248 Dr Turner on the Cyanuret of Mercury, <$•£. 



of doing this, is to operate in a large glass vessel, and to draw 

 off the supernatant liquid daily with a syphon. It is remarka- 

 ble that the ferro-cyanate of the peroxide of iron does not 

 subside well, unless an excess of the salt of iron be present ; 

 and, consequently, after repeated washings with fresh water, 

 by which the free salt of iron is removed, the Prussian blue 

 loses its power of subsiding, and remains suspended in the 

 liquid. This is a sign that the edulcoration has been carried 

 to a sufficient extent. The pure ferro-cyanate is then dried 

 on a sand bath. 



The readiest mode of forming a persalt of iron, is by ad- 

 ding nitric acid to a solution of the proto-sulphate, and boiling 

 it for a few minutes. A measured drachm and a half ©f nitric 

 acid, specific gravity 1.4, is sufficient for an ounce of the pro- 

 to-sulphate. A few drops of sulphuric acid should after- 

 wards be added, to prevent the formation of a sub-salt. 



Sulpho-cyanate of Potash. 

 In preparing the sulpho-cyanate of potash, according to the 

 method recommended by Vogel, it is difficult to obtain it quite 

 pure, except by continuing the operation for a considerable 

 length of time. An accident which occurred to my friend, 

 Mr John Home, while making this salt in my laboratory, led 

 me to the following modification of the process, by which it is 

 rendered more speedy and effectual. Mix the ferro-cyanate 

 of potash in fine powder, with an equal weight of sulphur, 

 and after putting the mixture in a porcelain capsule, place it 

 just above a pan of burning charcoal, so that it may be ex- 

 posed to a very strong heat, but short of redness. The mix- 

 ture speedily fuses, takes fire, and burns briskly for one or 

 two minutes, during which it should be well stirred. The 

 combustion soon ceases spontaneously ; and the dark-coloured 

 residue, on being dissolved in water and filtered, yields a very 

 pure and neutral sulpho-cyanate of potash. To insure the 

 decomposition of all the ferro-cyanate of potash, I generally 

 allow the mass to remain in a fused condition for a few mi- 

 nutes after the combustion has ceased, previous to withdraw- 

 ing it from the fire ; but this precaution is not necessary, if a 

 strong heat has been employed in the first instance. 



