226 Dr. Marcet on an Aluminous Chalybeate spring 



Although I found it expedient, before advancing farther in the 

 examination of the water, and in order to regulate my steps in the 

 progress of the inquiry, thus to trace the various plans which seemed 

 adapted to the purpose, yet I apprehend it would be superfluous to 

 detail here in regular succession all the trials which arose from these 

 diflTerent methods. I shall therefore confine myself to such as belong 

 more immediately to my object ; and in relating them, shall consider 

 singly and under separate heads, the various ingredients of the 

 water, stating, as I proceed, the proportions in which they were 

 ultimately obtained. 



§ VIII. Sulphat of Iron, 



The presence of iron, in the state of sulphat, having been abun- 

 dantly proved by the preliminary experiments, the next step was, 

 to ascertain the proportion of this salt in a given quantity of the 

 water. The first reagent which I tried for this purpose, was prussiat 

 of potash ; but after many trials which afforded uncertain and dis- 

 cordant results, I convinced myself that this test, however useful 

 for detecting the presence of iron, is quite inappropriate when our 

 object is to ascertain the quantity of that substance.^ 



* 



* Prussiat of potash, as a precipitant of iron, is liable to the following objection? : — 



1st. It is apt, although apparently well prepared and crystallized, to precipitate cer- 

 tain earthy substances, and in particular alumiuc; this I found distinctly to happen in 

 two experiments in which the mixture was heated. 



2dly. If the solutions be used cold, and if the metal be not highly oxydatcd, some 

 of the Prussian blue unavoidably passes through the filters; or if no filters be used, it 

 subsides but slowly and imperfectly. 



Sdly. If the solutions be heated, the prussiat of potash is itself decomposcdj and yields 

 a quantity of oxyd of iron which vitiates the results. 



