280 Mil. Tennent on the Yellow Prussiate of Potash Cake. 



The following paper was communicated by Dr. R. D. Thomson : — 



XLIL- — Examination of the Yellow Prussiate of Potash Cake. By 

 Mr. Hugh Brown Tennent, Laboratory Assistant, Glasgow College. 



Yellow prusstate op potash or ferrocyanide of potassium was dis- 

 covered by Macquer, and used as a test for persalts of iron. It was 

 introduced into calico printing by Charles Macintosh, Esq., of Campsie, 

 and was first manufactured upon the large scale at Campsie.* The process 

 consists in igniting substances rich in nitrogen, such as, hoofs, horns, 

 dried blood, woollen rags, &c, with carbonate of potash, in iron pots. 

 The fused mass is then digested in water, and the yellow salt crystallized 

 out. As much difference of opinion has existed respecting the composi- 

 tion of the black cake, when it is removed from the iron pots, previous to 

 its digestion in water, the following experiments were made for the 

 purpose of throwing some light on the nature of its constituents, and as 

 the analyses are somewhat complicated, every step has been carefully 

 described. 



50 grains were digested in cold water, and filtered, the filter being 

 previously dried at 212° and weighed, it was then washed, until the 

 washings gave no reaction with nitrate of silver. The filter was again 

 dried and weighed, when it gave 40*50 of insoluble matter, showing the 

 presence of 59*5 per cent, of soluble salts and water. 



I. Analysis of the Soluble Salts. 



1. Estimation of the Carbonate of Potash. — To the solution which passed 

 through the filter, as previously described, chloride of barium was added, 

 which gave a bulky white precipitate, consisting of carbonate and sulphate 

 of barytes, and caustic barytes. To determine the true quantity of carbonate 

 of potash, the precipitate, after being rapidly washed under cover and 

 ignited, was introduced into a flask containing a tube filled with hydro- 

 chloric acid, the bottle was then closed with a cork, through which a tube 

 filled with chloride of calcium passed. The whole was then weighed and 

 the weight carefully noted; the bottle was then shaken so as to overturn 



* The following is an interesting note from Mr. Clarke regarding the commercial 

 history of the prussiates : — 



" Campsie Alum Works, 2d October, 1848. 

 u My Dear Sir,— The period when prussiate of potash and prussian blue were first 

 begun to be manufactured here, was in 1807-8. The works were erected in 180G-7; and, 

 when I came here in 1809, the manufacture of both these articles was in operation, and 

 had been so for fully a year before. The prussiate was for some time, at that period, 

 sent to the calico printers and dyers in a liquid state, with printed directions for the 

 proper mode of applying it; but there was also a crystallization of it made when I came 

 here, and in this form it came very soon to be preferred, and the liquid was abandoned. 

 " I am, my Dear Sir, yours truly, 



M ROBT. CLARKE. 

 " Dr. R. D. Thomson." 



