552 



Prof. G. G. Stokes on certain 



[Dec. 20, 



involving considerable difficulty. We can do little towards classifying 

 the rays with which we are working unless we can form a pure spectrum. 

 A refraction-spectrum is the most convenient ; but the only substance 

 known which would be approximately suitable for forming the prism, 

 lens, &c. required for such a spectrum, and for confining liquids, is rock- 

 salt, of which it is extremely difficult to procure perfectly limpid speci- 

 mens of any size ; and even rock-salt itself, as Professor Balfour Stewart 

 has shown, is defective in transparency for certain kinds of radiant heat. 

 Then, again, the only suitable measuring-instrument for such researches, 

 the thermopile, demands a thorough examination with reference to the 

 coating to be employed for absorbing the incident radiation. Hitherto 

 lampblack has been used almost exclusively for the purpose ; and it is 

 commonly assumed, in accordance with certain of Melloni's results, that 

 lampblack absorbs equally heat-rays of all kinds. But the experiments 

 by which Melloni established the partial diathermancy of lampblack 

 prove that rays exist for the absorption of which that substance is un- 

 suitable. 



On calling on Mr. Crookes after the above was written, I was surprised 

 to find that all his mica radiometers behaved towards a heated glass shade 

 in the opposite way to that he had given me, going round positively in- 

 stead of negatively. Mr. Crookes showed me and gave me a specimen of 

 the kind of mica he employs, as eminently convenient for manipulation. 

 It is found naturally in a condition resembling artificially roasted mica. 

 Tt is not, however, quite so opaque for transmitted light, nor of quite 

 such a pearly whiteness for reflected light as that which has been arti- 

 ficially roasted at a high temperature. The mica radiometer that Mr. 

 Crookes first gave me, which I will call M 1? was, Mr. Gimingham told 

 me, the only one they had made with roasted mica. 



Mr. Crookes was so kind as to give me, for comparative experiment, 

 a mica radiometer, which I will call M 2 , made from the natural foliated 

 mica. It revolves a good deal more quickly than M x under the influence 

 of light ; it also gets more quickly under way, indicating that the mica is 

 thinner. When covered w T ith a hot glass it revolves positively, as already 

 remarked ; there is, however, but little negative rotation when the glass 

 is removed. 



The difference in the thickness and condition of the mica sufficiently 

 explains the difference of behaviour of M l and M 2 . Any radiant heat 

 incident on the white face that reaches the middle. of the mica, whether 

 it afterwards is absorbed by the mica or reaches and is absorbed by the 

 lampblack, tends to heat the second or blackened face more than the first, 

 and therefore conspires with the heat incident on the lampblack, and 

 absorbed by it, to produce positive rotation ; and the smaller thickness 

 and less fine foliation of the natural mica are favourable to the transmis- 

 sion of radiant heat to such a depth. 



