EXPANSION OF BODIES BY HEAT. 95 



t/iemic, or opaque for heat, to others* that the capability 

 of permeating glass increases with the temperature of 

 the ignited body and that rays which have passed one 

 screen traverse a second more readily. It would, 

 however, appear that something more than a mere 

 elevation of temperature is necessary to give terrestrial 

 heat-radiations the power of passing through glass 

 screens, or, in other words, to acquire the properties of 

 solar heat. 



To give an example. The heat of the oxy-hydrogen 

 flame is most intense, yet glass obstructs it, although it 

 may be assisted by a parabolic reflector. If this flame 



tained might not be ascribed to the heating of the screen ; but 

 such is the rapidity with which heat is propagated through a 

 thin plate of glass, that it is extremely difficult, if not impossible, 

 to observe the state of the thermometer before it has been 

 affected by the secoudary radiation from the screen. The method 

 employed by M. Delaroche, of observing the difference of effect, 

 when a blackened glass screen and a transparent one were made 

 successively to intercept the radiant heat, is liable to an obvious 

 error. The radiant heat would find a quicker passage through 

 the transparent screen; and, therefore, the difference of effect 

 was not due to the transmitted heat, but to the heat radiated 

 from the anterior surface. The truth contained in M. Delaroche's 

 fifth proposition is almost a demonstration of the fallacy of all 

 those that precede it. He found that ' a thick plate of glass, 

 though as much or more permeable to light than a thin glass of 

 worse quality, allowed a much smaller quantity of radiant heat 

 to pass.' If he had employed very thick plates of the purest 

 flint glass, or thick masses o*f fluid that have the power of trans- 

 mitting light copiously, he would have found that not a single 

 particle of heat was capable of passing directly through trans- 

 parent media." Sir D. Brewster, On new properties of heat as ex- 

 hibited in its propagation along platen of glass. Philosophical 

 Transactions, vol. cvi. p. 107. 



* Proposal of a Nfw Nomenclature for the Science of Calorific 

 Radiations, by M. Melloni. Bibliotheque Universelle de Geneve, 

 No. 70. Scientific Memoirs, vol. iii. part 12. Many of the terms, 

 as Diaihermasy, or transparency for heat ; Adiathermasy, opacity 

 for heat; Thermochroic, coloured for heat, and others, are valuable 

 suggestions of forms of expression which are required in dealing 

 with these physical phenomena. 



