and the Sulpho-ajanute of' Potash. 247 



the last drop. The success of the operation depends entirely 

 on the proportions which are employed. The most conve- 

 nient proportion is eight parts of the purified Prussian blue, 

 well dried on a sand bath, to eleven of the peroxide of mer- 

 cury. This quantity of the peroxide of mercury, without 

 beino- in excess, decomposes the ferro-cyanate completely ; 

 and the weight of the cyanuret which is obtained, somewhat 

 exceeds that of the peroxide employed in its preparation. The 

 ratio of eight to twelve gives an excess of the peroxide ; in 

 consequence of which the solution acquires an alkaline reac- 

 tion. Two inconveniences arise from this circumstance. In 

 the first place, the cyanuret of mercury does not crystallize 

 properly ; and, in the second, the excess of mercury occasions 

 some of the peroxide of iron to be dissolved, which colours 

 the solution, and renders the cyanuret of mercury impure. 

 This fact I have observed repeatedly. If, after decomposing 

 eight parts of purified Prussian blue by eleven of the peroxide 

 of mercury, one part more of the latter be added, the solu- 

 tion, from being neutral and colourless, acquires an alkaline 

 reaction and a yellow colour, and deposits peroxide of iron 

 when it is evaporated. 



The most economical method of obtaining pure ferro-cya- 

 nate of potash, is by direct combination of its elements. The 

 best Prussian blue, which in Edinburgh costs a shilling an 

 ounce, yields, after being purified and well dried, little more 

 than half its weight of pure ferro-cyanate of iron ; while the 

 ferro-cyanate of potash, which is the most expensive material 

 in its manufacture, may be purchased in Glasgow at the rate 

 of three shillings and sixpence per pound. The formation of 

 pure ferro-cyanate of iron from the ferro-cyanate of potash, 

 is very simple to the practised chemist ; but, as there are 

 one or two points of delicacy in the process, it may not be su- 

 perfluous, in a pharmaceutic point of view, to state briefly the 

 different steps of it. In principle, it consists merely in mix- 

 ing the ferro-cyanate of potash, dissolved in a large quantity 

 of water, with some persalt of iron, taking the precaution to 

 have an excess of the latter, and washing the resulting ferro- 

 cyanate of the peroxide of iron with successive portions of 

 water, until the edulcoration is complete. The best method 



