256 Dr Colquhoun on the Assay 



From these general rules regarding the fusibility of mix- 

 tures of silica, lime, magnesia, and alumina, it is plain that no 

 compound of them, more simple than the ternary, is at pre- 

 sent of any practical use in metallurgy. Upon these com- 

 pounds, Mr Kirwan has instituted a series of valuable expe- 

 riments, which lead to the following conclusions. 



1. When lime, magnesia, and alumina are the component 

 parts of the mixture. 



If lime preponderate, and the proportions be 3 lime, 2 mag- 

 nesia, and 1 alumina, the compound may be vitrified in the 

 furnace. Whenever the whole mass is thus melted and run 

 into a glass, the highest state of liquefaction is attained which 

 the smelter can desire. If the proportions only approximate 

 to those just mentioned, the mass may be softened under ex- 

 treme heat, so as to form a porcelain or an enamel, but it will 

 not vitrify. 



If magnesia preponderate, the compound will be infusible 

 in every degree of heat below 1 Q&^ Wedgwood. 



If alumina preponderate, and be equal to the larger of the 

 two other ingredients, and at the same time is thrice as abun- 

 dant as the smaller, the mixture may form a porcelain at 1^0° 

 Wedgwood. >moii.;> 



'ir^« In a mixture of lime, magnesia, and silica. Mft 



If the lime preponderate, there are various proportions in 

 which a fusible compound may be composed. n* 



If the magnesia preponderate, the mixture is infusible, jnl 



If silica preponderate, few fusible compounds can be pr6»j 

 pared. 



3. In a mixture of alumina, magnesia, and silica. 



If the alumina predominate, only a porcelain can be produ- 

 ced. 



If the magnesia predominate, not even an imperfect fusion 

 can be effected. 



If the silica be the most abundant ingredient, there are va- 

 rious mixtures which will yield a porcelain, and one that will 

 even vitrify. This occurs when the mass consists of 3 silica, 

 2 magnesia, and 1 alumina. 



4. In a mixture of alumina, lime, and silica. rl- 

 If the lime preponderate, the product will be a glass, a por- 



