94 



radiations from a bright fire or a mass of incandescent 

 metal are entirely obstructed by this medium. If we 

 place a lamp or a ball of glowing hot metal before a 

 metallic reflector, the focus of accumulated heat is soon 

 discovered ; but if a glass mirror be used, the light is 

 reflected, but not the heat ; whereas, with the solar rays, 

 but little difference is detected, whether vitreous or 

 metallic reflectors are employed. It is well known that 

 glass lenses refract both the light and heat of the sun, 

 and they are commonly known as burning-glasses : the 

 heat accumulated at their focal point being of the highest 

 intensity. If, instead of the solar beam, we employ, in 

 our experiments, an intense heat produced by artificial 

 means, the passage of it is obstructed, and the most 

 delicate thermometers remain undisturbed in the focus of 

 the lens. Glass exposed in front of a fire becomes 

 warm, and by conduction the heat passes through it, 

 and a secondary radiation takes place from the opposite 

 side.* It has been found that glass is transcalescent, or 

 diathermic, to some rays of terrestrial heat, and adia- 



4. The rays emitted by a hot body differ from each other in 

 their faculty "to pass through glass. 



5. A thick glass, though as much or more permeable to light 

 than a thin glass of worse quality, allows a much smaller quan- 

 tity of radiant heat to pass. The difference is so much the less as 

 the temperature of the radiating source is more elevated. 



C. The quantity of heat which a hot body yields in a given 

 time, by radiation to a cold body situate at a distance, increases, 

 cateris paribus, in a greater ratio than the excess of temperature of 

 the first body above the second. Journal de Physique, vol. Ixxv. 



* Sir David Brewster differs from the conclusions arrived 

 at by Delaroche. He thus explains his views : " The inability 

 of radiant heat to pass through glass, may be considered as a 

 consequence of its refusing to yield to the refractive force ; 

 for we can scarcely conceive a particle of radiant matter freely 

 permeating a solid body, without suffering some change in its 

 velocity and direction. The ingenious experiments of M. Prevost, 

 of Geneva, and the more recent ones of M. Delaroche, have been 

 considered as establishing the permeability of glass to radiant 

 heat. M. Prevost employed moveable screens of glass, and re- 

 newed them continually, in order that the result which he ob- 



