* 

 FOURTH SERIES. POWDERED SURFACES. 17 



The difference here seems imperceptible, the differences, such as they are, being 

 in opposite directions. The results in the last column are from single experi- 

 ments (November 28. 1839). 



35. Action of Powders. Adhering to the idea (12) that the action of a smoked 

 surface was due to the mechanical action of a number of minute opaque points 

 distributed over a transparent body, it occurred to me almost at the commence- 

 ment of these experiments, to try the effect of powders artificially sifted on such 

 a surface. Any ingredient, however, which could make the powders adhere to 

 the surface, would have vitiated the experiment, by introducing its own proper 

 diathermancy. I therefore included the powders between two polished plates of 

 rock-salt, closed at the edges with wax. The preliminary experiment (27), to 

 shew that the salt surfaces, in the state in which I commonly employed them, 

 exercise no perceptible influence on the quality of the transmitted heat, was 

 evidently a very important one for the conclusions I meant to draw. It was, as 

 I have stated, quite satisfactory. 



36. The first experiments which I made with powders (December 6. 1839), were 

 with Chalk and Alum, finely dusted between two plates of salt. I selected the 

 chalk on account of its absolutely uncrystalline and opaque character ; and alum, 

 because its power of stopping rays of heat of low temperature was so very 

 great, that I judged that if the influence as a mechanical modifier of surface 

 should prove predominant, and allow as much, or more, heat of low than of high 

 temperature to pass, the mechanical influence of a substance in fine powder would 

 be clearly established. 



37. Now, the result at which I arrived, and which was entirely conformable 

 to my anticipation, may serve to shew the caution requisite in drawing conclu- 

 sions from limited data, however apparently conclusive. The surfaces powdered 

 with chalk suffered rather more heat of low than of high temperature to pass 

 (viz. 34.5 per cent, dark heat, and only 30.5 of heat from LOCATELLI lamp, trans- 

 mitted through a thick glass-lens), whilst the salt strewed with alum appeared quite 

 indifferent to the kind of heat incident,* (transmitting only 17 per cent, of both, 

 thus shewing that the powder Avas in considerable quantity). I concluded, therefore, 

 with apparent reason, that the chalk having no specific action, or being (most pro- 

 bably) opaque or athermanous, the powder of it acting mechanically, allowed low- 

 temperature-heat to pass in excess, whilst in the case of alum, the specific action 

 was entirely counteracted by the mechanical action of the powder. I simply stated 

 the fact amongst others detailed in the preceding pages, in aMemorandum presented 

 to the Royal Society of Edinburgh on the 16th December 1839,f and a few days after, 

 in a slightly different form, communicated to M. ARAGO, and printed in the Comptes 

 Rendus de I'Academie des Sciences, 6th January 1840. On the 28th December, I 



* Yet an alum plate of a certain thickness transmits no less than 27 per cent, of the one kind 

 of heat, and no sensible portion of the other (MELLONI). 

 t See note page 1 of this paper. 



