VOL. VII.J PHILOSOPHICAL TllANSACTIONS. 693 



same, as may be seen by two of his letters written to the editor from Cam- 

 bridge, Jan. 18 and 29, 1671-72 to this effect, viz. 



1. That for a fit metalline substance he would give this caution; that while 

 men seek for a white, hard and durable metalline composition, they resolve not 

 upon such a one as is full of sinall pores, only discoverable by a microscope. 

 For though such a one may, to appearance, take a good polish, yet the edges of 

 those small pores will wear away faster in the polishing than the other parts of 

 the metal ; and so however polished the metal may appear, yet it shall not reflect 

 with such an accurate regularity as it ought to do. Thus tin glass mixed 

 with ordinary bell metid makes it more white, and apt to reflect a greater quan- 

 tity of light ; but its fumes, raised in the fusion like so many aerial bubbles, 

 fill the metal full of those microscopical pores. But white arsenic both blanches 

 the metal, and leaves it solid without any such pores, especially if the fusion 

 has not been too violent. What the Stellate Regulus of Mars (which I have 

 sometimes used) or other such like substance will do, deserves particular exa- 

 mination. 



He adds this : that putty, or other such like powder with which it is 

 polished, by the sharp angles of its particles frets the metal if it be not very fine, 

 and fills it full of such small holes as he speaks of. Wherefore care must be 

 taken of that, before judgment be given, whether the metal be throughout the 

 body of it porous or not. 



'2. Not having tried many proportions of the arsenic and metal, he does not 

 affirm, which is absolutely best, but thinks there may conveniently be used any 

 quantity of arsenic, equalling in weight between a sixth and eighth part of the 

 copper, a greater proportion making the metal brittle. 



The way was this: he first melted the copper alone, then put in the arsenic, 

 which being melted, he stirred them a little together, taking care in the mean 

 time, not to draw in breath near the pernicious fumes. After this, he put in 

 tin, and again as soon as that was melted, which was very suddenly, he stirred 

 them well together, and immediately poured them off. He knows not, whe- 

 ther by letting them stand longer on the fire after the tin was melted, a higher 

 degree of fusion would have made the metal porous, but he thought the way he 

 proceeded to be the safest. He adds, that in that metal which he sent to 

 London, there was no arsenic, but a small proportion of silver; as he remem- 

 bers one shilling in three ounces of metal. But he thought withal, that the 

 silver did as much harm in making the metal soft, and so less fit to be polished, 

 as good in rendering it white and luminous. At another time he mixed arsenic 

 one ounce, copper six ounces, and tin two ounces : and this an acquaintance of 

 his has, as he intimates, polished better than he did the other. 



