Dec. 19, 1884.] 



♦ KNOWLEDGE ♦ 



503 



to intimate that so far as the proper circulatioa of 

 the current is concerned, the number of plates is of no 

 moment. The student mu&t not forget, as it is known 

 many are apt to, that the shape of the plate, or its divisions 

 into any number of parts (all of which are metalically 

 connected) is immaterial. What really governs the 

 strength of the cui-rent is the amount of surface exposed ; 

 the larger the surface, the stronger the current, the more 

 silver will there be dissolved, and the more deposited. It 

 is, nevertheless, not always advisable to have a large sur- 

 face exposed ; this must be governed by the size of the 

 object, and by its composition. There is one point to be 

 observed in connection with the suspension of the silver 

 plate from the brass rod. The contact must not only be 

 sure, but the greatest precaution must be taken to guard 

 against the solution becoming impurified by means of the 

 wires used to suspend the silver. If copper-wire is used, 

 its connection with the plate should be at a point well out 

 of reach of the liquid. Lead and other metals are used for 

 this purpose, but perhaps the best method is to have one 

 or more lugs or extensions from the silver plate which 

 shall be long enough to bend over the brass rod, and so 

 allow the silver plate to haug down and make the necesary 

 contact in virtue of its own weight. When the silver plates 

 are cast, this method may be easily adopted; but if sheet- 

 silver is used, the adoption is still possible. In this case 

 the plate should, instead of being cut square or rectangular, 

 have one or two lugs or exteni-ions left on of the necessary 

 length. When the metal is too thick to bend, it may be 

 attached to the rod by means of a stout clamp, or binding- 

 screw. If it is wished to plate two sidts of an object 

 simultaneously, it may be done, if the current is strong 

 enough, by using three parallel rods instead of two, and 

 suspending silver plates from the two outer ones. These 

 two rods should be placed in connection one with the other. 

 If a round object is to be plated, it may be more advan- 

 tageous to use a cylindrical .silver plate. However, in such 

 ■cases the student must be guided and controlled by the 

 exigences of circumstances. The objects themselves must 

 be suspended from the rod in connection with the zinc or 

 negative pole of the battery. Tliey may be suspended by 

 copper wires, which should, however, be only just thick or 

 stout enough to support the objects in the solution. If 

 thick wires are used, there is great probability that an un- 

 pleasantly-striking dark line will be left round the part of 

 the object covered by the wire. The object to be coated 

 should manifestly be entirely immersed, otherwise only a 

 portion of it will receive the deposit. That part of the wire 

 which envelopes the object and is immersed in the solution 

 receives also a deposit of silver, but this is rather advan- 

 tageous than otherwise. 



Great care should be taken to prevent pieces of foreign 

 metal falling into the solution. Should, for example, a 

 piece of copper wire fall in, it would enter into chemical 

 action with the liquid, precipitate a qu mtity of silver, and 

 be itself dissolved. This would probably cause a deal of 

 trouble. Nor should the objects be immersed before the 

 other portion of the circuit is completed, otherwise con- 

 siderable trouble may ensue. If a copper object were 

 immersed and left hanging without a current being sent 

 through, a portion of it would be dissolved, and a corre- 

 sponding quantity of silver would be deposited, which, 

 however, would be of a non-adhering nature, and conse- 

 quently it would be found that the subsequently properly 

 deposited coating of silver would peel or strip oU'. Even 

 with this precaution, however, the "stripping" will some- 

 times occur, due generally to a want of proper care in 

 regulating the strength of the current. It is advisable, more 

 especially in early experiments, to remove the object from 



the bath, and apply the scratch-brush to it. If it is inclined 

 to strip, this operation will show it. This, though, is a little 

 anticipatory. Another precaution worthy of notice is to 

 avoid placing a fresh object in a bath in which a partly 

 finished object is being deposited upon. Should this be 

 done, there is a chance of local action being set up between 

 the fresh and the partly-finished objects, as we should have 

 two different metals immeised in a liquid and connected 

 externally by a metallic rod (all the conditions necessary to 

 set up a current of electiicity between the metals). 



On first placing the objects in the bath, the silver plate 

 should be lowered gradually until a thin film of silver 

 covers the whole of the object, when the plate, otherwise 

 known as the anode, may be lowered to its full extent. 

 The amount of surface then exposed should not, however, 

 exceed approximately the suiface of the object. If the 

 first film of silver is deposited very rapidly, the current is 

 demonstrated to be too strong, and, in that case, there is 

 great danger of the deposit stripping, whence the necessity 

 for gradually introducing the anode ; that is to say, 

 gradually increasing the strength of the current from the 

 minimum until the requisite strength is obtained. When a 

 considerable thickness of silver is required, as is the case 

 with objects likely to be subjected to considerable wear and 

 tear, the articles are taken out after a few hours' working, 

 and their sui faces scratch brushed, by "which means a more 

 or less crystalline deposit, is, to a great extent, if not 

 entirely, prevented. Were the oViject allowed to remain in the 

 solution continuously until a very strong deposit had accu- 

 mulated, it would most likely be of a crystalline character. 

 The occasioual scratch-brushing and subsequent immersion 

 may prevent this. 



It is, of course, easy to ascertain how much silver is 

 being deposited by first weighing the unsilvereJ article, and 

 then weighing at each i-emoval from the solution, until the 

 requisite quantity of silver is recorded as having been 

 added. 



{To hs continued.) 



AUTOMATIC ARCTIC EXPLORATION. 



THE Chicago Current says : — Probably the most wonder- 

 ful thing in connection with the whole sad history of 

 Arctic exploration is the lecent discovery of an ice-floe in 

 the waters of Davis' Strait — west of Greenland — which had 

 drifted from a point in the Arctic Ocean north-east of the 

 Lena delta — where the crew of the Jeannette divided into 

 three parties and took to the open waters — to the southern- 

 most point of Greenland, and north again to Bafiin's Bay. 

 Upon this floe were a corpse and many indubitablerelicsof the 

 expedition, including an article of wearing apparel marked 

 with the name of Seaman Noros, who, it will be remembered, 

 in company with Seaman Kindermann went a few miles ahead 

 of poor De Long, and lived to write the most extraordinary 

 experience ever penned by a human hand. Had these two 

 simple seamen been able to tell, in the Siberian tongue, that 

 their comrades were only eleven miles back, the whole De 

 ' Long party would have lived to join Melville and Danen- 

 hower. 



Now, the floe discovered by the Greenlanders has, perhaps, 

 crossed directly over the North Pole. From the Jeannette 

 floe to the southern point of Greenland, in a direct line 

 across the Pole, is 3,.500 miles, but bjrway of the northern 

 shore of Asia and Europe — past Cape Northeast, Nova 

 Zembla, Spitzbergen, and Iceland, and north again into 

 Baffin's Bay — would be a distance of at least 6,000 miles. 

 Scientifically, the life of amoving ice-flae for so many years, 

 and its migration from one side of the world to the other, 



