Fourth Series. — Powdered Surfaces. 1 1*7 



vie with the diamond scratches on the salt surface, which 

 presented, under the microscope, an irregular furrow, pro- 

 bably nearly ^Q^dth of an inch in mean breadth, — I recurred 

 to the project of using the metals in powder. It was evident, 

 from the experiments on depolished and scored surfaces, that 

 the irregularity of these streaks had nothing whatever to do 

 with the phenomenon of checking rays of high refrangibility 

 and admitting others. Sand- paper scratches, 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 metallic, to which I attached very considerable 

 importance, for it was quite conceivable that the metallic sul- 

 phurets and other substances employed for the fictitious me- 

 tallic powders called gold, silver, and copper bronzes, might 

 have specific diathermancies which might injure the experi- 

 ment. I at length succeeded in obtaining silver by precipi- 

 tation, and copper from Daniell's battery; and with some 

 difficulty I procured from a large manufacturer coinage silver 

 and gold, reduced by mechanical trituration to a perfectly 

 impalpable and beautifully metallic powder. These expen- 

 sive preparations are now wholly superseded by the admirable 

 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 experi- 

 ments on metallic powders, which (with the exception of tin) 

 may be considered as perfectly impalpable, adhering to the 

 dry finger, and undoubtedly metallic. 



Per- 



