62 SEVERAL IMPONDERABLE PRINCIPLES IN THE SUNBEAM. 



223. We have seen that the appearance of green matter in water, and the produc- 

 tion of chlorophyl in leaves, are the same phenomenon. In the largest trees all the 

 solid matter found in their branches, stems, bark, and various other parts, was originally 

 fabricated in the leaves, for it was in them that carbonic acid was decomposed and its 

 solid material fixed. Once thus introduced into the interior of the vegetable system, 

 it passes through a multitude of changes; from the descending sap the different corn- 

 pounds are removed, and lodged in their proper places. It is in this way that the 

 starch, sugar, gum, woody fibre, fibrine, albumen, caseiue, essential oils, resins, &c., are 

 all produced. First one and then another change is impressed on the descending juice, 

 and first one and then another special compound is removed from it ; but the point 

 from which all this cycle of changes begins may be traced back into the leaf, to the 

 decomposition of carbonic acid and production of chlorophyl, under the influence of 

 light. 



224. A ray of light, as is well known (137), contains, under ordinary circumstances, 

 several distinct principles. 1st. A principle which impresses the organ of vision with 

 a specific sensation, and which is therefore spoken of explicitly as LIGHT ; of this there 

 are several modifications : one which produces in our eyes the sensation of a red col- 

 our ; another, a yellow ; a third, a blue; and these, conjoined in proper proportions, give 

 rise to a white. 2d. A principle of radiant heat, the well-known characters of which 

 are the power of producing expansion in bodies, of communicating a feeling of warmth; 

 in this the phenomena of coloration have been traced as in the former case. 3d. A 

 principle of chemical force, which seems to be intimately associated with light, but dif- 

 fers from Ji rt principle in wanting the power of affecting our organs of vision. From 

 this intimate association with light, I have suggested for this principle the name of Ti- 

 THONIC RAYS, in allusion to the fable of Tithonus and Aurora. In this, as in the for- 

 mer, the quality of coloration has already been traced. 4th. A principle of phospho- 

 rescence, which has the distinctive character of causing certain substances, when sub- 

 mitted to its influence, to shine for a short time after with a transient light ; this prin- 

 ciple also is invisible to the eye, but differs from the tithonic rays in the fact that, un- 

 der certain circumstances, transparent glass is opaque to it. 



225. Of these constituents, visible and invisible, in the solar beam, which is it that 

 has charge of that digestive function of plants which we are now considering ? Is it 

 the light or heat, the tithonic or the phosphorescent ray 1 



226. Several years ago, experiments were made by SENNEBIER for the purpose ef de- 

 termining this question ; his results seemed to show that the violet rays contained the 

 active force. More recently, in France, M. MORREN commenced an inquiry of a sim- 

 ilar kind, and came to the conclusion that the luminous rays, and more especially the 

 yellow, were the cause of the phenomenon. In 1836, Dr. DAUBENY published, in the 

 Transactions of the Royal Society, a very extensive series of experiments, the result 

 of which went to show that the leaves of plants decomposed carbonic acid, and turned 

 green most rapidly in the yellow ray, the other colours, orange, green, red, blue, indigo, 

 and violet, producing the same effect, more and more slowly in proportion as their illu- 

 minating power was less. The plan which was followed by these different chemists 



