DESIGN. 31 



i. e., plants that are cultivated solely either for their flowers or 

 for their fruit. 



The exact manner in which light acts upon plants has been 

 studied by Dr. Daubeny, and others, and especially Mr. Hunt. 

 The result of these inquiries is given thus, in the Gardener's 

 Chronicle, of August 16th, 1845 : " Assuming, with Sir David 

 Brewster, that the prismatic spectrum consists of only three 

 primitive colors, namely, red, yellow, and blue, it is ascertained, 

 by experiment, that the maximum of heating power is found on 

 the confines of the red rays ; that the largest amount of light 

 is given by the yellow rays ; and that the chemical power exists 

 most strongly amidst the blue rays, of the spectrum. If we take 

 a deep red glass, which has been colored with the oxide of gold, 

 it will be found that the quantity of light which passes through it 

 is very small ; and, by using photographic paper, it may be ascer- 

 tained that the amount of that principle which produces chemi- 

 cal change is also very little, whereas the heat rays suffer no 

 interruption. A deep yellow glass, or a cell filled to the thick- 

 ness of an inch with a solution of bicromate of potash, intercepts 

 the chemical rays, but admits of the permeation of all the lumi- 

 nous rays, and offers but little interruption to the calorific rays. 

 If, however, we cover a pane of this yellow glass with another 

 of pale green bottle glass, the passage of the heating rays is 

 much impeded. A deep blue glass, such as is used for finger- 

 glasses, colored with oxide of cobalt, or a solution of oxide 

 of copper in ammonia, has the property of admitting freely 

 the passage of all the chemical rays, whilst it obstructs both 

 the heat and light radiations. Experiments conducted with 

 colors thus obtained led Mr. Hunt to the following conclusions : 



( 1 . ) Light which has permeated YELLOW media. LIGHT RAYS. 

 In nearly all the cases the germination of seeds was prevented, 

 and even in the few cases where the germination was com- 

 menced, the young plant soon perished. The germination 

 seemed referable to the action of the heat rays which had passed 

 the medium employed, rather than to the light. Agarics, and 

 several varieties of fungi, flourished luxuriantly under this influ- 

 ence. Although the luminous rays may be regarded as injuri- 



