1866.] 



of a Disk by rapid Rotation in vacuo. 



297 



its usual amount ; but the heating effect was as nearly as possible the 

 same in both cases, being 32 divisions of the scale in both. 



On all these grounds it would appear that the heating effect cannot be 

 due to vibration of the disk. 



21. It is hardly necessary to mention that the heating effect cannot be 

 due to radiation and convection from the wheelwork, which is no doubt 

 slightly heated during the experiment, for the mass of this metallic matter 

 is so great, that we cannot imagine it to be heated more than 1° Fahr, 

 Now the radiation from this against the back of the disk may certainly be 

 neglected, while the convection must be very small, since in the experi- 

 ments the pressure of the air was very small. Besides, the heating effect, 

 as will be seen shortly, was found to be independent of the pressure. 



,22. It has thus been shown that the disk is heated during the experi- 

 ment, and that this heating effect — 



(1) Is not due to rotation under the earth's magnetic force ; 



(2) Is not due to conduction of heat from the bearings ; 



(3) Nor to radiation or convection from the wheelwork ; 



(4) Nor to vibrations of the disk. 



And in view of the large and constant nature of this heating effect it may 

 be asserted that it cannot be sensibly due, either to one of these causes 

 singly, or to their combined effect. 



23. It will now be shown that the heating effect is independent both 

 of the density and chemical constitution of the residual air and vapour 

 around the disk. 



In art. 16, if we compare together the 7th, 8th, 9th, 10th, 11th, and 

 12th sets of experiments, we shall see that the heating effect was sensibly 

 the same, whether the residual gas was atmospheric air, or hydrogen cr 

 coal-gas. 



As hydrogen diffuses very quickly, it might perhaps be supposed that 

 when heated by rotation it might find access to the pile through the rock- 

 salt cover more easily than heated atmospheric air, so that while the whole 

 effect might appear the same in hydrogen as in air, yet only part of that 

 in hydrogen mJght be due to radiant heat, the remainder being due to 

 heated gas which had obtained access to the pile. This was, however, dis- 

 proved by an experiment, which showed that by blackening the interior 

 of the cone, the effect upon the pile was just as much diminished in a 

 hydrogen vacuum as in an air vacuum ; and hence in both cases the whole 

 effect is due to radiant heat. 



But, besides the residual gas, it may with truth be supposed that there 

 is always more or less of aqueous vapour, and also a little of the vapour of 

 oil, and perhaps of the vapour of mercury in the receiver. iVs regards the 

 hygrometric state of the residual air and its influence on the disk, this would 

 appear to be of the following nature : — 



(1) When the vacuum has just been made, there is generally a hygro- 

 metric difference between the air and the surface of the disk, on account of 



