342 Messrs. Stewart and Tait on the Heating of a Disk, $c. [June 15, 



the indication afforded by the galvanometer is reduced to about one-fourth 

 of the amount with the blackened disk. 



11. It only remains that the heating effect proceeds from the disk, and 

 since the heat-indication afforded by the galvanometer-needle remains nearly 

 constant for some time, this effect must be due to the heating of the whole 

 substance of the disk. 



1 2. Presuming, therefore, that the entire substance of the disk is heated, 

 the next point is to ascertain the cause of this heating effect. 



Now, in the first place, it cannot be due to conduction of heat from the 

 bearings, for in some of the experiments the disk was insulated from its 

 bearings by means of a plate of ebonite, and the result was the same. 



Again, it is not due to revolution under the earth's magnetic force, for 

 Professor Maxwell has kindly calculated the effect due to this cause under 

 the conditions of the experiment, and he finds it infinitesimally small. Nor 

 is the effect due to the condensation of vapour of water upon the surface of 

 the disk. In some of the experiments, when the vacuum was newly made, 

 there appeared to be a strictly temporary effect, due probably to moisture, 

 which increased the range of the needle, but only during the time when the 

 motion was taking place, for it very soon assumed its permanent position. In 

 other experiments, when the air was very dry, there appeared to be a tempo- 

 rary cold effect of a similar description ; but in all cases when the vacuum was 

 kept long enough for the sulphuric acid to act, the only effect was a perma- 

 nent one in the direction of heat, and this is that which has been described in 

 these experiments. This permanent heating effect cannot, therefore, be due 

 to the condensation of aqueous vapour, and indeed it is impossible to sup- 

 pose that in the presence of sulphuric acid so much vapour should remain 

 suspended in air of so low a tension as to produce a permanent effect so 

 very considerable by its deposition. 



13. In this endeavour to account for the heating effect observed, it would 

 appear that we are reduced to choose between one of two causes, or to a 

 mixture of the two. 



(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 constitutes heat. 



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



14. Now, if it be an air effect, it is not one which depends upon the mass 

 of air. For (art. 7) the effect for a vacuum of 0'3 in. is as large as for one 

 of O65 in. ; and also (art. 9) the effect for a vacuum of 0'37 in. is as large 

 as for one of 0'60in. ; and further, in some approximate experiments, the 

 effect produced upon a wooden disk, in a vacuum of 4*0 in. and 2'0 in., was 

 found to be the same as in one of 0'5 in., or very nearly so. It may there- 

 fore be presumed that only a very inconsiderable portion of the effect ob- 

 served depends upon the mass of air left behind. It would, however, ap- 

 pear, from the views of Professor Maxwell and Mr. Graham, that there is 



