396 



SltVER COINS. 



alcohol takes 

 op. 



and as such, perhaps the definition of it by the terni of an 

 extracto-resin may be adopted without impropriety. 

 Oxigenation of P.S. — I have observed that the action of oxigen on some of 

 matter which *^^ Other resinous bodies is very remarkable. It is well known 

 that by digesting mastich in alcohol, a partial solution only is 

 formed, and there remains an elastic substance, Vi'hich is 

 generally said to possess the properties of pure caoutchouc ; it 

 appears however to differ from this substance in becoming hard 

 when dried by exposure to air. Moreover, I have remarked 

 that the part of mastich which remains dissolved by alcohol, 

 may be again precipitated by water, and, on examination, I 

 found the precipitate to possess the properties of a pure resin : 

 but when a stream of oxy-muriatic acid gas was made to pass 

 through the solution, a tough elastic substance was thrown 

 down, which became brittle when dried, and was soluble in 

 boiling alcohol, but separated again as the solution cooled : its 

 properties, therefore, somewhat approached to those of the 

 •original insoluble part. 



vr. 



On Siker Coins. % Th o m a s Th o m s o n , 31. D. F-M-S. 5. 

 Communicated by the Author, 



Silver as a 

 medium of ex- 

 change. 



Analysis of 

 silver coins. 



The method. 



ILVER has been employed by most nations as a medium 

 of exchange. The ancients appear to have coined the metal 

 pure, and the same practice is still followed, I believe, in 

 Some of the Eastern nations. But in Europe it is always al- 

 loyed with copper j pure silver being considered as too soft 

 for coin. 



The analysis of silver coins is not attended with much diffi- 

 culty. The metals which they contain are silver and copper, 

 and almost always a little gold. The method which I use to 

 separate and estimate the relative weights of these metals was 

 the following : 



1. The silver coin was first well cleaned by means of soap 

 or an alkaline ley. It was afterwards weighed, and its speci- 



•fic 



