No. XXII.] APPENDIX. 305 



After having procured suitable originals, with proper means of dupli- 

 cation, the next process which we have to consider is that of obtaining 

 perfect moulds. Where the original is of wood, gutta-percha is generally 

 employed, but it is necessary that the mould should be used as soon as 

 made, as it will shrink gradually till it is no longer fit for the purposes 

 required. When gutta-percha is employed it is blackleaded, by the process 

 described by Murray, who was rewarded by a medal from the Society of 

 Arts. It is placed in the solution, and the copper grows over it. All 

 blacklead is not equally good, and when it has remained in the air for 

 some time we find it advisable either to heat the blacklead or use a little 

 bisulphuret of carbon, or other volatile fluid, to drive oiF the adherent air. 



• Occasionally, when we have metal originals, and are pressed for time, 

 we employ clichee moulds, but we never employ them when they can be 

 avoided. The Britannia, I have already stated, is engraved on steel, and 

 moulds are made from it by striking it upon pure soft lead, fixed upon 

 brass plates, by which process very perfect moulds are secured. 



For aU our other originals, when we desire perfection, we rely upon 

 dectro-moulds, and electro-moulds alone. Por this purpose the original is 

 placed in the precipitating trough, and a thick electro-mould deposited. 

 There is very little risk of adhesion, and very little difficulty, with moderate 

 care, in obtaining a perfect mould. I need hardly mention that it would 

 be a serious matter to place the original on the wrong side, for great would 

 be the horror of the operator, on peeping into the trough, to see its costly 

 original to have wasted away, instead of receiving the deposited metal. 



When the electro-mould is sufficiently thick, a wire is soldered to it : 

 it is waxed on the back and sides, and used for the deposition of the dupli- 

 cate. In the use of the electro-mould there is much risk of adhesion, which 

 requires skill to prevent. Sometimes we employ the film of air which I 

 have already described in my ' Electro-Metallurgy; ' sometimes we employ 

 with good success the vapour arising from snlphuret of ammonia, a process 

 which has been specially devised for the purposes of the Bank. In both 

 these cases the moulds are insei-ted into the solution in a dry state, 

 and little bubbles of air are apt to adhere, and be carried down into the 

 solution, to the great detriment of our electro-cast. Upon pondering over 

 this inconvenience, I thought it woxild be desirable to have a process 

 whereby the mould coxild be inserted in a wet state. After some thought 

 and many experiments, it occurred to me that we might use the layer of 

 metal in the infinitely divided state in which it is employed in my battery. 

 With care many metals in that state will answer, but I give the preference 

 to platina. When the process is carefully performed, I have seen the most 

 perfect success attend the platinizing process. Nevertheless, commonly 

 enough, without care, we find that there is a liability for little adhered 

 drops of water to be carried down, which in the electro prqpess have 

 been covered with metal, and the casts show slight indents, which are 

 fatal to success. Upon the whole I regard this process as an addition 

 to our knowledge, and it is particularly applicable to deeply-cut wavy 

 line work. 



The castB of the Britannia are generally deposited so thick in the 

 compound trough that they can be turned down to the required form and 

 size. Other subjects are generally backed with solder, and turned to their 

 proper thickness. In cases where the lines are very thin, and at the same 



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