on the Chemistry of Geology. 25 



course to this mode of reasoning. Substances, he said, which 

 are inappreciably soluble in one state, may be freely dissolved 

 in another. Silex in the finest powder may be boiled in water 

 without perceptible solution; but if presented to that solvent 

 while in the nascent state, it was freely dissolved. Substances 

 in the act of being formed from their elements, or of separa- 

 ting from previously existing combinations, do not possess that 

 force of aggregation which properly belongs to them, and in 

 such states of transition they have a peculiar aptitude to com- 

 bine with other bodies. This property is observed more or less 

 in all bodies ; but silica offers one of the most striking illustra- 

 tions of it. Siliceous earth, in its nascent state, is freely solu- 

 ble in water and in various acid and saline solutions, which do 

 not perceptibly dissolve ordinary flint, however finely it may 

 be pulverized ; and the alkalies and alkaline carbonates, which 

 dissolve silex even in its solid condition, take it up while 

 nascent in far greater quantity. Now in the decomposition of 

 felspathic rocks, which had been referred to in the first part 

 of the lecture expressly with a view to that subject, the silex 

 was exposed to the united action of water and alkali at the 

 moment of passing from the state of combination which con- 

 stitutes felspar, and would be expected to be freely dissolved. 

 That it was so, might be proved by a comparative view of the 

 constitution of porcelain clay and felspar. He would repre- 

 sent their composition, he said, by a formula expressive of the 

 number of equivalents of each element; though in doing so, 

 he did not mean to assert that porcelain clay was strictly an 

 atomic compound. Thus, 



Felspar. Porcelain Clay. 



(Po + 3Si) + (Al + 9 Si); (Al + Si Si). 



The lecturer stated that the porcelain clay referred to, was 

 a sample from Villarica, which he had analysed during the 

 course of the winter. Besides aluminous and siliceous earth, 

 it contained 21*3 per cent, of water. Mr. Rogers of Phi- 

 ladelphia had obligingly analysed for him some porcelain 

 clay from the vicinity of Mont Dor in Auvergne, which had 

 a similar constitution. Berthier and Rose had likewise ana- 

 lysed porcelain clay from other localities, and each found the 

 ratio of the two earths to be nearly 2 equivalents of alumina 

 to 3 of silica. Its constitution accordingly appeared subject 

 to very slight variation. The formulae showed that every 2 

 equivalents of alumina, present in porcelain clay along with 

 3\ of silica, corresponded in the original felspar, from which 

 it was derived, to 12 equivalents of silica and 1 of potash. 



Third Series. Vol. 3. No. 13. July 1833. E 



