203 



colour and fine earthy appearance of the fracture, bring to 

 mind the case recorded by Sir David Brewster, of a piece 

 of smoky quartz, the fracture of which was absolutely 

 black, and yet was quite transparent to transmitted light, 

 and whose blackness, he found, arose from the surfaces 

 of fracture, consisting of a fine down of short and slen- 

 der filaments of transparent and colourless quartz, the dia- 

 meter of which was so small (not exceeding the one-third 

 of the millionth part of an inch), that they were incapable 

 of reflecting a single ray of the strongest light. In describing 

 this. Sir David Brewster predicted, that " fractures of quartz 

 and other minerals would yet be found which should exhibit 

 a fine down of different colours depending on their size." 



It seems, therefore, extremely probable, that the cause of 

 the near approach to blackness in the fracture and filings of 

 this alloy, arises from the excessive minuteness of its parti- 

 cles, and thus fulfils the foregoing prediction ; the brownish 

 tinge being produced by the reflexion of a little red light.* 



The polish and power of reflecting hght of the anomalous 

 alloy are not quite so great as those of the normal, but are 

 still remarkable ; and, as it seemed a matter of some interest 

 to determine whether both reflected the same quantity or in- 

 tensity of light at equal angles, the author endeavoured to 

 ascertain this point as respects heat, by means of Melloni's 

 pile for the galvanometrical determination of temperature, 

 assuming, as suggested to him by Professor Mac CuUagh, 

 that what would be true of heat in this respect, would also be 

 so of hght ; but from the small size of the reflecting surfaces 

 he had at his command, he found it impossible to arrive at 



• Since this paper was read, Professor Lloyd suggested to the author, the ana- 

 logy between the appearance of the powder and filings of the anomalous alloy and 

 Platina Mohr, and those powders obtained by reduction of other metals by hydro- 

 gen. None of these, however, are coherent, which constitutes the peculiarity in 

 the present case. 



VOL. ir. R 



