FOURTH SERIES. POWDERED SURFACES. 21 



tried in different specimens, the powder more or less thickly strewed, and at dif- 

 ferent times, 



46. One circumstance in particular raised a doubt as to the result of my 

 former conclusion, where it seemed most incontrovertible. I had argued, that 

 if alum in powder arrested equally all kinds of heat, the mechanical action of 

 the powder must have opposed and destroyed the specific action of the alum (36.) 

 I was gradually, however, led to admit, that, in the state of powder, most diather- 

 manous bodies are almost equally opaque, or, rather perhaps, I should say, equally 

 indifferent to the kind of incident heat (i. e. colourless in optics). 



47. So far as the eye could judge of the proportion of obstacles in sur- 

 faces strewed with different kinds of powders, there did not seem any very 

 marked peculiarity in their transparency for heat. A surface dusted with alum 

 or citric acid appeared to transmit nearly as much as one strewed with powdered 

 rock-salt. Nor could this arise merely from the minute thickness of the substance, 

 which is well known to produce in heat, as in light, an approximation to a Colour- 

 less character ; for the proportion stopped by the powder was always a large 

 fraction (usually from fths to ^ths) of the incident heat. The opacity, then, is 

 the result of the innumerable reflections and interferences which scatter and stifle 

 the transmitted heat ; and this is almost equally effectually done, whatever be 

 the nature of the substance. On reflection, therefore, this general result does hot 

 appear surprising. I will quote one experiment, in particular, in illustration of it. 



48. When I was at a loss to procure fine metallic fibres, I thought of em- 

 ploying a diaphragm irregularly covered with fine threads of spun glass, with a 

 view (just as in the case of the alum powder) of ascertaining how far the me- 

 chanical condition of the glass might modify its well known qualities with re- 

 spect to the transmission of heat. When Locatelli lamp-heat, having been trans- 

 mitted by thick plate-glass, fell upon the spun-glass fibres, forming an irregularly 

 reticulated diaphragm, no more than 47.5 per cent, of the incident heat was trans- 

 mitted. Now, Ave know perfectly from the experiments of DE LA ROCHE and 

 MELLONI, that, after passing through such a thickness of plate-glass, an addi- 

 tional film, the thickness of the glass fibres used, would produce no sensible re- 

 sistance to the farther passage of the heat, excepting only its superficial reflec- 

 tion. The loss of 52.5 per cent, of the heat was therefore due to the scattering and 

 stifling of heat by Reflection at the surfaces of the fibres, Refraction through their 

 cylindric surfaces, and Interference. We cannot, therefore, be surprised, if the 

 refracted part of the heat reaching the pile (the only portion very materially 

 affected by the nature of the medium) should not greatly alter the quantity of 

 different sorts of heat indicated by the galvanometer. Accordingly, we find, that 

 heat from a dark surface of brass warmed by an alcohol lamp, had 44 per cent, 

 transmitted under the same circumstances ; and even hot water had 42 per cent, 

 although a small thickness of glass is sensibly opaque for that kind of heat. 



VOL. xv. PART i. F 



