HEFLECtlNG TELESCOPE. 



S3 



except those of copper, brass, tin, silver, and arsenic. I tried 

 no semi-metal, except the lattei", which whitens copper, and 

 unites intimately with it: because it is stated, in the treatise of 

 the j^rt of Assaying, by the observant and accurate Cramer, 

 that a,ll the semi-metals rise in flowers, during the fusion : 

 which would certainly make the metal porus. On this ac- 

 count, I would have rejected the brass, because of the zinc 

 contained in it ; but that it seemed to render the composition 

 whiter, and less apt to tarnish, than it would be without it. It 

 will have little tendency to rise in flowers, if the speculum-me- 

 tal be fused, with the lowest heat requisite, and if the brass be 

 of the best kind ; because, in this, the zinc is more perfectly 

 united with the copper, and both are purer. I used, for this 

 purpose, the brass of pin-wire : and, because the quantity of -pj^^^j.^^^^ 

 it was only the one eighth part of the copper employed, which, 

 I imagined, would receive too fierce a heat, if put alone into 

 the melted copper ; I first added to the brass, in fusion, about 

 an equal quantity of the tin, and put the mass cold into the 

 melted copper ; supplying afterwards the remainder of the tin, 

 and then the arsenic ; the whole being generally in the follow- 

 ing proportion : viz. 32 parts best bar copper, previously 

 fluxed with the black flux, of two parts tartar, and one of nitre, adopted?"* 

 4- parts brass, 16| parts tin, and l| arsenic. I suppose, with 

 others, that, if the metal be granulated, by pouring it, when first 

 melted, into water, and then fused a second time, it will be less 

 porous than at first. 



In this process, whatever metals are used, and in what pro- How fo deter- 



portions soever, the chief object is, to hit on the exact point of "^'"^ the Sa- 

 . -' ./-,,, turationofthe 



saturation of the copper, &c. by the tin. For, if the latter be tin and copper. 



added in too great quantity, the metal will be dull-coloured 

 and soft; if too little, it will not attain the most perfect white- 

 ness, and will certainly tarnish. It is too late to discover the im* 

 perfections of the metal, after the mirrors are cast and polished ; 

 and no tokens given of them (that I know) are sufficiently free 

 from ambiguity. But I observed the following, which proved, 

 in my trials, at first view, indubitable marks of the degree of 

 saturation ; and I think it fit to describe them particularly, as 

 they have not, to my knowledge, been noticed by others. 



When the metal was melted, and before I poured it into the 

 ^ask, I always took about the quantity of an ounce of it, with a 

 Vor.XVr.— Jan. 1807.^No. 65. P 



