1819.] some new Combinations of Pntssic Acid. 449 



When in order to make the soluble prussiate of mercury, we boil 

 prussian blue with red precipitate, a change of bases takes place 

 by double affinity. The mercury has a very great affinity for 

 prussic acid ; and the iron of prussian blue, which is at a mini- 

 mum of oxidation, has likewise a great affinity for oxygen. 

 Hence it happens that the iron deprives the acid of oxygen, and 

 passes into the peroxide ; while the mercury seizes the pure 

 prussic acid without oxygen and without iron. In this last state 

 the prussic acid drawn from mercury has no cyanic power, and 

 remains mixed with sulphate of iron without forming a prUssiate. 



I shall terminate this memoir by an experiment, the result of 

 which gives great probability to this conjecture. 



If the prussic acid, when it combines with the peroxides, has 

 really the property of absorbing a portion of their oxygen, and 

 of bringing them to the state of protoxide, it ought to act in the 

 same way on the oxide of the red prussiate of copper, and make it 

 pass to blue, on the supposition that in the blue prussiate the 

 oxide of copper is at a minimum of oxidation. 



To determine this point, I mixed beautiful red prussiate of 

 copper (newly precipitated and washed on the filter) with water. 

 I added a little muriatic acid, and boiled the liquid. The colour 

 appeared to become more red. I then threw in some crystals of 

 prussiate of potash, and had the pleasure of seeing the mixture 

 speedily pass to the finest blue. 



This experiment seems to me to furnish a double proof, both 

 that prussic acid does not produce the blue, except when it is 

 oxidized, and that the bases of the metallic prussiates are at the 

 minimum of oxidation. We may, therefore, distinguish three 

 different well marked states in prussic acid. 1. The pure acid 

 without oxygen and without iron, such as it exists in the soluble 

 prussiate of mercury. 2. The ferruginous acid without oxygen, 

 such as it exists in the prussiate of potash and in the green 

 prussiates. 3. The ferruginous and oxygenized acid, such as it 

 exists in all the blue prussiates. 



Article VII. 



Observations on Gehlenite, made during a Series of analytical 

 Experiments upon this Mineral ; which prove that it contains 

 Potass. By Edward Daniel Clarke, LL.D. Professor of 

 Mineralogy in the University of Cambridge, &c. In a Letter 

 to the Editor. 



(To Dr. Thomson.) 



SIR, Cambridge, Oct. 4, 1819. 



Among the more remarkable properties of minerals is that of 

 their getatinizatiou in acids. This property, which is confined 

 Vol. XIV. N°VI. 2F 



