384 THIRD REPORT — 1833. 



Since the rays which have been transmitted by citric acid and 

 some other substances are those only of greatest refrangibility, 

 these have, relatively to most substances, a great penetrating 

 power. In the remarkable case of green glass, where rays of 

 a low degree of refrangibility are most easily transmitted, the 

 reverse shovdd be the case. Accordingly, out of 100 parts of 

 heat transmitted by citric acid, white-, violet-, red-, orange-, yel- 

 low-, blue-, and indigo-coloured glass transmitted vai'ious quan- 

 tities of heat, varying from 89 to 28 parts ; whilst two specimens 

 of green glass only transmitted 6 and 2 parts. What is most re- 

 markable is, that while for all the glasses but green the trans- 

 missive power is three times greater than when the heat of in- 

 candescent platinum was directly incident upon them, the effect 

 with that particular glass descended from 23 or 24 per cent, 

 nearly to 0. The reason is obvious ; for the whole of the rays 

 of low refrangibility emitted by the platinum, and for which 

 alone the green glass was transparent, had been stopped by the 

 interposition of the plate of citric acid, which had, as it were, 

 sifted it free from these rays. 



Hence M. Melloni concludes that green glass is the only kind 

 which has coloration for heat, if we may use the expression ; 

 the others acting upon it only as a more or less transparent 

 glass of uniform tint does upon light. 



On Thermo-Electricity. By John Prideaux. 



In examining the theory of Thermo-electricity, Mr. Prideaux 

 satisfied himself — 



1. That thermo-electricity is conveyed through long wires 

 with the same promptitude as common or voltaic electricity ; 

 and that it does not contain caloric as an element, differently 

 from those principles : — 



2. That bringing two heated wires into contact in the bulb 

 of a large air thermometer, so as to generate a thermo-electric 

 current there, does not accelerate the cooling of the wires, and 

 thus that caloric is not consumed in its production ; nor does a 

 thermo-electric current become developed when caloric is ren- 

 dered latent, as in the fusion of a metal : — 



3. That making the surfaces of a thermo-electric pair iden- 

 tical, by tinning them all over before contact, does not impair 

 their powers (but rather the reverse) ; and that large plates of 

 copper and tin, or tin plate, heated together, separated from 

 metallic contact only by fine gauze, or a few threads spread over 

 the surface, will not produce the slightest deviation of the 

 needle, although metallic communication being made between 



