VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 55 



A small proportion of zinc is sometimes added to allays of copper by tin ; on 

 some occasions, on account of colour, on others perhaps to render the copper still 

 less oxydable and more fusible ; and on other occasions, as I have found on inquiry, 

 it is added from erroneous theory, or mere caprice. No one could tell me the use 

 of zinc, which in some manufactories is added, in making gun metal. 



Tin might be used also to render copper less clingy, or more brittle, for the 

 purpose of writing on it, or marking it with lines and figures, as on mathematical 

 instruments : but the allay with zinc is now preferred for these purposes, as I sup- 

 pose on account ©f its being less hard than allays with tin, and yet sufficiently brittle; 

 on account also of its golden colour ; and also on account of its being still more 

 difficultly oxydable by air and water. 



The scabbard metal contained a rather larger proportion of tin than the celts, 

 N° 1 and N° 1 ; namely, being -^ th of its weight. Copper allayed by zinc would 

 have been sufficiently hard and strong, and on other accounts preferable to the allay 

 of copper with tin. This is however one proof of the extensive use of this last com- 

 position among the ancients. 



The art of allaying copper with an earth-like substance; which, within a little more 

 than the last 50 years only, we have learned was an ore of a metal, namely, zinc; 

 was known perhaps in the time of Aristotle, and certainly of Pliny ; for the latter 

 informs us, that this composition resembles orichalcum ; and after his time it was 

 called orichalcum. Thus the native and factitious orichalcum were confounded. 

 The ancients do not appear to have used the allay of copper by zinc, except for 

 mere ornaments, to resemble gold. It is much more extensively employed by the 

 moderns, and the allay of copper with tin is less extensively used : 1st, because the 

 former is cheaper than the allay of copper with tin ; 2dly, because it is now gene- 

 rally understood that it preserves its colour longer ; 3dly, because it is easier to 

 work it into various forms, and especially for philosophical instruments ; few of 

 which were probably made by the ancients. 



The composition in common use, which contains the greatest proportion of tin, 

 is called speculum metal The requisites of this metal are compactness, uniformity 

 of texture, whiteness, sufficient strength to prevent its cracking in cooling, and to 

 bear polishing without breaking. Mr. Mudge found the whole of these properties 

 attainable in the greatest degree, by a little less than 1 part of tin with 1 parts of 

 copper. But for very large instruments, such as the 40-feet telescope of Dr. 

 Herschel, the proportion of tin must be less than in small instruments, on account 

 of the property of brittleness. The compound of equal weights of copper and tin is 

 so brittle, that it is not easy to conceive to what useful purpose it can be applied. 

 The allays of tin with copper, by which I mean those compounds of copper and tin 

 in which the tin is in greater quantity than the copper, I believe, have not been 

 examined. It is said, indeed, that tin allayed with a very small proportion of cop- 

 per has been employed for tinning, to save much of the expence of tin; for a much 

 thinner coat of this compound can be spread than of tin. 



