through different solid and liquid bodies. 333 



If we look at a luminous body through a very clear plate of glass, 

 we perceive but a very slight or insensible diminution of the clear- 

 ness of the body, whatever may be in other respects the force or in- 

 tensity of the liglit which emanates from it. The rays of solar light, 

 or those which issue from a glow worm, pass with the same facility 

 through a plate of glass or through any transparent medium what- 

 ever. It is not thus with respect to radiant heat : for we have just 

 seen that in regard to glass, Iceland spar, rock crystal, and in general, 

 all diaphanous bodies, the rays are transmitted in quantities which 

 diminish with the temperature of the source, so that they are all in- 

 tercepted when they proceed from a body heated to about 100° C. 

 There was then every reason to believe that this phenomenon de- 

 pended on the nature of the agent which constitutes heat, and that 

 consequently no body could exist which was truly diathermanous, 

 that is to say, a substance which acts upon rays of heat proceeding from 

 different sources, as every diaphanous medium acts upon luminous 

 rays whatever may be their origin. Rock salt has completely chang- 

 ed the received opinions on this head and established an unexpected 

 connection between those two great natural agents heat and light. 



The constant action of rock salt upon all sorts of radiant heat 

 may be usefully applied in a great many cases. Suppose we wish 

 to ascertain whether the rays of obscure heat are susceptible of re- 

 fraction, — place a receiver full of boiling water at a certain distance 

 from the thermo-electric pile and out of the direction of its axis ; 

 the rays emanating from the receiver cannot then enter the tube and 

 the galvanometer remains motionless ; but by placing a prism of 

 rock salt properly before the tube, we perceive the needle instantly 

 to leave its position of equilibrium, — proving that obscure heat is re- 

 jfracted like light. If we wish to propagate to great distances the 

 action of a hot body of small dimensions, — fasten it at the focus of 

 a lens of rock salt which refracts the radiant heat and sends out the 

 rays parallel to the axis, forming a true pharos of heat. The ope- 

 ration may be reversed when we wish to render sensible the rays 

 which proceed from a very feeble source, — ^the lens will then re- 

 ceive the rays and cause them to converge to the thermoscopic body. 

 Melloni has in this manner obtained by his instmment and by simple 

 air thermometers, very marked signs of heat from very distant ves- 

 sels containing warm water. In short, rock salt may be made to act 

 upon rays of heat either obscure or accompanied by light, in the 



Vol. XXVIl.— No. 2. 30 



