on the supposed Diathermancy of Rock- Salt. 425 



A thermopile is placed at a distance from the source of heat, 

 the radiation from which causes a deflection of the galvano- 

 meter-needle. This arrangement completed, the substances to 

 be examined are introduced between the source and the pile, 

 their different powers of absorption and transmission being 

 determined by the different values of the deflections. Taking 

 a single instance, it is assumed that a plate of ice ^ of an 

 inch thick absorbs all the incident radiation from copper heated 

 to 400° C. ; while a rock-salt plate of the same thickness 

 transmits 92*3 per cent, of the total radiation from the same 

 source of heat. That the ice does absorb the heat is proved by 

 the liquefaction of the substance ; but that 92"3 per cent, of 

 the total radiation passes through the salt is not, I think, equally 

 certain ; in fact the experiment points, not to diathermancy, 

 but to the unequal absorptive powers of the different substances 

 examined. 



In 1869* Professor Magnus endeavoured to account for the 

 diathermancy of rock-salt by saying :— " The great diather- 

 mancy of rock-salt does not depend on a small absorbing- 

 power for different kinds of heat, but upon the circumstance 

 that it emits only one kind of heat, and only absorbs this one, 

 and that almost all other bodies at a temperature of 150° C. 

 emit heat which only contains a small portion, or none at all, 

 of the heat which rock-salt emits." There will be little doubt 

 that this conclusion is based upon the doctrine of periods, 

 although the result of another experiment with fluor-spar, 

 noticed in the same paper, is fatal to that conclusion. 



It is less, I think, from an experimental point of view than 

 from unsuccessful attempts to explain why a solid substance 

 should be diathermanous, that any one would be led to doubt 

 the value of Melloni's conclusions ; and here I would state 

 that the experiments recorded in this paper, by which opposite 

 results have been obtained, were not originally suggested by 

 any apparent inefficiency in the mode of experiment adopted 

 by this great philosopher. 



The apparatus by which these results have been obtained is 

 as follows: — Two thermometers, each 3 inches long, with bulbs 

 ■jfa of an inch diameter, registering from 0° to 200° C. One 

 of these thermometers is enclosed in a rock-salt case 3^ inches 

 long, bored out so that the bulb stood at a distance of -j^ of 

 an inch from the salt ; the scale of the thermometer is plainly 

 visible through the salt case, which consists of two pieces, as 

 shown in lig. 1, afterwards cemented together with a thin film 

 of transparent glue. The sides of the case are -^ of an inch 



* Philosophical Magazine for November 1869. 



