252 Report of' Dr Twncr'it Lectw^ on 



tion. Berthier and Rose had likewise analyzed porcelain x-kly 

 from other localities, and each found the ratio of the two earths 

 to be nearly two equivalents of alumina to three of silica. Its 

 constitution, accordingly, ap]:)eared subject to very slight varia- 

 tion. The formulae shewed that every two equivalents of alu- 

 mina, present in porcelain clay along with three and a half of 

 sihca, corresponded in the original felspar, from which it was de- 

 rived, to twelve equivalents of silica and one of potash. Hence 

 the quantity of silica carried off in solution was enormousiio now 



The lecturer then went on to explain how it happened that 

 silica, existing in solution, was deposited so as to constitute mi- 

 nerals. One obvious principle, he stated, was the molecular at- 

 traction which exists between similar particles of matter, as was 

 proved by facts without number. Its existence was attested by 

 the globular form assumed by water, oil, mercury, and other 

 liquids ; by the separation from one another of salts in crystal- 

 lizing out of mixed solutions ; by the formation of crystals du- 

 ring the slow deposition of vapour, as when camphor was sub- 

 liming slowly in a glass bottle, the particles attaching themselves 

 to one another, rather than spreading uniformly over the sur- 

 face on which they collect ; and by the tendency of like mole- 

 cules to get together and cohere while intermixed with a mass 

 of dissimilar matter, rendered liquid by heat, as when particles 

 of titanium diffused in a furnace, through a mass of iron, seek 

 out each other and form regular crystals, or when minerals crys- 

 tallize out of melted lava or basalt : so from solutions of silex, 

 whether strong or dilute, the particles are disposed to adhere 

 together whenever they cease to remain in solution. 



Another principle applicable to this question, was the follow- 

 ing : Whenever substances, insoluble in their ordinary state, 

 were dissolved by the force of favourable circumstances, such so- 

 lutions were very prone to decomposition. They formed in- 

 stances of peculiarly unstable equilibrium. The slightest dis- 

 turbing causes, as agitation, change of temperature, or the affi- 

 nity, though slight, of some other body for the solvent, — would 

 in such cases put an end to the solution. Illustrative examples 

 of this principle were afforded by solutions of tin, titanium, and 

 peroxide of iron, in a neutral state. He might probably quote 



