TINNING, PLATING, &C. 326 



to think, that if the Sydonians had only invented the art of using 

 a flat piece of glass as a speculum, without knowing how to give it 

 a metallic coating, on which its excellency chieily depends, they 

 would not have merited the mention which Pliny makes of them ; 

 for their looking-glasses must have been inferior to the metallic 

 mirrors then in use at Rome. There seems to be but one objec. 

 tion of any consequence to this conclusion : had the method of 

 giving a metallic covering to plates of glass been known at least to 

 the Romans, (for it might have been known in Asia long before it 

 was known in Italy), it seems probable, that the metallic specula 

 would have fallen into general disuse, much sooner than there is 

 cause to think they did, for it would have been much easier to 

 make a looking-glass, than to polish a metallic mirror ; and the 

 image from the glass would have been superior to that from the 

 metal, and on both accounts the mirrors would have become un- 

 fashionable. 



The first mode of fixing a coat of tin on a tooking.glass, I sus. 

 pect to have beenthat of pouring the melted metal on the glass; 

 and I have some reason, not now to be insisted on, to think, that 

 this mode was not disused in the fourteenth century. Baptists 

 Porta lived in the fifteenth, and died towards the beginning of the 

 sixteenth century ; he gives us a very accurate description* of the 

 manner in which looking-glasses were then silvered ; it differs from 

 that now in use only in this, that the tin-foil, when silvered, was 

 taken up and gpntly drawn upon the glass. J. Maurice Ilolfman 

 published his Acta Laboratorii Chemici, in 1719 ; he there speaks t 

 of a mixture of one part of tin with three of quicksilver, which 

 some time ago, he says, was usually applied to the back surfaces of 

 looking-glasses ; although the Venetians did then make looking, 

 glasses by pouring quicksilver upon tin. foil placed on the back 

 surface of the glas. This mode of silvering ihe glass was not then 

 invented by the Veii"tians, as appears from what Baptista Porta 

 had advanced above two hundred years before ; though the mode 

 of silvering the tin-foil, when laid upon the glass, was an improve, 

 inent on tliat prescribed by Baptista Porla, just as the mode now 

 in use is a great improvement on that practised by the Venetians 

 in the time of Hotfman. 



The men who are employed in silvering looking-glsses often 



* Alaj. Nat. 1. iv. c. zviii. f Id * P- 245. 



