74 IT IS NOT RADIANT HEAT NOR THE TITHOXIC RAY. 



To them all is apparently given a power of producing expansion in bodies, and to each 

 one, probably, its own specific chemical powers. The experiments of Sir J. HERS- 

 CHEL seem already to give proof of this fact. 



That the decomposition of carbonic acid by leaves is not due to yellow heat, may 

 be proved by causing the active light to pass through a solution of bichromate of pot- 

 ash, which is of an orange-yellow colour. This ray, thus treated, appears to carry on 

 the decomposition with nearly the same activity as the direct solar beam (Ar., 788). 

 In an experiment which I made, using it in a stratum of certain thickness, it seemed to 

 transmit the yellow and orange light with very little loss ; but acting more energetically 

 on the calorific ray, it transmitted of it only -26. Had this heat been the cause of the 

 decomposition, the rapidity with which the action took place should have been pro- 

 portionally reduced. 



272. From such results, it is to be inferred that radiant heat generally, and the yel- 

 low rays of heat especially, do not produce the decomposition of carbonic acid gas in 

 the structure of vegetable leaves. 



273. This narrows the question down to the inquiry, whether it be the yellow ray 

 of light or the yellow tithonic ray ; for, as has been observed, the phosphorescent rays 

 may be left out of the discussion. 



274. Experiments conducted on the same principle, and nearly in the same way, 

 with that already cited, serve to determine this point. If a beam which has passed 

 through a solution of bichromate of potash retains its power, as we know by experi- 

 ment, it remains for us to inquire whether that solution acts as feebly on the yellow 

 tithonic ray as it does on the yellow ray of light. We have already shown (251) 

 that this is not the case, for the tithonic ray undergoes an abundant absorption, its 

 force being greatly diminished, so that bromide of silver, which is easily changed by 

 the yellow tithonic ray, undergoes, in this disturbed and absorbed beam, a slow decom- 

 position. In the same way, therefore, that we determine that it could not be radiant 

 heat, we also determine against the tithonic rays. The decomposition of carbonic 

 acid, the production of chlorophyl, and the greening of plants go on with great rapid- 

 ity under that yellow solution, because, although it has absorbed, and therefore removed 

 these imponderable agents, it allows the active light to pass with little or no diminution. 



275. The observation made in 237 is now understood. Analysis by the prism 

 serves only to point out in what particular region of the spectrum given phenomena 

 are produced ; it therefore narrows our discussions down within certain limits. By 

 introducing, in addition, the action of absorbent media, we are enabled to point out, 

 with a certain amount of precision, the exact agent which is involved. This use of 

 absorbent media conjointly with prismatic analysis, which was introduced into these 

 inquiries in 1837 (Ap., CH. X.), may be expected continually to yield interesting results. 



276. To the LIGHT, and more especially to the yellow light of the sun, we are to 

 impute the production of this, the most interesting phenomenon of organic chemistry. 

 Rays which come from artificial sources, such as lamps and gas flames, can also bring 

 it about to a degree corresponding with their intensity. 



277. Whether that peculiarity of light by which it gives to us the sensation of spe- 



