232 Royal Society. 



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 might be due to radiant heat, 

 the remainder being due to heated gas which had obtained access to 

 the pile. This was, however, disproved 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. 

 As 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' 

 hygrometric difference between the air and the surface of the disk, on 

 account of which there is a strictly temporary effect, either in the 

 direction of heat or cold, at the surface of the disk, owing probably to 

 condensation or evaporation of small quantities of aqueous vapour ; 

 but this effect disappears the moment the motion is stopped, leaving 

 behind the permanent effect apparently unaltered. 



(2) This temporary effect disappears when the disk has been left 

 for some hours in the vacuum. 



Next, with regard to vapour of oil, we cannot suppose its effect to 

 be so large or so different in character and constancy from that of 

 aqueous vapour as to account for the effect observed. Add to this 

 that the effect takes place with an uncoated metallic disk probably 

 to the same extent as with a coated one. The same remark may be 

 made with regard to vapour of mercury. The effect would therefore 

 appear to be independent of the chemical nature of the residual gas 

 and vapour around the disk. 



In order to prove that this effect is also independent of the pres- 

 sure of the residual gas, it is only necessary to refer to the whole 

 body of experiments which have been described, to see that between 

 4 inches and 0*25 inch there is no perceptible variation in the effect 

 observed. 



24. The following generalization may now be made : — 



(1) If a perfectly true aluminium disk (without vibrations) be 

 made to rotate in a vertical plane at the earth's surface, after the 

 manner herein described, there will be an increase of the tempera- 

 ture of the disk, which is not due to communication of heat from 

 the bearings or machinery, nor to the earth's magnetic force. 



(2) This heating effect is independent of the density and chemical 

 constitution of the residual air and vapour which surround the disk. 



(3) It is probable that the quantity of heat developed in disks of 

 similar extent of surface and similar circumstances of motion is the 

 same. For, in the first place, the quantity of heat developed in three 

 aluminium disks, '05, '0375, -025 of an inch in thickness respect- 



