ALUMINOUS CHALYBEATE SPRING IN I. OF WIGHT* 03 



for this purpose was prussiate of potash ; but after many 

 trials (which afforded uncertain and discordant 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*. 



Fifty grains of residuef dried at the temperature of be- Residuum of 

 tween 170 and 180, (as described in sect. VI,) and there- ?H w , a ' e r 

 fore equal to ten ounces of the water, were boiled in sue- potash, 

 cessive solutions of the potash, so as to saturate all the acid 

 contained in. that residue, and to dissolve the alumine. The 

 remaining solid residue, which had passed first to a dark 

 green, and some houra afterward to a dark brown or nearly 

 black colour, was dissolved in nitric acid, and the solution treated with 

 evaporated to dryness, after which a red heat was applied, '«t«cacid 

 in order to bring the iron to a state of peroxide, and thus 

 render it insoluble in the same acid. The mass being now 

 treated with nitric acid, in order to separate the lime and 

 magnesia supposed to be mixed with the oxide of iron, and 

 the whole being thrown into a filter, the clear solution wa9 

 found still to contain a good deal of iron. This last solution 

 was, like the former, evaporated to dryness, and to the resi- 



* Prussiate of potash, as a precipitant of iron, is liable to the fol- Objections to 

 lowing objections:— prussiate of 



1st. It is apt, although apparently well prepared and crystallized, P ot ash. 

 io precipitate certain earthy substanc> s, and in particular alumine; 

 this I found distinctly to happen in two experiments, in which the 

 mixture was heated. 



2(lly. If the solutions be used cold, and if the metal be not highly 

 oxidated, some of the Prussian blue unavoidably passes through 

 the niters; or if no niters be used, it subsides but slowly and imper- 

 fectly. 



3dly. If the solutions be heated, the prussiate of potash is itself 

 decomposed, and yields a quantity of oxide of iron, which vitiates the 

 results. / 



f By the word residue, thus generally used, is always meant the resi- 

 due of the water under examination, dried at the temperature of be- 

 tween 170 and ISO 3 . And in comparing a quantity of residue with a 

 corresponding portion of the water, the average proportion of 80 - 5grs 

 for each pint (sect, VI, 2) is always assumed as the standard of com- 

 parison. 



due, 



