/// the Isle of Wight. 229 



cipitate carefully washed, dried, and ultimately heated to redness 

 \vith a little wax in a platina crucible, weighed 7,^ grs. 



3. It will be observed that between this and the former result 

 there was a difference of 0,4 grs. in the quantity of oxyd of iron 

 contained in 50 grs. of residue. But when it is considered that in 

 the first of these analyses, a small quantity of iron was positively 

 detected in the acetic solution, which, from the best estimate I could 

 make, would have brought the quantity of iron very near that ob- 

 tained in the second process, it will readily be admitted that the 

 coincidence was such as to authorize me to consider the last result 

 as sufficiently accurate.* 



4. If therefore we consider 7,2 grs. of peroxyd of iron, as the 

 quantity of this metal contained in 50 grs. of the residue, which 

 corresponds to 11, 59 grs. of the oxyd for 80,5 grs. of residue (that is 

 for each pint of the water, according to the average before esta- 

 blished, § VI. 2), we shall be able to infer the quantity of sulphat of 

 iron contained in the water. 



^' "5. In order to do this, however, it was necessary to ascertain by 

 a comparative experiment the proportion of oxyd which a known 

 quantity of sulphat of iron yields by a process similar to that which 

 I have just described. For this purpose, 50 grs. of transparent 

 crystallized green sulphat of iron were dissolved in water, and 

 treated with carbonat of ammonia as long as any precipitate appeared^ 



* In one experiment in which the iron was precipitated from a similar quantity of 

 residue, by prussiat of potash, and the prussiat of iron roasted with wax, the quantity 

 of oxyd obtained amounted to 11 grs. from which I infer either that a portion of the 

 oxyd of iron, always contained in prussiat of potash, must have been precipitated with 

 the Prussian blue, or that the prussiat of iron was not completely decomposed in the 

 process in question, or that some earthy substance was precipitated along with the 

 iron, 



