174 APPENDIX. [No. VII. 



course it is a perfect facsimile, and therefore tHs mettod -would be of the 

 greatest importance to bankers for their notes, and is far superior to Mr. 

 Perkins's apparatus for the multiplication of plates, because in that case 

 they almost invariably require to be touched up afterwards, and therefore 

 absolute identity is destroyed. The cost of their manufacture would be 

 trifling, being merely the value of the zinc * dissolved in the battery, and 

 a pound of zinc of the value of sixpence would produce a copper-plate 

 weighing about two pounds ; and I trust that copper will again, from its 

 beauty, take the place of steel engravings. 



So much for the precipitation of the copper : and the next thing to 

 which I have to direct your attention, is a mode of making copper-plate 

 engraving without an engraving in the first instance. This is done by 

 drawing upon a smooth piece of copper (such as a plate used for engrav- 

 ing) with any thick varnish or pigment insoluble in water, and then 

 exposing the plate in the usual way to the influence of the current, when 

 first copper will be thrown down upon the uncovered parts and will 

 gradually grow over the drawing, and the electrotype when removed will 

 be ready for printing. A practical difficulty arises in the application of 

 this for the arts, as unless very thick oil paint is used, sufficient depth is 

 not obtained to hold the ink. However, judging from the sharpness of 

 the edges of the lines, I have but little doubt that this difficulty may be 

 overcome by those who are accustomed to drawing ; and it possesses, as 

 an additional advantage to its cheapness, the valuable property of not 

 requiring the artist to reverse the design. An opposite effect to this may 

 be produced by placing a piece of copper similarly drawn upon at the 

 oxygen end of the battery, when the metal will be acted upon, leaving a 

 drawing in basso-rilievo. ^ 



No. VII. 



ON THE PEEROSESQUICTANUBET OP POTASSIUM. By Alfred 

 Smee, Esq., Surgeon. (' London and Edinburgh Philosophical Maga- 

 zine and Journal of Science,' September 1840.) 



The action of chlorine upon the ferrocyanuret of potassium is a subject 

 of much interest to the chemist, and has not been examined to any extent 

 in this country. It therefore has been my endeavour to investigate this 

 action carefully, and to see under what circumstances the change from the 

 ferrocyanate into the ferrosesquicyanuret takes place ; and the methods 

 which are here detailed to obtain this latter salt uncontaminated with 

 impurities, will be found free from the difficulties and uncertainties 

 attending on the present mode of preparing it. 



When a current of chlorine is passed through a solution of ferrocya- 

 nate of potassa, or an aqueous solution of that gas is added to it in certain 

 quantities, the persalts of iron are not precipitated. This solution has no 



* The zinc in the fluid might be precipitated as a carbonate, for which there 

 is great demand in the arts, and thereby the expense of the electrotype would 

 be further diminished. 



