March i, 1900] 



NATURE 



429 



Until the advent of the dynamo and the storage battery, 

 methods of electrical analysis or of galvanic deposition could 

 not profitably be employed, at any rate on a very large scale ; 

 now, however, with the cheapening of production, very much has 

 been made possible, which but a few years ago would have been 

 scouted as Utopian. Is it realised by those who admire the 

 splendid photographic reproductions which are met with on 

 every hand, even in penny illustrated journals, that the majority 

 of these are reproduced by means of electrical processes ? A 

 scene, e.g. the leaving of volunteers for the seat of war, is 

 photographed, and in a few hours by means of electrical stereo- 

 type is transferred to a metal plate and is ready for the printer. 



Those interested in metallurgy knows how difficult it is to 

 obtain absolutely pure copper by furnace methods, even after 

 many processes of refining, the copper still contains small 

 quantities of impurities from which it is only with great diffi- 

 culty freed. When copper is required for electrical purposes, very 

 small quantities of impurities considerably redvice its conducting 

 power. For this reason then it is important to obtain perfectly 

 pure copper ; and as the impurities often consist of gold and 

 silver, endeavours have been made to obtain a process which 

 would produce absolutely pure copper, and at the same time 

 leave the gold and silver in a workable condition. Thanks to 

 electro-chemistry, such a process has been devised and is now 

 very largely used. The copper which has either been cast into 

 plates or bars, or granulated, is placed in a bath of copper 

 sulphate and connected with an electrical machine as the anode, 

 a plate of pure copper forming the kathode. On the current 

 being passed, the copper at the anode dissolves and is deposited 

 on the kathode in the pure condition ; the impurities— gold, 

 silver, antimony, arsenic, etc., dissolving in the bath or re- 

 maining on the anode as a sponge, finally falling to the bottom 

 of the bath forming a muddy sediment (" anode sludge "). 



The electrolytic deposition of copper is also made use of in 

 the manufacture of weldless copper tubing. In a bath of copper 

 sulphate, granulated copper is niade the anode, the kathode con- 

 sisting of metallic spindles, the thickness of which is determined 

 by the size of the tube it is desired to produce. The spindle is 

 made to revolve at a very high rate of speed ; by this means the 

 copper, which ordinarily is deposited in a more or less crystal- 

 line condition, forms a dense and even deposit, which, when 

 used for boiler tubes, &c., is capable of sustaining very high 

 pressures without bursting. 



Although it has not up to the present been found practicable to 

 obtain zinc commercially from its ores by electrolytic methods, 

 notwithstanding the many attempts which have been made, it is 

 interesting to note that zinc galvanising by electrolysis is now 

 being used to a very considerable extent, in place of the old 

 method of hot dipping, the zinc, as in the case of the copper 

 deposition just described, being made the anode in a weak acid 

 bath, the material to be galvanised the kathode. The galvanis- 

 ing of the inner surfaces of tubes has always been very difficult ; 

 in the electrolytic method the difficulties are overcome. The 

 bottoms of ships, torpedo-boats and other large surfaces are now 

 frequently galvanised by means of the electrical galvanic process. 



The advantages which this method has over the hot dipping 

 process are obvious. In the latter, many tons of zinc have to 

 be kept in a state of fusion over long periods, the expense of 

 fuel being very great, and the loss of zinc through alloying and 

 oxidation very considerable. In the old process the tensile 

 strength of the iron and steel is said to be diminished. In the 

 new process it is claimed that the quality is in no way de- 

 preciated. 



Probably '* the man in the street " would point to the manufac- 

 ture of aluminium as being the triumph of electrical chemistry. 

 Certainly, here is a triumph, and it will perhaps be interesting to 

 briefly trace the development of the industry since the discovery 

 of the metal by Wohler in 1827. He obtained it as a grey 

 powder by heating aluminium chloride with potassium. In 1856, 

 Bunsen prepared it by electrolysis of the double chloride of 

 sodium and aluminium. Deville, in 1854, obtained it by the 

 action of sodium on this same double chloride, but owing to the 

 high price of sodium, aluminium cost, in 1857, 5/. the pound ; 

 even in 1888 the price was over 2/. per pound. In this year the 

 Netto-Castner process for the manufacture of sodium was per- 

 fected, and as a consequence the price of aluminium at once fell 

 to 15^-. the pound. Shortly after this the electrical methods 

 were employed, and now aluminium may be obtained at less 

 than 2s. the pound ; the cost would probably be still further 

 reduced if the metal itself were of more general use. 



NO. 1583, VOL. 61] 



Calcium carbide, which is now being largely manufactured by 

 heating a mixture of limestone and coke in an electrical furnace, 

 has made it possible to obtain, at a stoall cost, acetylene gas, 

 which, were it not for the restrictions that are placed upon the 

 storage of the carbide, would probably be used to a very much 

 greater extent. The extraordinary brilliancy of the light which 

 this gas gives causes one to wonder that attempts have not been 

 made to employ it for street lighting. 



In gold, silver, and nickel plating, electrolysis on a small 

 scale has been employed for a number of years, and within the 

 last few years it has been successfully used on a large scale for 

 gold extraction. In the Mc Arthur- Forrest cyanide process for 

 obtaining gold from the " tailings," a double cyanide of gold 

 and potassium is obtained ; from this double cyanide the gold 

 is precipitated by means of metallic zinc, the gold so obtained 

 having to be refined and purified. By the Siemens-Halske 

 process the cyanide solution is electrolysed by means of weak 

 currents, the gold being deposited in a ourer form than when 

 precipitated by means of zinc. In this process the anodes are 

 of iron, and the kathodes on which the gold is deposited of 

 lead ; an amalgam of lead and gold being obtained, from which 

 the latter is recovered by cupellation. By this method the gold 

 is obtained purer, and the quantity of cyanide employed is much 

 less than is the case in the original cyanide process. There is 

 little doubt that in a short time a very large percentage of the 

 " tailings," both in Australia and Africa, will be worked by means 

 of electrical processes, and it is not impossible that a process will 

 be devised for the treatment of the auriferous quartz, though, up 

 to date, attempts in this direction have not been commercially 

 successful. 



Either with the electric furnace or by electrolysis, it is now 

 possible to obtain practically all the metals from their oxides or 

 salts, e.g. chromium, by heating the sesquioxide with aluminium 

 in an electric furnace ; magnesium, by electrolysis of fused car- 

 nallite ; the rare metals yttrium, lanthanum, and cerium have 

 also been isolated by electrolysis of their fused chlorides. An 

 interesting process for purifying tin might be mentioned here. 

 The metal obtained from American sources often contains ap- 

 preciable quantities of gold. A very neat method for separating 

 the tin from the gold has lately been patented. The gold-con- 

 taining tin forms the anode in a bath of sodium sulphide, the 

 kathode being a strip of pilre tin ; on the current being passed, the 

 tin at the anode dissolves, forming a thio salt, from which it 

 is deposited, pure, on the kathode. The gold with other im- 

 purities hangs on the anode in a spongy form, or falls to the 

 bottom of the bath, as "anode sludge," from which it is readily 

 extracted. 



As showing the many-sided character of electro-chemistry, an 

 interesting process for obtaining accurately reflecting mirrors 

 might be mentioned. Many attempts have been made to pre- 

 pare perfect metallic mirrors, in which the use of glass could be 

 done away with, and on a small scale for lamps the attempts 

 have been fairly successful ; but where a moderately large and 

 true reflecting mirror is required, attempts to substitute cast, 

 spun or stamped metal for glass have always been unsatisfactory. 

 The difficulty seems to have been overcome in a process brought 

 out by Mr. Cowper-Coles. A glass mould is obtained, the 

 convex side of which is accurately shaped and polished to form 

 a true parabolic or other reflecting surface. On the prepared 

 surface a metallic coating of silver is deposited by chemical 

 means ; it is then polished, and a backing of copper is deposited 

 to any desired thickness, by making the silver the kathode in a 

 bath of copper sulphate, the mould being at the same time 

 rotated in a horizontal position. The copper adheres firmly 

 to the silver, and as soon as sufficient has been deposited the 

 glass mould is placed in cold water, which is gradually warmed. 

 The unequal expansion of the metal and the glass causes the two 

 to separate, yielding a concave surface of silver on copper, 

 exactly corresponding to the mould, which requires no further 

 polishing. As, however, silver when exposed to atmospheric 

 conditions rapidly tarnishes, metallic palladium is electrolytically 

 deposited on its surface. Palladium is not affected by atmo- 

 spheric changes, being practically unoxidised even at high tem- 

 peratures. Its reflecting power, moreover, is but little inferior to 

 that of silver. 



A metallic reflector prepared after this method was recently 

 tested at Portsmouth, a number of rifle bullets being fired 

 through it ; it was even then found that the beam was but 

 slightly affected. Whereas one shot fired at a glass reflector 

 smashed it to pieces. It is obvious then that such mirrors should 



