing like effect is not the 

 distinguishing quality of the tithonic rays, nor can the term 

 chemical be any more applied to them than to either of their 

 acknowledged distinct companions. Unless therefore chemists 

 are content to admit that a species of heat may exist devoid, 

 of the power of expanding bodies, of giving the sensation of 

 warmth, and of being transmitted by conducting processes ; or, 

 unless they admit that light can exist in such a modified condi- 

 tion as to produce in our eyes the sensation of darkness, they 

 will have to admit these tithonic rays as constituting a fourth 

 imponderable agent. The name they may take is not a matter 

 of importance, that which is least trammelled by hypothesis 

 is best. It is not the object of the papers I have written in 

 this Journal to show merely that a class of invisible rays exists 

 in the spectrum ; that has been known for a long time ; but it 

 is to point out the true relation of these rays to other bodies 

 and other forces in the world, to assert for them their title of 

 a fourth distinct imponderable agent, and to secure for them 

 the admission of that title by giving them a name. 



When the leaves of plants are placed in water from whicli 

 all air has been expelled by boiling, and exposed to the sun's 

 rays, no gas whatever is evolved from them. When they are 

 placed in common spring or pump water bubbles quickly 

 form, which when collected and analysed prove to be a mix- 

 ture of oxygen and nitrogen gases ; from a given quantity of 

 water a fixed 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 rapidity, 

 and large quantities of gas are evolved. 



The obvious inference which seems to arise from these facts 

 is, that all the oxygen collected is derived from the direct de- 

 composition of carbonic acid. We shall presently examine 

 whether this is the correct inference. 



Having, by long boiling and subsequent cooling, obtained 

 water free from dissolved air, I saturated it with carbonic acid 

 gas. Some grass leaves, the surfaces of which were carefully 

 freed from any adhering bubbles or films of air by having 

 been kept beneath carbonated water for three or four days, 

 were provided. Seven glass tubes, each half an inch in dia- 

 meter 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 inserted side by side in a 

 small pneumatic trough of porcelain. It is to be particularly 

 remarked that the blades were of a pure green aspect as seen 



M 2 



