80 QUARTERLYJOURNAL. 



tion of the physical constitution of the sunbeams and the pris- 

 matic spectrum, we shall detain the reader a moment only with 

 the contents of chapter sixth, in which it is satisfactorily and very 

 beautifully shown which of the rays in the sunbeam decompose 

 carbonic acid, and at the same time turn the vegetable organs to 

 green. 



It is probably known to all of our readers, that light, as it comes 

 to us from the sun, contains several distinct principles, each prin- 

 ciple impressing the visual organs in its own specific manner, each 

 producing a sensation which we call light ; thus, one produces 

 red light, another yellow, and another violet light. These princi- 

 ples are separated from each other when light passes through a 

 triangular prism of glass, and each color, when the rays have 

 passed through the glass, occupy upon a plane upon which they 

 are received, a certain position, but shading gradually into each 

 other. In common language, the rays are called different kinds 

 of light 



As it has been shown that light exerts a very important influ- 

 ence upon plants, an interesting inquiry arose, viz : by which of 

 the rays of light are vegetables most affected ? Is it the compound 

 white light, or is it one individual ray, which decomposes carbonic 

 acid, and gives the green color to them 7 To give a satisfactory 

 answer to these questions many experiments have been made with 

 the different kinds of rays. In some experiments, light was made 

 to pass through colored glass, or colored solutions — as the yellow, 

 red, orange, violet. By a series of experiments made by colored 

 glass, the fact has been discovered that the different kinds of light 

 act very differently upon plants ; thus, yellow light gives the green 

 to vegetables by decomposing carbonic acid, whereas plants ex- 

 posed to the action of the other rays, the same effects were Ic's'; 

 and less in the proportion to their illuminating power. 



As colored glass does not transmit a pure ray, or one unmixed 

 with the others, Professor Draper first employed the rays separated 

 by the triangular prism of glass, and exposed plants directly to 

 each kind of light, by a set of tubes. By this arrangement the 

 Ijo-ht was made to fall upon the plants growing in boxes, and ar- 

 rano-ed in such a way that a comparative estimate could easily be 

 made of the effects of each ray upon the same plant. 



This plan of experimenting resulted in a very gratifying manner, 



