108 



GLASS. 



spectrum was thrown upon it, the other surface was washed 

 with strong sulphuric ether. By the evaporation of the ether, 

 the points of calorific action were most easily obtained, as these 

 dried off in well defined circles, long before the other parts pre- 

 sented any appearance of dryness. By these means it is not 

 difficult, with ease, to ascertain exactly the conditions of the 

 glass, as to its transparency to light, heat, and chemical agency, 

 (actinism.) 



" The glass thus chosen is of a very pale yellow green color, 

 the color being given by oxide of copper, and is so transparent 

 that scarcely any light is intercepted. In examining the spec- 

 tral rays through it, it is found that the yellow is slightly dimin- 

 ished in intensity, and that the extent of the red ray is diminished 

 in a small degree, the lower edge of the ordinary red ray being 

 cut off by it. It does not appear to act in any way upon the 

 chemical principle, as spectral impressions, obtained upon chlo- 

 ride of silver, are the same in extent and character as those 

 procured by the action of the rays which have passed ordinary 

 white glass. This glass has, however, a very remarkable action 

 upon the non-luminous heat rays, the least refrangible calo- 

 rific rays. It prevents the permeation of all that class of heat 

 rays which exists below, and in the point fixed by Sir William 

 Herschel, Sir H. Englefield, and Sir J. Herschel, as the point 

 of maximum calorific action, and it is to this class of rays that 

 the scorching influence is due. There is every reason to con- 

 clude that the use of this glass will be effectual in preserving 

 the plants, and at the same time that it is unobjectionable in point 

 of color, and transparent to that principle which is necessary for 

 the development of those parts of the plant which depend upon 

 external chemical excitation, it is only partially so to the heat 

 rays, and it is opaque to those only that are injurious. The 

 absence of the oxide of manganese, commonly employed in all 

 sheet glass, is insisted on, it having been found that glass, into 

 the composition of which manganese enters, will, after exposure 

 for some time to intense sun-light, assume a pink hue, and any 

 tint of this character would completely destroy the peculiar 

 properties for which this glass is chosen. Melloni, in his in- 

 vestigations on radiant heat, discovered that a peculiar green 



