in the Isle of Wight. 245 



was attended with some difficulty, and required a considerable quan- 

 tity of water) there always remained a small proportion of earthy 

 matter, which resisted all solvents, caustic potash excepted. This 

 insoluble matter, I had thought from some of the first trials, 

 amounted to about 1 gr. in 100 of the residue ; but from some 

 subsequent experiments in which the silica was separated by caustic 

 potash, there appeared to be reason to suppose that this estimate 

 was rather over-rated. I shall relate the process, to which, after 

 various trials, I gave the preference. 



2. 50 grains of residue being boiled with very dilute muriatic 

 acid, a white flocculent substance remained undissolved, upon which 

 neither acid nor water could make any impression. This substance 

 being separated and boiled in a solution of caustic potash, readily re- 

 dissolved with the exception of a few particles of highly oxydated 

 iron which subsided. Muriat of ammonia * being added to the 

 clear alkaline solution in sufficient quantity to saturate the whole 

 of the potash with muriatic acid, the white flocculent substance re- 

 appeared, which, after being well washed and heated to redness, 

 weighed between 0,3 and 0,4 gr. This substance when heated 

 with alkali ran into a vitreous globule, and muriatic acid being poured 

 upon this, the alkali was dissolved, and the earthy matter remained 

 untouched. It was therefore silica, the quantity of which may be 

 estimated at 0,7 gr. in a pint of water, f 



* This precipitant, which was, I believe, first proposed by Mr. Chenevix, is much 

 more appropriate than acids, because if an excess of acid be incauf.oiuly added, the 

 precipitate is re-dissolved; whilst with muriat of ammonia, an excess of the test is 

 attended with no inconvenience. 



+ The presence of silica was also shewn, and its quantity attempted to be ascertained 

 by the following process. A portion of residue was boiled in caustic potash : this dis- 

 solved not only the silica, but also the alumine; both these earths were precipitated 

 from the alkaline solutions by muriat of ammonia, and separated 3 muriatic acid being 



