CALORIFIC TRANSPARENCY. 



(55 



Although we do not fear that heat and light can be 

 confounded in the mind, so different are their pheno- 

 mena, we have heat rays, as from dark hot iron, 

 which give no light, while in the full flood of the lunar 

 rays the heat is scarcely appreciable by the most delicate 

 instruments ; yet it is important to show how far these 

 two principles have - been separated from each other. 

 Transparent bodies have varied powers of calorific trans- 

 parency, or transcalescence : some obstructing the heat 

 radiated from bodies of the highest temperatures almost 

 entirely even in the thinnest layers ; whilst others will 

 allow the warmth of the hand to pass through a thickness 

 of several inches. Liquid chloride of sulphur, which is of 

 a deep red colour, will allow 63 out of 100 rays of heat 

 to pass, and a/ solution of carmine in ammonia, or glass 

 stained with oxides of gold, or copper, rather a greater 

 number ; yet these transparent media obstruct a large 

 quantity of light. Colourless media obstructing scarcely 

 any light, will, on the contrary, prevent the passage of 

 calorific rays. Out of every hundred rays, oil of turpen- 

 tine will only transmit 31, sulphuric ether 21, sulphuric 

 acid 17, and distilled water only 11. Pure flint glass, 

 however, is permeated by 67 per cent, of the thermic rays, 

 and crown glass by 49 per cent. The body possessing 

 the most perfect transparency to the rays of heat is diapha- 

 nous salt-rock, which transmits 92, while alum, equally 

 translucent, admits the passage of only 12 per cent.* 



* The following table of the rays penetrating coloured glass has 

 been given by Melloni, in his memoir On the Free Transmission of 

 Radiant Heat through Different Bodies : 



40 

 34 

 33 

 33 

 26 

 23 

 19 

 30. 



