ON METALS. 



per was known long before the age of Pliny; it is mentioned 

 by Aristotle*, incidentally, when he is describing a method of 

 rendering copper white, but not by tin ; and from its great utility, 

 it will probably never fall into disuse. We have ceased, indeed, 

 since the introduction of glass mirrors, to use it in the way the 

 ancients did; but it is still of great use amongst us, since the specula 

 of reflecting telescopes are commonly made of it. Mr. Mudge 

 has ascertained +, not only the best proportion in which the copper 

 and tin should be mixed together, but has found out also a method 

 of casting the specula without pores. He observes, that the per. 

 fection of the metal, of which the speculum should be made, 

 consists in its hardness, whiteness, and compactness. When the 

 quantity of tin is a third of the whole composition, the metal then 

 has its utmost whiteness ; but it is at the same time rendered so 

 hard that it cannot be polished without having its surface splin- 

 tered and broke up. After many experiments, he at length found 

 that fourteen ounces and one half of grain tin];, and two pounds 

 of copper made the best composition ; an addition of half an ounce 

 more tin rendered the composition too hard to be properly polish, 

 ed. The casting the metal so as that it may be compact and with, 

 out pores, is a matter of the greatest consequence; he hit upon 

 the manner of doing it by accident. His usual way of casting a 

 speculum metal, was to melt the copper and to add the tin to the 

 melted copper : the mass when cast was seldom free from pores. 

 After having used all his copper in trying experiments to remedy 

 this defect, he recollected that he had some metal which had been 

 reserved, when one of the bells of St. Andrew had been re-cast : 

 he added a little fresh tin to it, and casting a metal with it, it 

 turned out free from pores, and in all respects as fine a metal as 

 he ever saw. Upon considering this circumstance, he proceeded 

 to form a metallic mass in the usual way, by adding tin to melted 

 copper; this mass was porous, it was in the state of the bell. metal 

 he had tried ; and upon re-melting it, it became, as the bell-metal 



* DC iMirab. f Philos. Trans. 1777. p. 296. 



| Grain tin is worth ten or twelve shillings per hundred more than mine tin, 

 because it is smelted from a pure mineral by a charcoal fire ; whereas mine tin 

 if usually corrupted with some portion of mundick, and other minerals, and is 

 always smelted with a bituminous fire, which communicates a harsh, sulphu- 

 reous, injuriousquality to the metal. Pryce, Min. Corou. p. 137. Mr. Mudge 

 probably used u hat is called grain-tin iu UlC shops, or the purest sort, which is 

 usually sold in piece* like icicles. 



