244 Dr. Marcet en an Aluminous Chalybeate Spring 



S. It appears therefore that the aggregate of the analytical results 

 would indicate 79 grs. of ignited sulphat of barytes, instead of the 

 74 grs. obtained by a single direct operation. This difference I 

 apprehend to be in a great degree owing to my estimate of the 

 proportion of acid in sulphat of alumine being over-rated, from the 

 circumstance of not having been able to obtain a neutral sulphat 

 of alumine in the experiment j ust related from which that estimate 

 was deduced. 



§ XVI. Silica, 



I. During the various solutions of the residue in acid, I had re- 

 peatedly observed that, besides the selenite, (the solution of which 



for 21 grs. of ignited sulphat of barytes. From which it may be inferred^ that the 3,8 

 grs. of ignited alumine found in a pint of the water, were combined with a quantity of 

 acid equal to 17,7 grs. of ignited sulphat of barytes. But it is assumed in this compu- 

 tation that the artificial sulphat of alumine subjected to analysis, was in the same state 

 of combination as that which exists in the water, a supposition which may not be strictly 

 accurate. 



J The quantity of sulphat of barytes produced by the precipitation of a given quan- 

 tity of sulphat of lime, was ascertained in the following manner : some pulverized 

 crystals of native selenite, apparently perfectly pure, were dissolved in water and 

 "afterwards slowly precipitated by evaporation. The object of this previous operation 

 vas to obtain the sulphat of lime in a state more fit for subsequent re-dissolution. 

 Fifteen grains of this selenitic residue were dissolved in water, slightly acidulated by 

 muriatic acid, in order to supersede the necessity of using a large quantity of water; 

 and the solution, after being neutralized by pure ammonia, was precipitated by muriat 

 of barytes. The sulphat of barytes thus obtained, weighed, after careful edulcoration 

 and ignition in a platina crucible, 26,75 grs. which is equivalent to 175,6 grs of barytes 

 for 100 grs. of ignited sulphat of lime. 



II According to Dr. Henry 100 grs. of crystallized sulphat of magnesia give 111 grs. 

 of ignited sulphat of barytes. See Philos. Trans. 1810, p. 114. 



§ These proportions were deduced from the following experiment : 40 grs. of crys* 

 tallized sulphat of soda, being dissolved in water and precipitated by nitrat of barytes, 

 the sulphat of barytesj well edulcorated and ignited, weighed 29,1 grs. 



