56 M. MELLONI ON THE IMMEDIATE TRANSMISSION 



This apparatus being placed horizontally on a table, the centre C was 

 brought within a little distance of the bottom of a vertical prism(N) of 

 rock salt, so that when the ruler C D was properly placed the refracted 

 parcel of hot rays fell on all the points of the linear pile. 



By establishing the electric communications with the galvanometer 

 and moving the ruler over the graduated arc, the point at which the 

 deviation of the magnetic index attained its greatest value was easily 

 determined. The radiating source was then changed while everything 

 else was allowed to remain in the same state. We had now a calorific 

 action more or less intense than the preceding; but in order to obtain 

 the maximum of effect it was necessary to slide the ruler in one direction 

 or the other. Thus, for instance, when I commenced the experiment 

 with the incandescent platina, that is, when I had found the correspond- 

 ing position of the pile that gives the greatest galvanometric deviation, it 

 was necessary to move the ruler about two lines towards B, on the side 

 to which the 7nost refrangible rays are directed, if I substituted the 

 Locatelli lamp for the platina. But if I substituted for the platina a 

 plate of copper heated to 390° I was obliged to slide the ruler three lines 

 towards A, in the direction of the less refrangible rays. The action of 

 the boiling water in this experiment was too feeble to be compared with 

 that of any of the three other sources. 



The refraction and constant transmission of the calorific rays through 

 the rock salt being placed beyond the possibility of doubt, we imme- 

 diately see the use that may be made of this substance in investigating 

 the nature of radiant heat. If, for instance, it is proposed to propagate 

 to great distances the action of a heated body of small dimensions, we 

 are now certain that we have only to place the body at the focus of a 

 lens of rock salt, which will refract the calorific rays and make th(!m form 

 a real pharos of heat by issuing in a direction parallel to the axis. Is 

 it desired that extremely feeble rays emanating from any source should 

 be rendered perceptible? Let them be received on a lens of this sub- 

 stance having a thermoscopic body placed in its focus. In this manner 

 we may, Avith the aid of an ordinary differential thermometer with small 

 balls, obtain very decided indications of the heat issuing from a vessel 

 filled with tepid water and placed at a great distance. In short, rock 

 salt formed into lenses and prisms acts upon calorific rays in a manner 

 perfectl}^ analogous to that in which optical instruments act upon lumi- 

 nous rays. It constitutes then the trne glass of radiant heat, and there- 

 fore the only glass that should be employed in appreciating the effects 

 of its intensity. All other transparent bodies are but partial and in- 

 complete transmitters of heat, totally intercepting calorific rays of a 

 cei'tain kind. It is easy to conceive, from these considerations, with 

 Avhat serious disadvantages those persons have had to contend who have 

 undertaken to investigate the composition of solar heat with common 



