294 Chemical Examination of an antient Speculum. 



A hundred parts of the metal of this antient mirror con- 

 sisted therefore of 



Grains. 



Copper - - - 63 



Tin , . . 32 



Lead - , - 6 



100 



Trom this analysis it appears that the antients, in the 

 composition of their, mirrors, employed the same kind of 

 mixture as that used at present for the specula of tele- 

 scopes. The quantity of lead is here so small that it is 

 hardly worth notice, since it would produce no alteration 

 of anv importance. Besides, it is probable that the lead 

 found in this case, did not belong to the formula of the 

 component parts, but arose merely from the adulteration 

 of the tin, as tin no doubt was sold at a much higher price 

 among the antients than it is among us. Pliny complains 

 of this adulteration of tin in the foUowiug passage (lib. 

 xxxviii. cap. 6.) : Plumbum candidum. That this expres- 

 sion means tin has been mentioned already — quad ceri in- 

 coquehatur hnprobiores nigra tertiporabant. 



The proportion pf the metals also in these antient mir- 

 rors corresponds nearly with that used at present for the 

 specula of telescopes, and consisted usually of two parts 

 copper, and one part of tin. 



Our artists, however, are accustomed to pmploy other 

 metallic additions, such as silver, zinc, antimony, arsenic, 

 but these substances are added in so small quantities, that 

 they cannot be considered as essential to the composition : 

 the intention of adding them is partly to render the metal 

 more secure from rust, and partly to make the mixture 

 more fusible. It is with the latter view that arsenic is 

 added, and it iii some measure answers the purpose. The 

 antients, who were not acquainted with arsenic, could de^ 

 rive" no benefit from its quality of rendering metals more 

 fusible. 



Metallic mirrors, since the invention of glass ones, have 

 been entirely disused as articles of furniture. But a glass 

 mirror, properly speaking, is metallic ; for it is not the glass 

 but the amalgam of tin, placed at the back of itj which 

 reflects the miage of the object presented to it. 



LIII. 4n 



