M. E. Pringsheim on the Radiometer. 113 



layers is redly translucent. According to Tyndall*, a plate of 

 rock-salt coated with, lampblack, through, which not a glimmer 

 of the light from the most brilliant gas-flame could penetrate, 

 transmitted 38 per cent, of the rays emanating from a vessel 

 of boiling water. 



Besides the absorption, the internal heat-conductivity of the 

 substances made use of for radiometer-vanes is of exceeding 

 importance, since it tends to remove the difference of tempe- 

 ratures of the two sides, and consequently to weaken the most 

 important cause of the motion. Direct methodical experi- 

 ments on this subject have not yet, so far as I know, been 

 made, although differences in the action of metal and mica 

 radiometers have frequently been referred to the difference of 

 conductivity of the two substances. To investigate this more 

 closely, the simple way presented itself of making, in my 

 above-described apparatus, a series of similar experiments 

 with mica and a metal. First the same mica plates which 

 had served for the experiments on the influence of the glass 

 case were smoked on one side over a turpentine-oil flame, and 

 then introduced in the old way into the apparatus, after which 

 the latter was completely exhausted. On irradiating the clear 

 side with sunlight, it was so forcibly propelled against the 

 light that the spot of light which served as index fell quite 

 outside of the scale, and reading-off was impossible. At all 

 events the lampblack, heated by absorption of the rays trans- 

 mitted through the mica, acted just as if it had undergone 

 direct heating. 



On irradiation by the flame of a Bunsen burner, however, a 

 peculiar phenomenon presented itself. Namely, at the first 

 instant of the irradiation an energetic negative deflection oc- 

 curred, as the irradiated clear side moved towards the source 

 of light ; then a brief oscillation to and fro followed ; and after 

 that the mica plate gradually passed over to a fixed position 

 of rest corresponding to a positive deflection, consequently to 

 a receding of the irradiated side of the mica. If the irradia- 

 tion was then interrupted suddenly by the interposition of an 

 opaque screen, the same play repeated itself in the opposite 

 direction : first the vane was suddenly and energetically de- 

 flected with its blackened side going before ; it then swung 

 backwards and forwards for a few seconds ; and after that it 

 quite gently returned to its initial position of rest. 



The following are the corresponding positions of the spot 

 of light: — 



* Tyndall, ( Heat as a Mode of Motion,' German translation, 3rd ed 

 p. 407 



