1865.] of a Disk by Rotation in vacuo. 341 



tion produced by this exposu're being divided by the number of degrees by 

 which the disk was hotter than the pile, we have at once the value in terms 

 of the galvanometric scale of a heating of the disk equal to 1 Fahr. 



7. The following sets of experiments were made with blackened aluminium 

 disk and rock-salt in the cone. 



No. of ob- No. of 



servations turns of Heat Tension 



No. of in each Time at handle at indication. of air in 



set. set. full speed. full speed. Fahr. inches, 



I. 3 30 20 0-85 0-3 



II. 4 30 20-5 0-87 0'3 



III. 4 30 20 0-81 0-3 



IV. 3 30 20 0-75 0'65 



8. A piece of wood precisely similar to the rock-salt plate was next in- 

 serted into the fitting of the latter, and after rotation there was no indica- 

 tion whatever. Hence the above effect (art. 7) is due to radiant heat, and 

 not to currents of heated air reaching the pile. Again retaining the rock- 

 salt, the interior of the cone was covered by black paper, and the effect 

 upon the pile was very much diminished : this also goes to prove that the 

 effect (art. 7) is due to radiant heat ; and it now remains to discover whether 

 this radiant heat comes from the rock-salt or from heated air, or from the 

 surface of the disk. 



9. The following sets of experiments were made with blackened alumi- 

 nium disk, but without rock-salt. 



No. of ob- No. of 



servations turns of Tension 



No. of in each Time at handle at Heat of air iii 



set. set. full speed. full speed. indication. inches. 



V. 3 30 20 6-92 0-37 



VI. 3 30 20 0-93 0'60 



And when a black paper cover was introduced into the cone, other things 

 remaining as before, the indications of the galvanometer were greatly dimi- 

 nished. The effect produced without rock-salt is therefore also a radiant 

 heat effect ; and as the indications (in terms of temperature) are as large as 

 when rock-salt was used, we may conclude that the effect of art. 7 was to 

 no perceptible extent due to heating of the rock-salt, otherwise it would 

 have been diminished when the plate of rock-salt was taken away. Besides, 

 as rock-salt is a bad radiator and a good absorber of its own heat, the plate 

 would have had to be heated perhaps as much as 15 or 20, in order to 

 furnish a radiation equal to 0'8 from the disk. On both these accounts it 

 is impossible to believe that the effect was due to heating of the rock-salt. 



10. Nor is it probable that the heating effect is due to radiation from 

 heated air, since in order that nearly dry air of such a tenuity might 

 give such a radiation, it would require to be heated enormously. But 

 another proof that the effect is not due to air is afforded by removing the 

 black from the aluminium disk and leaving it a rough metallic surface, when 



