502 



• KNOWLEDGE ♦ 



[Dec. 19, 1884. 



It will be granted tliat a theory which leaves no loop or 

 part of a loop without an explanation at once complete and 

 simple, well deserves all the admiration which men have so 



ling awarded to the labours of Copernicus, Kepler, and 

 Xewton. 



(To ie continued.) 



E L E C T R - P L A TI N G. 



By W. Slingo. 



XV.— PREPAEIXG THE BATH, &c. 



G1 ERMAN silver articles having been subjected to the 

 r cleansing processes, require jet another operation 

 prior to immersion in the depositing bath. This consists 

 in dipping them into a tolution of nitrate or cyanide of 

 mercury. The former is more generally adopted, and is 

 prepared by dissolving an ounce of mercury in nitric acid, 

 diluted with three timf s its volume of distilled water. The 

 quantity of acid rr quired varies with its quality. As it is 

 desirable that there should not be either too much metal 

 or tco much acid, the best plan, perhaps, is to place the 

 mercury in a vessel, and then pour the dilute acid over it 

 until the whcle of the metal is dis>olved and converted 

 thereby iDto nitrate of mercury. When this is accom- 

 plished, water is added iintil the bulk is increased to one 

 gallon, when a littlo additional nitric acid may be poured 

 in. The immersion of the German silver articles in this 

 solution results in a film of mercurv, of a greyish or 

 blackii-h colour, being deposited upon them. The charac- 

 teristic brightness is produced by brushing the film softly. 

 As mercury oxidises sonjewhat n adily, it is essential that 

 the article should be immediately transferred to the deposit- 

 ing bath. In the event of the paits of an article being 

 soldered or leaded together, yet another process is 

 necessary. By simply sulijecling the solder, &c., to the 

 abovementiontd operation it lemaiLS obdurate, and will 

 rarely take a deposit of silver. If, however, a stronger 

 mercury solution is applied to the solder or other joint, the 

 necessary metallic film is deposited ; but a better way, 

 perhaps, is that recommended by Watt. The edge or joint 

 is immersed in a s lution of sul[jhato of copper, and one end 

 of a rod of iron being held in the hand, the other end is made 

 to touch in succession the various parts of the joint. By 



so doing, electrolytic action is set up, the sulphate of copper 

 is decomposed, and a bright metallic cojiper film is deposited 

 upon the solder. After well linsing, it is ready for the 

 silvering bath. Another plan which answers well, more 

 particularly if we have a joint other than an edge to deal 

 with, is to attach one or two thin iron wires to the handle 

 of a camel-hair brush, and moistening the latter with a 

 weak solution of sulphate of copper, pass it slowly over the 

 joint. The iron wires should be bo arranged that, travelling 

 with the brush, they come into contact with the successive 

 parts of the soldering immediately after they have been 

 wetted by the brush. The same action then takes place as 

 would result from immersion in the sulphate of copper, a 

 copper film being deposited on tlie solder. 



To coat brass or copper with a deposit of silver, the same 

 series of operations should be gone through as in the case of 

 German silver. 



Turning now to the electrolytic cell, the presence of the 

 cyanide prevents the use of that form recommended for 

 the sulphate of copper solution, and it would be the best 

 economy to procure an earthenware bath. However, as 

 has been previously pointtd out, the adoption of the ortho- 

 dox rectangular form is not in any way obligatory, and 

 vessels of other shapes may be more cheaply purchased. 

 They will, in all probability, require a somewhat larger 

 quantity of solution, but that is of no great moment, seeing 

 that it does not materially deteriorate. The relative 

 dimensions of the bath must to some extent be determined 

 by the shape of the object to be plated, so that it would be 

 well for the student to make up his mind as to the kind of 

 work he contemjilates turning out before he provides 

 himself with his bath. Fer some objects he would require 

 a tall, long, and narrow bath, for others a sliort, wide one, 

 while other?, again, might as well be suited l>y a circular 

 bath as by any other. 



Having determined the kind of work to be done, and 

 procured the bath, the next thing is to procure a couple, or, 

 if necessary, more, bright brass or copper rods, about two 

 inches longer than the bath and a quarter of an inch in 

 diameter. To one end of each rod should be attached a 

 binding-screw, for the purpose of making connection, by 

 means of copper wire, with the terminals of the battery. 

 If the student has the facilities, he may accomplish this 

 object by tapping one end of each rod, and then making a 

 screw to fit. The wire may then be clamped between the 

 screw-head and the end of the rod. 



These rods then become the terminals of the battery. 

 From one of them — that is, from the one in connection 

 with the copper, or positive, pole of the battery — must be 

 susjjended the silver plate or plates intended to form the 

 anode of the electrolytic bath. Only fine silver should be 

 used, as impurities may impair the deposit, and they will 

 certainly injure the solution. The number and form of 

 the silver plates must depend upon the shape of the article 

 to be plated. They should, however, be of considerable 

 thickness, for it must be borne in mind that for every atom 

 of silver deposited upon the object, a corresponding atom 

 should be dissolved from the plate connected with the 

 battery. If this docs not take place, then the atom de- 

 posited is withdrawn from the solution, and, no compen- 

 satirg action taking place, the solution cannot fail to be 

 proportionately weakened. In fact, were this to continue, 

 we should soon find hydrogen or some other metal accu- 

 mulating over the surface of the object instead of silver. In 

 the case of a flat bath, used for plating medallions or such- 

 like objects, flat plates of silver answer best. When the 

 bath is long as well as narrow, it may be advantageous to 

 uf e a number of such plates, but all should obviously be 

 suspended from the same rod. It is hardly necessary 



