ON SILEX, CLAY, ETC. 143 



I have made several experiments with alum, in order to 

 ascertain its effects when mixed with other substances. A 

 solution of alum is decomposed (a) by lime-water. If no 

 more lime-water be added than is exactly requisite for the 

 precipitation, the earth of alum forms a transparent 

 precipitate, of the appearance of boiled starch. If the clear 

 water is filtered, it proves to be a solution of gypsum. (&) 

 If more lime-water be added to the solution of alum than is 

 requisite for its precipitation, a white precipitate is formed, 

 but nothing gelatinous, as in the foregoing experiment. If 

 the whole be left together for a quarter of an hour, and 

 frequently agitated during this time, and if it then be 

 filtered, no gypsum, nay, not even lime, is found in the 

 supernatant water, unless there has been added too much 

 lime-water ; but it will be found to be pure water. I was at 

 first at a loss to imagine what was become of the gypsum. 

 (c) On examining the precipitate, I found it to consist of 

 earth of alum, selenite, and lime ; for, after putting it to 

 dissolve in muriatic acid, the gypsum, which is not so easily 

 soluble, remained behind. The clear solution being saturated 

 with caustic volatile alkali, a transparent gelatinous earth, 

 which was the earth of alum, was precipitated. Upon 

 straining it again, and pouring lixivium tartari to it, I 

 obtained a precipitate, which was lime. Hence I saw that 

 the lime and gypsum had separated from the water and 

 united with the earth of alum. 



In order to acquire a more distinct idea of this 

 phenomenon, (d) I precipitated a solution of alum with a 

 quantity of caustic volatile alkali, more than sufficient to 

 saturate the acid, in order that I might be quite certain of 

 separating all the vitriolic acid which might perhaps still 

 adhere to the precipitate. The precipitated earth was 

 edulcorated, and mixed with a solution of gypsum, that I 



