122 Professor Forbes's Researches on Heat. 



51. On the preceding table, I would observe, (1.) That the 

 pulverized crystalline 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 indiffer- 

 ence as a substance to the quality of the heat which it transmits 

 would at once leave the effect, if any, due to mechanical con- 

 dition, apparent. It even very evidently appears in this state 

 to transmit less freely heat of low than heat of high tempera- 

 ture. (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 opake 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 similar me- 

 chanical constitution. 



S3. 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 properties, to one of the same material mechanically 

 furrowed*. The contrariety of action of metallic powders 

 to those of opake earths, is as singular as it was unexpected. 

 I have already stated, however, my doubt whether a complete 

 investigation of the peculiarities of specific substances would, 

 at present, reward the necessary labour. I have made expe- 

 riments 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 shown to exercise no specific action 

 (34-.). The following table contains a few results not in- 

 cluded in preceding ones, and illustrating in several sub- 

 stances the quality of heat-colour 9 which in this paper we have 

 been considering. 



* To put this in the most clear point of view, I used and compared two 

 such plates in the same experiment. 



