NATURAL PHILOSOPHY. 173 



position exposed to the pile, and having a temperature higher 

 than that of the pile by a known amount. The deflection pro- 

 duced by this exposure being divided by the number of degrees 

 by which the disc was hotter than the pile, gives at once the value 

 in terms of the galvanometric scale of the heating of the disc 

 equal to 1° on Fahrenheit's scale. 



The disc of aluminum being blackened with a coating of lamp- 

 black, applied by negative photographic varnish, and rock salt 

 inserted in the cone, the following results were obtained : — 



To ascertain whether the radiant heat recorded was derived 

 from the rock salt, or from heated air, or from the surface of the 

 disc, the next series of experiments were tried : — 



EXPERIMENTS WITH BLACKED ALUMINUM DISC WITHOUT ROCK SALT. 



Time at Heat indications 

 full speed. Tahronheit. 



30 0.92 



30 0.93 



With certain modifications of the above experiments it was sat- 

 isfactorily proved that the effect was not due to heating of the 

 rock salt, or to radiation from heated air ; it must therefore be due 

 to the disc of aluminum, which seemed to have rubbed against 

 some matter which remained in the receiver alter the air was re- 

 moved. The question being "Was this ether?" the experiment- 

 ers further state that : — 



1. It may be due to the air which cannot be entirely got rid 

 of. 



2. It is possible that visible motion becomes dissipated by an 

 ethereal medium in the same manner, and possibly to nearly the 

 same extent, as molecular motion, or that motion which consti- 

 tutes heat. 



3. Or the effect may be due partly to air and partly to ether. 

 Not to leave the matter wholly undecided, it was suggested by 



Professors Maxwell and Graham that there is another effect of air, 

 namely, fluid friction, the coeflicie\it for which they believe to be 

 independent of the tension. 



It would appear, however, that the fluid friction of hydrogen is 

 much less than that of atmospheric air, so that were the heating 

 effect due to fluid friction, it ought to be less in a hydrogen vac- 

 uum. An experiment proved that the heating effect due to rota- 

 tion in a hydrogen vacuum was 22.5, while in an air vacuum it 

 was 23.5, and the authors are inclined to consider these numbers 



15* 



