220 Royal Society : — Messrs. Stewart and Tait on the 



(3) Besides the heating epect alluded to in (1) and (2), there 

 was found to be, when the vacuum had been recently made, a strictly- 

 temporary effect, sometimes in the direction of heat, sometimes in 

 that of cold, owing probably to the condensation or evaporation of 

 small quantities of aqueous vapour ; but this effect was only notice- 

 able during rotation, disappearing the moment the motion was 

 stopped. 



27. The experiments described in these communications w r ere re- 

 sumed in 1870. In the interval an addition had been made to the 

 apparatus, in virtue of which an ordinary carbonic-acid vacuum 

 might be subjected to the influence of a vessel containing potash 

 allowed to open in it, and thus to absorb as much as possible of the 

 remaining gas. 



On May 4 a carbonic-acid vacuum was obtained by this means 

 (pressure 0*05 in.). A disk made of cartridge-paper, when made 

 to rotate in this vacuum, gave a very perceptible result. Carbonic 

 acid was then allowed to enter the vacuum until the pressure 

 became 0*65 in. The consequence of this increased pressure was in 

 this instance an increase in the effect, which was probably of a per- 

 manent nature, inasmuch as it remained after three days. Unfor- 

 tunately the exact increase was not noted ; but it is believed that 

 the heat-indication became about three times as great in conse- 

 quence of the additional pressure. 



28. On May 11 another carbonic-acid vacuum was made (pres- 

 sure 0-12 in.), and at the suggestion of Professor Maxwell a sul- 

 phuric-acid gauge was placed in the receiver. A rotation was then 

 made ; and the result of the rotation was a hardly perceptible rise 

 in the sulphuric-acid gauge. We may therefore imagine that the 

 residual air was not greatly heated. 



29. On May 1 6 another carbonic-acid vacuum was obtained (pres- 

 sure 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 in- 

 crease of pressure of the residual air produced little or no effect. 



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

 cuum, pressure # 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 without 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 radiation, 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. 



