1873.] 



Disk by rapid Rotation in vacuo. 



311 



sent experiments the centre of the pile was made to tap the disk at a 

 distance of 1*5 in. from the rim. The amount and velocity of rotation 

 were represented b} r 30 turns of the handle, or 3750 turns of the disk, in 

 about 40 seconds. The following results were obtained, a result repre- 

 senting on an average somewhat more than four rotations : — 



Pressure of Heat-indication for 



residual gas, 



in inches. Dry hydrogen. Dry air. Dry carbonic acid. 



(A) 11-5 35-0 33-0 



(B) & . 7-5 15-0 15-0 



It would thus appear that the results derived by tapping are very 

 different from the radiation-results, inasmuch as in the former the effect 

 of the pressure and quality of the residual air is very apparent, while in 

 the radiation-results it is hardly perceptible. A probable explanation of 

 this will be given afterwards (art. 46) ; but in the mean time, in Aiew of 

 these results, it will be expedient to discuss them quite independently 

 and by themselves, with the view of ascertaining whether they can best 

 be explained by a gas-effect alone, or whether they likewise indicate a 

 residual effect independent of gas. 



34. With this object let us take i^thiS^ as representing the whole 



effect at a pressure of ^ in., due to whatever cause or causes. "We thus 

 obtain 



Dry hydrogen. Dry air. Dry carbonic acid. 

 "Whole effect at $ . . 9-5 25-0 24-0 



Again, let us suppose that (A)— (B) denotes the gas-effect for ^ in., and 

 we obtain 



Dry hydrogen. Dry air. Dry carbonic acid. 

 Gas-effect at 4-0 20-0 18-0 



Finally, let us regard as unknown residual effect the difference between 

 the whole effect and the gas-effect, and we obtain 



Dry hydrogen. Dry air. Dry carbonic acid. 

 Residual effect .... 5*5 5*0 6-0 



35. Similar experiments with the same galvanometer were made with 

 a disk of cartridge-paper, of which the pores were filled with solid 

 paraffin. And here we may mention that in all experiments with paper 

 disks a small wooden attachment was placed at some little distance be- 

 hind the disk and at the height of the pile, and against this the disk was 

 pressed during tapping — care being taken that the disk did not touch it 

 during motion, but only when it was pressed against it by bringing up 

 the pile. 



With this paper disk the amount and speed of motion were represented 

 by 30 turns of the handle, or 3750 turns of the disk, in 30 seconds. The 



