CAUSE OF THE PHENOMENON. 



rect agency in the matter ; it may bring about changes which, to a certain extent, 

 complicate the phenomenon, but it (Joes not produce the abstraction of any compound 

 of oxygen and carbon from carbonic acid. Apart from the agencies exercised by the 

 elements of the plant, agencies which are unquestionably of the utmost importance, the 

 decomposition is remotely brought about by the action of radiant matter. But non- 

 luminous heat, though capable of evolving gas, produces no change of its constitution ; 

 shall we, then, suppose that there is a difference, in point of quality, between the heat 

 given off by the bodies below ignition, and the heat of incandescent matter ? Or does 

 the light itself aid decomposition ? An experiment may be made which appears to me 

 to bear directly on the answer which should be given to this query. Let a beam of 

 light (^fig. 61), two inches in diameter, pass through an aperture in the shutter A B, and 

 fall upon any medium, c d, which absorbs a certain number of the rays of heat, as 

 bichromate of potassa, which may be so diluted as to absorb exactly 50 rays out of every 

 100. Having, by means of a good thermometer, g, measured this, let the beam of light 

 pass through a second trough, e f, containing the same solution of the same strength, and 

 its temperature again be taken, it will appear that the ray, instead of losing half its 

 heat, will contain nearly all of it ; or, in other words, the second trough exerts no action 

 on the passing beam. In an experiment tried after the manner here indicated, the ther- 

 mometer having shown a loss of 50 rays by the action of the first trough, fell only to 

 47, or gave a loss of three rays only as the action of the second trough; an action to be 

 referred, undoubtedly, to a degree of turbidness which does exist, to a small extent, in 

 the clearest solutions ; and also to the reflective and scattering action of the surfaces of 

 the troughs. Now the very same thing takes place in the case of light. A beam that 

 has passed through a green, or any other coloured glass, loses much of its intensity, but 

 if it pass through a second plate, of the same tint, the second loss is entirely dispropor- 

 tionate to the former ; and the reason of this is very apparent, for if the second plate had 

 been of a different colour, the ray might have been much more affected r or even entirely 

 extinguished. DELAROCHE made an identical observation in the case of non-luminous 

 heat, for he proved that a plate of glass obstructed a large portion of the rays falling 

 on it, but that a second plate allowed these rays to pass with far less loss. Now 

 these experiments would lead us to conclude that there are essential differences in ra- 

 diant heat analogous to the differences in light. The rays of heat given off by a can- 

 ister of hot water may be, to use an expressive solecism, violet heat, and a piece of 

 transparent glass may be able to transmit green heat only ; hence, in using two plates, 

 the absorptive action of the first has the largest share in producing the phenomenon, 

 the second transmitting nearly all which passed the first, an action identical with that 

 of coloured glass on light. Bodies, as their temperature rises, emit more and more rays- 

 capable of passing through glass, simply because they become of that class over which 

 the medium does not exercise an absorptive power. 



438. The general conclusion which we are to draw from these researches is, that 

 the decomposition of carbonic acid gas by vegetable matter is a very complex phenom- 

 enon, due to the combined action of three forces : 1st, the decomposing action of a tissue; 

 2d, to the impinging of radiant heat ; 3d, to chemical affinity, it being probable that any 



