310 



Messrs. Stewart and Tait on the Heating of a [May 15, 



gauge. "We may therefore imagine that the residual air was not greatly 

 heated. 



- 29. On May 16 another carbonic-acid vacuum was obtained (pressure 

 0*08 in.), and with an ebonite disk of about inch thickness, the heat- 

 indication was 16. Carbonic acid was introduced until the pressure was 

 1*6 in. ; but the indication was only 18. This result is in conformity 

 with our previous experiments, in which an increase of pressure of the 

 residual air produced little or no effect. 



30. The ebonite disk was likewise tried in a carbonic-acid vacuum, 

 pressure 0*04 in. ; and also in one, pressure 0*02 in., which was the 

 lowest we could obtain. The result of these experiments appeared to 

 show that in all probability the cartridge-paper disk radiated more than 

 twice as much as the thin ebonite disk. The experiments were put a stop 

 to by a collapse of the glass receiver during rotation, fortunately with- 

 out injuring any one present. 



31. In June 1871 the experiments were resumed. In the mean time 

 Mr. Beckley had fitted the apparatus with an arrangement working 

 through a barometer-tube, by means of which, instead of trusting to ra- 

 diation, the disk itself might, after rotation, be tapped by means of the 

 pile, which could be brought up to it and then withdrawn. By this 

 means a much larger effect might be obtained ; and it became possible, by 

 varying the adjustment, to find according to what law the heat effect 

 varies with the distance from the centre. 



32. These experiments were conducted in the following manner: — 

 The disk was first of all tapped before rotation several times ; at each 

 tapping the momentary swing of the needle was recorded, and the mean 

 of the readings was regarded as indicating the state of the disk with re- 

 spect to heat. 



The disk was next tapped after rotation, and the difference between 

 the readings before and after was taken as indicating the change in the 

 state of the disk produced by rotation. In the later experiments the disk 

 before rotation was kept in slight motion in order to equalize any ten- 

 dency to unequal heating of its various parts ; but this was not done in 

 the experiments of June 1871. In these experiments (June 1871) the 

 disk was of brown paper, and the results obtained were as. follows : — For 

 a carbonic-acid vacuum (pressure 0-065 in.) a swing of 307 divisions was 

 recorded, w T hile for a hydrogen vacuum (pressure 0*150 in.) a swing of 

 281 divisions was recorded. Each result was the mean of three rotations. 



33. The next experiments were made in January 1872. The galvano- 

 meter was one of Thomson's, but more adapted for battery currents, and 

 hence not in a very delicate state for these experiments ; the time of 

 vibration of the needle was 3 seconds. The disk used was of ebonite 

 (thickness about in.). The ebonite of this disk was completely black ; 

 and in this respect, as well as in being thinner, it differed from the 

 ebonite disk first used in the radiation-experiments (art. 17). In the pre- 



