66 CALORIFIC TRANSPARENCY. 



Black mica, obsidian, and black glass, are nearly 

 opaque to light, but they allow 90 per cent, of radiant 

 heat to pass through them ; whereas a pale green glass, 

 coloured by oxide of copper,* covered with, a layer of 

 water, or, a very thin plate of aluni, will, although 

 perfectly transparent to light, almost entirely obstruct 

 the permeation of heat rays. 



* " The physical characters -of this species of glass, which 

 acts so differently from the other species of coloured glass in all 

 the phenomena of calorific absorption, are, 1st, its intercepting 

 almost totally the rays which pass through alum ; 2nd, its entirely 

 absorbing the red rays of the solar spectrum. I have already 

 stated that their colouration is produced almost entirely by the 

 oxide of copper. 



" Thus, the colouring matters of the coloured glasses, while 

 they so powerfully affect the relations of quantity which the dif- 

 ferent rays of ordinary light bear to each other, exercise no elec- 

 tive action on the concomitant calorific rays. This curious 

 phenomenon is the more remarkable as the colouring matters 

 absorb almost always a very considerable portion of the heat 

 naturally transmitted by the glass. The following are, in fact, the 

 calorific transmissions of the seven coloured glasses referred to ; 

 the transmission of the common glass being represented by 

 100; red glass, 82'5 ; orange, 72'5 ; yellow, 55; bluish-green, 

 07.5; blue, 52'5; indigo, 30; violet, 85. The quantity of 

 heat absorbed through the action of the colouring substances is, 

 therefore, 17.5 in the red glass, 27.5 in the orange, 45 in the 

 yellow, 42-5 in the green, 47*5 in the blue, 70 in the indigo, 

 and 15 in the violet. Now, as these absorptions extinguish a 

 proportional part of each of the rays which constitute the calo- 

 rific stream transmitted by common glass, they may be compared, 

 ns we said before, with the absorbent action exercised on light 

 by matters more or less deeply brown or dark, when they are 

 immersed in water, or some other colourless liquid which dis- 

 solves, but does not affect them chemically." Annales de Chimie 

 et de Physique, torn. xl. p. 382. 



Guided by these principles, the author selected the glass 

 employed in glazing the Royal Palm-House, at Kew Botanical 

 Gardens, where it was desired to obstruct the passage of those 

 rays which have a particular scorching influence. Of this glass 

 a description was given at the meeting of the British Association 

 at Oxford, v/hich appears in the Transactions for that year. The 

 result has been all that could be desired not a single instance of 

 scorching having occurred during the three years which have elapsed. 



