



COATING OP IRON WITH COPPER. 93 



in commercial yellow prussiate of potash: 80 per cent, of cyanide 

 of potassium is the greatest proportion that this process can give. 

 We have occasionally obtained it at 78 per cent, from commercial 

 materials, but more generally at 70 and 72 per cent. ; and we have 

 found cyanide of potassum in the market containing as little as 49 

 per cent, of pure cyanide. 



The results of the manufacture of this salt on a large scale, from 

 the ordinary materials of commerce, show that 551bs. of yellow 

 prussiate, dried as directed above, yield 481bs. ; and 191bs. of car- 

 bonate of potash give 181bs. of dry salts ; in all 6 Gibs, of the proper 

 mixture. The crucible used was of this shape, capable 

 of holding from two to three pints : in general two 

 of them were used up in making the above quantity 

 of cyanide, even when great care was taken. One 

 great cause of the crucible giving way is the depth 

 of the fire, and openness of the bars of the grate. 33. 

 The bottom of the crucible, between each pair of 

 bars, fuses from the great heat concentrated near the opening. To 

 remedy this evil, a square tile of fire-clay should be laid upon the 

 bars upon which the crucible is to rest. The tile must not cover 

 all the bars, else the draught will be stopped an equal space must 

 be left at each side of the tile, which will preserve a regular heat 

 around the crucible. 



The quantity of clean cyanide of potassium obtained from the 

 above quantity of materials was about 381bs. ; the sediment scraped 

 out of the crucible, being put into water, yielded about Gibs, more 

 in solution, but of inferior quality good enough, however, for 

 precipitations, the cleaning of silver, and other general purposes in 

 the factory. 



It may be mentioned that in these operations the crucible is 

 never allowed to cool, but as soon as the sediment is scraped out, 

 it is again put into the furnace. If the iron sediment is not well 

 cleared out, it imbibes oxygen rapidly, and the charge next taken 

 from the crucible will have an excess of cyanate of potash, besides 

 lessening the capacity of the crucible. Generally speaking, how- 

 ever, even when the utmost care is taken, the last charge has more 

 cyanate of potash than the first. 



Cyanide of Copper. To prepare copper solutions by means of 

 cyanide of potassium, for covering iron and other positive metals, 

 there are several methods. 



First Method. To a solution of sulphate of copper, add by 

 degrees a salution of cyanide of potassium, which will give a 

 yellowish green precipitate, with slight effervesence. There will 

 be evolved a gas, having a most pungent odour, to prevent the 

 inhalation of which the most watchful carefulness has to be 



