397 



7th. When the compounds in a solution are not present in 

 eutectic proportions the order of consolidation, in certain 

 cases, depends upon the relative proportions of the constituents 

 not upon their relative fusibilities. Thus, in a solution con- 

 taining A and B, the order of crystallization may depend on 

 whether A or B is in excess of the eutectic proportion. 



Mixtures of fused silicates, such as those from which artificial glasses, 

 slags, and igneous rocks are formed, may be regarded as solutions. This 

 was maintained long ago by BUNSEN as regards the igneous rocks and by 

 BENRATH and others as regards artificial glasses. Prof. LAGORIO^ has 

 recently discussed the phenomena of igneous rocks from this point of view 

 in a paper from which we shall frequently quote in what follows. Silicate- 

 solutions differ from aqueous solutions in the readiness with which they 

 form an amorphous glass when cooled rapidly. This appears to be 

 connected with the fact that they may be readily over-cooled, and that 

 when in this state they are highly viscous, so that a rapid approach of the 

 molecules is prevented. The melting point of a glass is lower than that of 

 the same substance in a crystalline condition. A glass, therefore, results 

 from the solidification of an over-cooled liquid. 



The common varieties of artificial glass are amorphous mixtures of 

 various silicates, especially those of the alkalies and alkaline earths. They 

 are formed by fusing together certain constituents such as quartz, chalk, 

 and sulphates or carbonates of soda or potash. At the high temperature of 

 the glass-furnace silicic acid displaces sulphuric and carbonic acids, thus 

 liberating sulphuric anhydride (decomposed at a red heat into sulphur 

 dioxide and oxygen) and carbonic anhydride. When the ingredients 

 are mixed in suitable proportions the fused mass consolidates as an 

 amorphous glass. All kinds of glass may be devitrified, that is, may be 

 converted into a crystalline aggregate. This is done by maintaining the 

 glass for a long time at a temperature sufficient to soften the glass but not 

 sufficient to melt it. Different glasses behave very differently when sub- 

 jected to the conditions which determine devitrification. Some are 

 devitrified readily, others with difficulty. The potash-glasses are the most 

 obstinate in resisting devitrification. A consideration of the phenomena 

 of glass-making leads to the following conclusions according to LAGORIO. 

 Silicate-solutions are not easily saturated with silicates containing 

 potassium and aluminium or with silica itself. They are more readily 

 saturated with sodium silicates and still more readily with silicates of 

 lime and magnesia. In other words silicates of the alkaline earths 

 separate out most readily from silicate-solutions ; then follow sodium- 

 silicates and last of all come potassium-silicates and free quartz. 



These facts correspond very closely with what is known with regard to 



(1) Ueber die Natur der Glasbasis sowie der Krystallizationsvorgange im eruptiven 

 Magmen. T.M.M., Band VIII, p. 421. 



(2) Compare this with the fact that sulphurous and carbonic acids are frequently 

 exhaled in connection with volcanic action. 



