102 Dr. Thomson on [August, 



Article IV. 



On the Salt usually called Triple Pmssiate of Potash. By- 

 Thomas Thomson, M.D. F.R.S. Regius Professor of Che- 

 mistry in the University of Glasgow. 



My attention was accidentally drawn to this salt in conse- 

 quence of a visit, which I paid last winter to a prussian blue 

 manufactory belonging to my friend Charles Macintosh, Esq. of 

 Glasgow. This work is conducted in a manner that does great 

 credit to the well known genius and skill of the proprietor. I 

 was surprised to find in it the direct preparation of the triple 

 prussiate of potash on a large scale in most magnificent crystals 

 of the greatest purity and beauty. I was induced to make a few 

 trials on this salt, thus furnished in such abundance, chiefly 

 with the view of determining the component parts of the acid 

 which it contains. Mr. Porrett was the first who demonstrated 

 that this acid is of a peculiar nature ; and from the constituents 

 which he detected in it, he gave it the name of ferrureted 

 chyatic acid. I was induced from strong analogies to consider 

 this acid as a compound of an atom of cyanogen and an atom of 

 iron ; but it will be seen from the experiments which I have to 

 state, that Mr. Porrett's opinion, that hydrogen exists in it as a 

 constituent, is correct. Mr. Porrett's very ingenious and 

 important experiments and conclusions respecting this acid will 

 be found in the Phil. Trans, for the years 1814 and 1815. 



1 . The triple prussiate of potash when prepared in small quan- 

 tities usually crystallizes in cubes, or at least in a form which to 

 the eye appears cubic. But the large crystals formed in Mr. 

 Macintosh's manufactory are always square plates with bevelled 

 edges, usually about half an inch thick, and two or three inches 

 in diameter. If we suppose an elongated octohedron, like the 

 primitive form of the anatase ; and suppose further, that the apex 

 of each of the two four-sided pyramids of which the octohedron 

 is composed, is truncated very deep by a plane parallel to the 

 base of the pyramids, we shall have a pretty accurate idea of 

 the appearance of these crystals. The two faces of the truncated 

 pyramids, or the bevelled edges of the tables, are inclined to 

 each other at an angle of about 135°, I have examined several 

 hundred crystals without being able to detect any other form. 



The colour of the triple prussiate, as prepared in Mr. Macin- 

 tosh's manufactory, is a fine topaz yellow. It is transparent, 

 and when held between the eye and the light, appears green. 

 Its specific gravity is 1*833. Its taste is saline and cooling, and 

 by no means disagreeable. It is not brittle like many other 

 crystals, but splits into plates parallel to the bases of its pyra- 

 mids, and these plates admit of being bent without breaking. 

 Three cleavages may be perceived in it ; one very distinct, parallel 



