Heating of a Disk by Rotation in vacuo. 317 



by removing the black from the aluminium disk and leaving it a rough 

 metallic surface, when the indication afforded by the galvanometer is 

 reduced to about one-fourth of the amount with the blackened disk. 



1 1. 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. 



12. 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 infini- 

 tesimal ly 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 expe- 

 riments, when the air was very dry, there appeared to be a temporary 

 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 permanent 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 suppose that in the presence of sulphu- 

 ric 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 consti- 

 tutes 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 0*65 in. ; and also (art. 9) the effect for a vacuum of 

 0*37 in. is as large as for one of 0'60 in. ; and further, in some ap- 

 proximate 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 therefore be presumed that only a 

 very inconsiderable portion of the effect observed depends upon the 

 mass of air left behind. It would, however, appear, from the views 



