/// the Isle of Wight. 227 



1. Fifty grains of residue * dried at the temperature of between 

 170' and 180', (as described in § VI.) and therefore equal to ten 

 ounces of the water, were boiled in successive solutions of 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 hours afterwards to a dark brown or 

 nearly, black colour) was dissolved in nitric acid and the solution 

 evaporated to dryness, after which a red heat was applied in order to 

 bring the iron to the state of peroxyd, 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 oxyd of iron, and the whole being thrown into a filter, the clear 

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

 solution was, like the former, evaporated to dryness, and to the 

 residue, again heated to redness, acetic,f instead of nitric acid, was 

 this time added, and the solution filtered. The filtered fluid still 

 contained a quantity of iron, which however, from subsequent ex- 

 amination, appeared very inconsiderable. The oxyd of iron left 

 in the filter being roasted with wax and heated to redness, in order 

 to bring it to an uniform state of oxydation, weighed 6,8 grs.ij: 



* By the word residue^ thus generally used, is always meant the residue of the water 

 under examination, dried at the temperature of between 170° and 180". And in com- 

 paring a quantity of residue with a corresponding portion of the water, the average pro- 

 portion of 80,5grs. for each pint (§ VI. 2) is always assumed as the standard of com- 

 parison. 



+ The acetic acid, as well as the nitric, is said to be incapable of dissolving any 

 iron, which has been peroxydated by the process just described. In this instance a few 

 particles of oxyd were taken up by the acid ; but it is probable that if, instead of heating 

 the residue to redness only for a few minutes, the oxyd had continued exposed to a red 

 heat for half an hour or more, the whole of it would have become insoluble. 



X It may be asked in what state of oxydation the iron is after that operation ? \i 

 has gcuerally been supposed to be reduced to the state of protoxyd iu consequence of 



2 F 2 



