THE TECHNOLOGIST. [March 1, 1865. 



348 ON CHEMISTRY APPLIED TO THE ARTS. 



settle, and, lastly, dried and sold under the name of animal black. Its 

 chief uses are in the manufacture of blacking and printing ink. Another 

 manufacture which consumes a large quantity of animal refuse, especially 

 the horns, hoofs, &c, of too inferior a quality to be used for the pur- 

 poses described in my first lecture, is that of the yellow prussiate of 

 potash, a most important salt, for it is extensively used in calico print- 

 ing, silk and wool dyeing, and in the manufacture of the pigment called 

 prussian blue — for gilding silver, copper, and other inferior metals ; and 

 lastly, it is the source from which cyanide of potassium is procured, a 

 substance much employed in the art of photography. Let me now call 

 your attention to the manufacture of prussiate of potash, the greatest 

 portion of which is still prepared at the present day by the old process 

 devised by Dr. Woodward, F.R.S., in 1724. It consists in introducing 

 into large cast-iron pots American pearlash, melting it, closing the ves- 

 sel, and then setting the mass in motion by means of a revolving shaft. 

 At this period of the operation, hoofs, horns, and other animal refuse 

 are introduced in small quantities at a time. Under the influence of 

 heat and of the alkali, the nitrogen of the organic matters splits into 

 two parts, one part combining with the hydrogen to form ammonia, 

 which escapes, whilst the other portion unites with the carbon, pro- 

 ducing cyanogen, which remains combined with the potassium of the 

 potash. After several hours the operation is considered to be completed 

 and the melted mass is run out into small cast-iron receptacles ; when 

 cool, these are placed in large vats with water, and a jet of steam is in- 

 troduced, and the whole is kept on the boil for several hours, when the 

 cyanide of potassium is partly decomposed, giving rise to carbonate of 

 potash and to cyanide of iron, for not only has a portion of the iron of 

 the melting pots been attacked and combined with the mass, but a cer- 

 tain quantity of iron filings has been used during the operation. How- 

 ever, two parts of the cyanide of potassium combine with one part of 

 cyanide of iron, and the result is that a double cyanide, called ferro- 

 cyanide of potassium, or yellow prussiate of potash, is formed. The 

 liquors are then allowed to clear by standing, and the aqueous solution 

 is evaporated until a pellicle appears on its surface, when it is permitted 

 to cool, and the salt is deposited on strings which have been passed 

 through the crystallizing vat, and which facilitate the crystallization of 

 the prussiate salt. In consequence of the large amount of animal mat- 

 ter used as compared with the quantity of prussiate obtained, this salt 

 has always commanded a good price in the market, and has induced 

 many eminent chemists to try to devise cheaper processes for obtaining 

 it. To attempt here to give merely an outline of these various proposed 

 plans would involve so much technical description as would occupy far 

 too much time for this lecture, but I would recommend those interested 

 in this branch of manufacture to read the learned account given by Dr. 

 A. W. Hoffman, in his report on " The Chemical Products in the last 

 Exhibition," page 57, where they will find the process of M. Gauthier- 



