GLASS. 107 



was, therefore, thought desirable to ascertain if it would be pos- 

 sible to cut off these scorching rays by the use of a tinted glass, 

 which should not be objectionable in its appearance, and the ques- 

 tion was, at the recommendation of Sir William Hooker and 

 Dr. Lindley, submitted, by the commissioners of woods, &c., to 

 Mr. Hunt. The object was to select a glass which should not 

 permit those heat rays, which are most active in scorching the 

 leaves of plants, to permeate it. By a series of experiments, made 

 with the colored juices of the palms themselves, it was ascer- 

 tained that the rays which destroyed their color belonged to a 

 class situated at the end of the prismatic spectrum, which ex- 

 hibited the utmost calorific power, and just beyond the limits of 

 the visible red ray. A great number of specimens of glass, vari- 

 ously manufactured, were submitted to examination, and it was 

 at length ascertained, that glass tinted green appeared most likely 

 to effect the object desired, most readily. Some of the green 

 glasses that were examined, obstructed nearly all the heat rays ; 

 but this was not desired, and, from their dark color, these were 

 objectionable, as stopping the passage of a considerable quantity 

 of light, which was essential to the healthy growth of the plants. 

 Many specimens were manufactured purposely for the experi- 

 ments, by Messrs. Chance, of Birmingham, according to given 

 directions ; and it is mainly due to the interest taken by these 

 gentlemen, that the desideratum has been arrived at. 



" Every sample of glass was submitted to three distinct sets of 

 experiments. 



" First. To ascertain, by measuring off the colored rays of 

 the spectrum, its transparency to luminous influence. 



" Second. To ascertain the amount of obstruction offered to 

 the passage of the chemical rays. 



" Third. To measure the amount of heat radiation which 

 permeated each specimen. 



" The chemical changes were tried upon chloride of silver, and 

 on papers, stained with the green coloring matter of the leaves 

 of the palms themselves. The calorific influence was ascer- 

 tained by a method employed by Sir John Herschel, in his ex- 

 periments on solar radiation. Tissue paper was smoked on one 

 side, by holding it over a smoky flame, and then, while the 

 10 



