SOLVENT POWER OP WATER ON SILICEOUS MINERALS. 229 
both because under perfectly similar circumstances, when 
there was no water, no effect was produced, and because 
each pound of alkali would have had to dissolve, perhaps, 
forty pounds of silica. Mr. Jeffreys was informed by a mili- 
tary engineer of distinguished ability, to whom he related the 
experiment, that he had once observed a similar distinctive 
effect upon the brick casing of a kiln, by moisture getting in 
at an intense heat, though no scientific notice was taken of it 
at the time; and as coal was the fuel in this case, there was 
still less ground for supposing the action to be alkaline. 
Lastly, if alkali did play an appreciable part, the experi- 
ment would remain still sufficiently curious, as it showed an 
abundant vaporization of silica, by a fraction of its weight 
of alkali, without the influence of fluorine, a phenomenon 
which has, so far as the author is aware, been only obscure- 
ly manifested in minute quantities, as noticed by the late Dr. 
Macculloch. 
Ibid. 
