16 Elements of Mineralogy. 



this laft compound forms a menftruum ftill 

 more compound, which is able to diflblve 

 ftillmore of magnefia, for equal parts lime, 

 magnefia, and filex form a perfect glafsj 

 and hence equal parts of any three of the 

 iimple Earths will vitrify in a fufficient heat* 

 fo that calcareous Earth be one of the three ; 

 nay one part lime and one of argill will melt 

 2 or 2! of iilex, and other mixtures are more 

 or lefs fufible as they approach to this pro- 

 portion. 



The principal marks of Chymical Union are the 

 following. 



i ft * Afpeclfic gravity exceeding that of the 

 heavieft of the ingredients of the compound, 

 or even greater than the intermediate. But 

 it does not follow that, where fuch denfity is 

 tvanting, a chymical union does not exift, 

 for the peculiar ilructure of the compound 

 which does not admit water into its vacuities 

 may hinder this property from being obferved, 

 fo may alfo a certain quantity of water which 

 enters into the compofition, and cannot al- 

 ways be made fenfible. 



2 d * Transparency^ when this property is 

 found in combinations of Earths with each 

 other, or with faline fubftances, it indicates 

 a chymical union ; but fuch an union is alfo 



confiftent 



