M. E. Pringsheim on the Radiometer. 115 



It therefore appeared, as might have been foreseen, that by 

 the filtration of the light in passing through the glass plates 

 the first deflection, opposed to the action of the glass wall of 

 the apparatus, in comparison with the final deflection was 

 strengthened. 



Further, in all the observations the almost absolute accord- 

 ance in the magnitude of the first deflection, at the sudden 

 commencement and sudden cessation of the irradiation, is 

 remarkable. 



With respect to the explanation of these phenomena, there 

 cannot be any doubt that here two different and opposite 

 causes of motion confront each other, of which the first comes 

 in suddenly, the second gradually. 



The first, suddenly acting cause is the absorption in the 

 lampblack; the second, slowly appearing, is the heating of the 

 front (mica) face by conduction. Here, truly, the peculiar 

 case has happened, that that very substance which, as a bad 

 conductor of heat, we had selected in order as far as possible 

 to exclude the influence of conduction, shows this influence 

 with special distinctness. 



At the same time, however, this arrangement of the expe- 

 riment permits us to separate completely the action of the 

 absorption from that of the heat-conduction, because, in con- 

 sequence of the comparatively slight conductivity of mica, a 

 considerable time elapses before the effect of the conduction 

 commences. The process is doubtless as follows: — The rays 

 from the source of heat pass for the most part unabsorbed 

 through the thin plate of mica, and are strongly absorbed in 

 the coat of lampblack, in which, thin as it is, two sides are to 

 be distinguished — the front side, nearest to the source, and 

 the back side, furthest from it. The former absorbs more 

 strongly, and becomes in consequence more heated, than the 

 latter. Now, since the heat-conducting power of lampblack, 

 as Rumford* already showed by experiment and H. Weberf 

 concluded from his experiments on iron and German silver, is 

 extraordinarily little (according to Rumford, almost exactly 

 equal to that of sheep's wool), the heat of the front layer of 

 the lampblack is conducted away less by the lampblack than 

 by the mica ; so that after a time the front face of the mica 

 becomes warmer than the back of the lampblack. While this 

 back side of the lampblack is at first suddenly heated by ab- 

 sorption and therefore produces the sudden first deflection of 

 the vane, the heat conveyed to the mica by conduction causes 



* Phil. Trans. 1792, p. 42. See also Tyndall, * Heat' &c., German tr. ; 

 3rd ed. p. 276. 



t Pogg. Ann. cxlvi. p. 282 (1872). 



