t DECOMPOSITION OF CARBONIC ACID BY LEAVES. 



media, any indications of such a result, and therefore suppose that something of a 

 material character has been abstracted from the ray ; that it is really a loss, and not a 

 change. 



412. Being thus possessed of the means of depriving the beams of the sun of their 

 heat and their chemical force, I have proceeded to examine a variety of questions 

 of interest. A great many changes in the constitution of bodies, on their exposure to 

 light, are recorded in the books of chemistry and physics, but they are there imputed 

 to light in the aggregate, without any reference to its compound character: we shall 

 find there are changes due to the colorific ray, changes due to the calorific ray, and 

 changes due to the chemical ray. 



413. DECOMPOSITION OF CARBONIC ACID BY LEAVES. One of the most important 

 and extensive functions exercised by radiant matter from the sun is the decomposition 

 of carbonic acid by vegetable leaves, and the elimination of oxygen gas. Vegetable phy- 

 siology looks to chemistry for information, but hitherto the chemist has not possessed 

 the means of perfectly developing the matter. Its intrinsic importance entitling it to 

 investigation, I shall not offer any apology for passing from the direct object of this 

 paper to the mention of some facts necessary to the thorough understanding of the 

 matter. 



414. It would appear that there is a particular kind of combination, to which attention 

 has hardly yet been drawn, distinct from what is understood by chemical combination 

 and mechanical mixture. A pint of alcohol and a pint of water being mixed together, 

 the result will measure somewhat less than a quart, and the same might be indicated of 

 a variety of other liquids. No instance, I believe, is yet on record of a like penetration 

 of dimensions being observed in the case of gases ; if it exist at all, it exists to a very 

 small amount, and the change of volume which these bodies readily experience, by al- 

 teration of temperature and pressure, renders so minute an effect very difficult of detec- 

 tion. It has been supposed, judging from analogy, that the constituent gases of the 

 atmosphere, the uniting volume of which is always constant, are held together in this 

 manner, or that the whole volume is condensed and retained by some force of com- 

 pression. There are some experiments which indirectly prove this : sound passes 

 along different media with different velocities ; if a cannon were therefore discharged 

 at a distance, it should impress the ear with two distinct sounds ; the one coming along 

 the particles of nitrogen, and the other along the particles of oxygen. But it is well 

 known, from observations made directly on this point, that instead of there being any 

 reduplication of the sound, it comes clear, distinct, and alone; we have, therefore, to 

 infer that these two gases are held together in a state of compression. 



415. In making experimental investigations on this matter, two different courses may 

 be followed : first, we may measure the resulting volume after the mixture of known 

 volumes of the gases under trial ; or, secondly, we may ascertain whether any thermal 

 disturbance takes place during the act of their uniting ; the latter is the mode I have 

 followed in my researches. 



416. Take a cylindrical glass (A, fig. 53) two inches in diameter and four in height ; 

 close its upper extremity with a flat piece of wood by means of cement; in the centre 



