Materials for a History of the Prussiates. 341 



in order to preserve its base at its minimum. On the other 

 hand, we keep in ebullition over a lamp, a matrass, in which 

 has been placed fifteen or eighteen grains of prussiate of pot- 

 ash and two or three ounces of hydro-sulphuieted water : a 

 few seconds after the ebullition or the vapour of it has 

 driven off the air which occupies the vacuum of the matrass, 

 we drop into it some solution of the sulphate : there is a 

 precipitate immediately produced which makes the lio ( uor as 

 white as milk, and which remains so while the heat con- 

 tinues. This is the precipitate which I call luhite prussiate: 

 this is the prussiate obtained by Fourcroy, Vauquelin, Davy, 

 and all those, without doubt, who, having had regard to 

 the conditions which secure the success of it, have ascer- 

 tained that the base of the green sulphate could also become 

 the base of a prussiate different from that which has for its 

 base the oxide at the maximum. But as upon passing from 

 one combination to the other the black oxide does not al- 

 ways lose its disposition to be hyper-oxidated, we see that as 

 soon as the matrass is removed from the fire the atmosphere 

 reacts upon the milky-looking mixture, and rapidly ex- 

 hibits undulations in it which commence by shading it, and 

 which finish by giving it the deep shade of the most perfect 

 blue. 



We may also obtain this product in another manner. 

 Drop some grains of prussiate of potash into a verv dilute 

 and boiling solution of green sulphate, and we see a preci- 

 pitate appear, the white of which sustains the action of the 

 air for a short time longer. 



The following are a few additional processes, which, if 

 they add nothing to our conviction, are nevertheless inter- 

 esting from the variety of the means. 



Fill two glasses, one with nitrate of iron and the oilier 

 with green sulphate, both very dilute : afterwards drop into 

 them a crystal of prussiate of potash. In the first we find 

 the crystal is coloured at the same instant of so deep a blue 

 that it resembles black velvet. In the second it crackles, 

 disperses, and falls down in a white powder : but, asbefore 

 becoming the subject of the experiment it had imbibed at- 



Y 3 mospherie 



