EXPERIMENTS OF SENNEBIER, MORREN, AND DAUBENY. (53 



was to cause the plants to grow or to carry forward their decomposing action under 

 the influence of light which has passed through glass stained with the different colours 

 Thus, if young plants are made to grow beneath a shade of lemon-yellow glass, in a 

 few days it is seen that such light does not exert a destructive agency, but they grow 

 vigorously ; and if, simultaneously, comparative trials are made with other shades of 

 different colours, such as red, blue, green, &c., an estimate may be obtained of the com- 

 parative effects of rays of these different colours. 



227. But it is well known that lights thus produced are not monochromatic, but 

 contain a variety of different coloured rays ; for when the ray which passes through 

 them is examined by a prism, it is always dispersed into a variety of different colours. 

 Thus, the light which passes through the ordinary variety of blue glass, analyzed in this 

 way, is found to contain red, yellow, and an abundance of indigo and violet, although it 

 looks of a pure blue tint. The blue light which passes it is therefore not a simple, but 

 a composite blue, made up of many other colours, the blue predominating. And as 

 this observation applies to almost all sorts of coloured media, their colours being com- 

 pound and not simple, it is obvious that when we make experiments with them we 

 shall be liable to be led into error, unless we determine what rays they actually trans- 

 mit. Accordingly, Dr. DAUBENY determined this point in the case of each glass and 

 coloured medium that he used. 



228. About the same time that these experiments were made in England, I com- 

 menced some of the same kind in the south of Virginia (Ap., 413, 497, 506, 515-517), 

 where, during the summer season, the light is extremely brilliant. These served to 

 show that under a yellow solution, such as the bichromate of potash affords when dis- 

 solved in water, the leaves of plants turned green, and the decomposition of carbonic 

 acid gas was effected. 



229. Soon after this the British Association for the Advancement of Science, in 

 consequence of some results which had been obtained in England, which seemed to 

 countenance the opinion of SENNEBIER, that the violet rays direct the function of di- 

 gestion, appointed a committee to examine this matter at their expense. A number of 

 experiments were consequently made, very much in the same manner as those which 

 had been previously conducted by Dr. DAUBENY ; these seemed, however, to be at va- 

 riance with his conclusions, and showed, that while the violet tithonic ray is the ac- 

 tive agent which controls the process, the yellow ray of light is exceedingly injurious. 



230. But it is now known that these results, like those of SENNEBIER, are erroneous, 

 and that, as was first discovered by MORREN in France, and by DAUBENY in England, 

 it is yellow LIGHT which directs the digestion of plants ; that leaves can be greened, 

 and oxygen given off, and carbon fixed by light that has passed through a solution of 

 bichromate of potash, or through yellow glass (Ap., 784-792). 



231. The proper mode of conducting these experiments is to employ the solar spec- 

 trum itself, a process which, it will be seen (Ap., CH. XV.), was first resorted to iri 

 New-York. By some appropriate optical mechanism, a ray of the sun is transmitted 

 into a dark room through a circular hole in one of the shutters, and kept motionless 

 for several hours, although the sun may have apparently moved. a considerable distance 



