1873.] Heating of a Disk by rapid Rotation in vacuo. 309 



IV. " On the Heating of a Disk by rapid Rotation in vacuo" By 

 Balfour Stewart, M.A., F.R.S., Professor of Natural Phi- 

 losophy in Owens College, Manchester, and P. Gr. Tait, M.A., 

 Professor of Natural Philosophy in the University of Edin- 

 burgh. Received April 10, 1873. 



26. In two previous communications (Proc. Roy. Soc. June 15, I860, 

 and No. 88, 1866) to this Society, we gave an account of some experi- 

 ments which we had made upon the heating of a disk through rotation 

 in vacuo. In these experiments the increase of radiation of the heated 

 disk was observed by means of a delicate thermopile and galvanometer. 

 Three aluminium disks of various thicknesses and one ebonite disk were 

 used ; and the results derived from the experiments were as follows : — 



(1) The heatiug effect observed appeared to be independent of the 

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

 surrounding the disks. 



(2) The quantity of heat developed under similar circumstances of 

 rotation in three aluminium disks *05, '0375, *025 of an inch in thickness 

 respectively appeared to be the same, inasmuch as the relative thermo- 

 metric effect for these disks varied inversely as their thickness. 



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

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

 rary 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 noticeable during rotation, dis- 

 appearing the moment the motion was stopped. 



27. The experiments described in these communications were resumed 

 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 (pres- 

 sure 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 permanent nature, inasmuch as it re- 

 mained after three days. Unfortunately the exact increase was not 

 noted; but it is believed that the heat-indication became about three 

 times as great in consequence of the additional pressure. 



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

 0*12 in.), and at the suggestion of Professor Maxwell a sulphuric-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 



2 c 2 



