FOURTH SERIES. POWDERED SURFACES. 



19 



than which nothing can be more irregular, produced the effect, and that more 

 intensely as the surface became more coarsely and closely furrowed. Nay, it 

 occurs in natural tarnish, where there can be no linear arrangement of the points 

 affected. It seemed to me, therefore, that a surface covered with a metallic- 

 powder, presented the limit of a grating where the interstices were not required 

 to have any regular form. 



40. The next difficulty was to obtain impalpable powders indubitably metal- 

 lic, to which I attached very considerable importance, for it was quite conceivable 

 that the metallic sulphurets and other substances employed for the fictitious 

 metallic powders called gold, silver, and copper bronzes, might have specific 

 diathermancies which might injure the experiment. I at length succeeded in 

 obtaining silver by precipitation, and copper from DANIELL'S Battery ; and with 

 some difficulty I procured from a large manufacturer coinage silver and gold, re- 

 duced by mechanical trituration to a perfectly impalpable and beautifully metallic- 

 powder. These expensive preparations are now wholly superseded by the admi- 

 rable fictitious bronzes in use in the arts. These, together with metallic copper- 

 bronze, perfectly impalpable, furnished by the same individual, and a much coarser 

 tin powder used by druggists, formed the material of a very careful series of ex- 

 periments, which I extended over a very considerable period, and varied in a 

 great many ways. (1840, Jan. 28, &c.) 



41. The following table contains the results of my experiments on Metallic 

 Powders, which (with the exception of tin), may be considered as perfectly im- 

 palpable, adhering to the dry finger, and undoubtedly metallic. 



Per-centage of Heat from different Sources transmitted by Metallic Powders. 



42. These observations are confessedly very imperfect. I am persuaded, 

 however, that the apparent anomalies are not errors of observation ; other in- 



