FOURTH SERIES. POWDERED SURFACES; 



23 



talline bodies, such as rock-salt, alum, citric acid, and sulphur, exhibit no decided 

 tendency to transmit an excess of heat of low temperature, depending on their 

 powdery form. The carefully repeated experiment with rock-salt is, on this 

 point, very conclusive, since its indifference as a substance to the quality of the 

 heat which it transmits would at once leave the effect, if any, due to mechanical 

 condition, apparent. It even very evidently appears in this state to transmit less 

 freely heat of low than heat of high temperature. (2.) Galena, the crystallized 

 sulphuret of lead, in fine powder, appears to possess the qualities of gold, silver, 

 and tin (43.) (3.) Red lead, charcoal, chalk, and magnesia, all substances in 

 an opaque earthy condition, appear certainly to transmit an excess of Dark Heat. 

 I think it probable that this list might be extended to most bodies having a simi- 

 lar mechanical constitution. 



53. These distinctions, I am well aware, leave the causes of the difference of 

 character of powders, and the peculiarities of tarnished surfaces, nearly in the 

 same obscurity as before. In particular, I cannot but regard it as being singular, 

 that a surface covered with powdered salt has no analogy, but even opposite pro- 

 perties, to one of the same material mechanically furrowed.* The contrariety 

 of action of metallic powders to those of opaque earths, is as singular as it was 

 unexpected. I have already stated, however, my doubt whether a complete in- 

 vestigation of the peculiarities of specific substances would, at present, reward 

 the necessary labour. I have made experiments on a few fibrous substances, as 

 paper and membrane, which I thought might very probably act as tarnished sur- 

 faces do. There is an approximation to this, as will be seen, in the common 

 cambric or tissue-paper. In the kind of tracing-paper employed (which is made in 

 Paris, I believe, under the name of papier vegetal), there is evidently some foreign 

 matter introduced to produce the transparency, which modifies the transmission. 

 A close reticulation of cotton fibres has already been shewn to exercise no speci- 

 fic action (34.). The following Table contains a few results not included in pre- 

 ceding ones, and illustrating in several substances the quality of heat-colour, 

 which in this paper we have been considering. 



Per-centage of Heat transmitted by several Bodies. 



1 To put this in the most clear point of view, I used and compared two such plates in the same 

 experiment. 



