212 Mr JOHNSTON on the Combination of Chlorine 



added together and evaporated, a small quantity of a dark thick 

 liquid is obtained, similar in appearance to that occurring in the 

 preparation of the cyanide of mercury, having a peculiar smell, 

 approaching to that of weak chlorine, and being of a blackish by 

 reflected, but of a deep red by transmitted light. By farther 

 concentration, this liquid is partly decomposed, depositing a 

 green sediment, and by slow cooling gives crystals of a deep red 

 colour, in doubly oblique four-sided prisms, terminated some- 

 times by two or three planes, and not unfrequently acuminated 

 into pyramids. These crystals are insoluble in alcohol, unless 

 considerably diluted, but very soluble in water ; and the solu- 

 tion, which, even when very weak, is of a bright greenish-yellow 

 colour, has all the properties above mentioned. Sometimes the 

 crystals are deposited in very minute needles, when they are of a 

 bright golden-yellow colour, and sometimes in beautiful red tables. 



If this salt be reduced to powder, and treated with concen- 

 trated sulphuric acid, it gives off chlorine, and on the application 

 of heat hydrocyanic acid. Its solution with tartaric acid gives 

 crystals of bi-tartrate of potash ; and heated per se, in an open 

 crucible, it leaves an oxide of iron. It contains therefore chlo- 

 rine, cyanogen, potash, and iron. In having the first of these 

 for one of its constituents, it differs from the common prussiate 

 of potash. 



Having shown this salt to Dr THOMSON, I was referred by 

 him to a paper by LEOPOLD VON GMELIN in SCHWEIGGER'S 

 Journal, N. S. vol. iv. p. 325, in which he describes a salt in red 

 prisms, having properties precisely the same as those above stated. 

 The angles of the rhombus, which I find, by careful measurement, 

 to agree with those of the salt obtained as above, he states at 81 

 48' and 98 12', and their diagonals as 2:^3. The salts are 

 therefore identical, though, as we shall afterwards see, VON 

 GMELIN has mistaken its composition. 



To obtain this salt, he passes a stream of chlorine gas through 



