ACTION OF DIFFERENT RAYS. 461 



includes nearly all vegetable forms, for we may leave out of considera- 

 tion the fungi, a group which stands intermediately between plants and 

 animals. The distinctive character of a plant is to form, from carbonic 

 acid of the air, solid organic structures. The distinctive character of an 

 animal is, by the oxidizing processes going on in it, to restore the or- 

 ganic bodies which have served it as food to their original formless state. 

 The group referred to differs from true plants in feeding on matter al- 

 ready organized, and breathing like animals. It therefore does not re- 

 quire the influence of light to collect material for it, and bring it to the 

 proper state. In the development of this group the plastic principle is 

 alone concerned. 



Since the sunlight is composed of many differently colored rays and 

 different principles, it becomes an interesting inquiry which Powe * of dif 

 of these is the immediate agent in ministering to the nutri- ferent colored 

 tion of plants. In 1843, by causing plants to effect the de- ra y sofli s ht - 

 composition of carbonic acid in the prismatic spectrum, I found that the 

 yellow is by far the most effective, the relative power of the various col- 

 ors being as follows : 



Yellow, Blue, 



Green, Indigo, 



Orange, Violet. 

 Bed, 



My experiments on the production of hydrochloric acid by the direct 

 union of chlorine and hydrogen under the influence of light, Phenomena of 

 both artificial and solar, and also on the decomposition of ihe action of 

 pe'roxalate of iron, from which carbonic acid is readily disen- growth of & 

 gaged, conclusively establish the fact that the primary con- P lants - 

 dition essential for the chemical action of light is the absorption of some 

 particular ray. If the physical condition of substances otherwise easily 

 decomposable is such that they transmit light without absorbing any, no 

 chemical change ever ensues in them, and the same condition obtains in 

 cases of combination. Thus oxygen and hydrogen can not be made to 

 unite, even by the most intense radiation, because neither of them ex- 

 ert any absorptive action ; but chlorine and hydrogen unite with energy, 

 because the chlorine absorbs the indigo ray. 



The same experiments prove that the amount of decomposition or oth- 

 er work done by light is always proportional to its quantity ; hence, by 

 the aid of converging mirrors and lenses, chemical changes can be ac- 

 complished with great rapidity. These instruments, however, when even 

 of the largest size, are unable to produce any other effect than would be 

 brought about by a feebler ray if applied sufficiently long. The great- 

 ly increased intensity of light which they can present does not enable us 

 either to bring about combinations or decompositions of substances which 



