CATARACTS, AND INUNDATIONS. 195 



by evaporation. M. Gioanetti made the same observation by 

 evaporating the Prussian alkali, purified, by the method of Baunie, 

 to dryness : he has proposed two processes for obtaining this liquor 

 in a state of purity, and totally exempt from iron; the one consists 

 in supersaturating the Prussian alkali with distilled vinegar, evapo- 

 rating it to dryness by a gentle heat, dissolving the remaining mass 

 in distilled water, and filtrating the solution; all the Prussian blue 

 remains on the filter, and the liquor which passes through contains 

 none at all. The other process consists in neutralizing the alkali 

 with a solution of alum, from which after filtrating, the sulphat 

 of potash is separated by evaporation. These two liquors do not 

 afford a particle ot Prussian blue with the pure acids, nor by evapo- 

 ration to dryness. The lime water, saturated with the colour- 

 ing matter of Prussian blue, which is a prussiat of iron, does 

 not require these preliminary operations : when poured on a so- 

 lution of sulphat of iron, it immediately forms pure Prussian blue, 

 without any mixture of green. Acids only precipitate a few parti- 

 cles of Prussian blue from this re-agent ; it therefore does not 

 contain iron, and consequently is preferable to the Prussian alkalis, 

 in the assay of mineral waters. This phenomenon doubtless de- 

 pends on the action of the lime, which, when dissolved in water, 

 is far from having the same efficacy on iron as alkalis have. This 

 prussiat of lime seems to be exceedingly well adapted to distinguish 

 ferruginous waters, whether they be gaseous or sulphuric. la 

 fact, the carbonic gas, which holds iron in solution in waters, being 

 of an acid nature, decomposes Prussian lixiviums by the way of 

 double affinity, as well as sulphat of iron. Fourcroy tried prussiat 

 of lime on Spa waters, and these of Passy, and he immediately 

 obtained a very perceptible blue in the former, and very abundant 

 in the latter. This, therefore, is a liquor vtry easily prepared, 

 which does not contain the smallest portion of Prussian blue, and 

 is exceedingly well calculated to exhibit 'the presence of small quan- 

 tities of iron in waters. It is a kind of neutral salt, formed by the 

 prussic acid, or the colouring part of the blue and lime. 



Nut-galls, as well as all other bitter and astringent vegetables, 

 such as oak bark, the fruit of the cypress tret, the husks of nuts, 

 &c. have the property of precipitating solutions of iron, and exhi- 

 biting that metal of different colours, according to its quantity, its 

 slate, and that of the water in which it is dissolved. This colour 



O 2 



