;8o 



NA TURE 



[FeCKUARY 20, 1908 



COPPER MIRRORS. 

 AT E'lALLIC mirrors have been known from very early 

 •'■^•*- times, and references to tliem arc scattered through 

 ancient literature. They were frequently elaborately 

 decorated, and many of them possess the greatest interest 

 as objects of art. Looking-glasses coated with an 

 .imalgam of quicksilver and tin came into use about the 

 middle of the fifteenth century, but it is not known by 

 whom they were originally invented. The details of their 

 manufacture were, for long, carefully guarded as trade 

 secrets, and were not made public until about a hundred 

 years later. The process then described is in all essentials 

 that still employed, wherever it has not been abandoned 

 on account of the danger to workers from mercurial 

 poisoning. Tin amalgam mirrors were most extensively 

 used during the latter half of last century, but at the 

 present time in England and Germany they are no longer 

 made, as mirrors obtained by the actual deposition of 

 metallic silver upon glass have displaced them. 



This gradual but complete transformation of an 

 important industry had its origin in an observation made 

 by Liebig when investigating the properties of aldehyde, 

 which he had recently discovered. He found that if a 

 solution of silver nitrate to which some drops of ammonia 

 had been added was warmed with the new compound, the 

 silver oxide was immediately reduced, and that the reduc- 

 tion was accompanied by a peculiar phenomenon, the metal 

 attaching itself to the glass in the form of a thin reflecting 

 layer. 



Liebig apparently did not at the time realise the import- 

 ance of his discovery^ in relation to mirror making. This 

 was first done by Thomas Drayton, of Brighton, who eight 

 years later, in 1843, patented a process for manufacturing 

 looking-glasses by a similar reduction of a silver solution 

 by oil of cloves. His process did not prove a commercial 

 success, and was soon abandoned in favour of one worked 

 out by Liebig, in which milk-sugar was used as the 

 reducing agent, and by various modifications of which all 

 mirrors are now made. 



Drayton's method at the time, however, excited wide- 

 spread interest, and Faraday lectured upon it at the Royal 

 Institution, silvering a number of large glass vessels during 

 the lecture, to the great delight of his audience. 



Faraday about this time made the interesting observation 

 that a mirror-like deposit of copper upon glass could be 

 obtained by heating plates of glass in a liquid made by 

 dissolving a little oxide of copper in olive oil. Copper 

 mirrors obtained thus are generally lacking in brilliancy, 

 and if of any size are liable to be stained and discoloured in 

 patches by decomposition products of the oil. Further, as 

 the deposition of the metal only takes place at a tempera- 

 ture above that at which the oil decomposes, the process 

 is excessively disagreeable to carry out, and as the oil 

 is spoiled it is somewhat costly. 



The writer has recently discovered ' that copper can be 

 deposited upon glass from aqueous solution in a film as 

 brilliant as a similarly deposited silver one if a suitable 

 reducing agent be employed. Such a reducing agent is 

 found in phenyl hydrazine, which has the power of readily 

 abstracting oxygen from copper oxide, leaving the copper 

 in the metallic state, and being itself oxidised to benzene, 

 nitrogen, and water. 



To obtain a copper mirror by this process it is best to 

 heat a mixture of one part of freshly distilled phenvl 

 hydrazine and two parts of water until a clear solution is 

 obtained, and to add about half its bulk of a warm 

 saturated solution of cupric hydroxide in strong ammonia. 

 Nitrogen is freely evolved during the addition, and the 

 cupric is reduced to cuprous hydroxide, which remains dis- 

 solved in the ammoniacal liquid, and does not undergo 

 any appreciable further reduction until heated. A hot 

 10 per cent, aqueous solution of potassium hydroxide is 

 iTxt to be added until a slight permanent precipitate of 

 cuiirous hydroxide is produced. If the colourless or pale 

 yellow liquid thus made be cautiously heated in contact 

 with a perfectly clean glass surface, metallic copper is 

 deno.sited upon it in the form of a thin, coherent, perfectly 

 reflecting lamina. 



Ipl'*^'-1,'^°'' "*' T>eD05;ting Copper upon Glass from Aqueous S Unions in 

 a t hin Knlliantly Reneriina Film, and thns producing a Copper Mirror." 

 Read before the Roy.il Society, Nov -"^ - 



As nitrogen is evolved during the reduction, and as 

 tarry bye-products are formed in small quantity and float 

 with the benzene produced to the surface of the liquid, if 

 flasks or tube are to be coppered, devices must be adopted 

 to keep the inner surfaces completely covered by the 

 liquid from which the metal is being deposited, whilst 

 allowing the gas to escape. 



To obtain a film of sufficient thickness to be permanent, 

 it is best to allow it to remain for an hour or so in 

 contact with the warm reducing fluid, and not to pour off 

 the latter until it has cooled to the temperature of the air. 

 The surface of the deposited copper should then be well 

 washed, 6rst with water and afterwards with alcohol and 

 ether, and finallv should be protected from the slow 

 oxidising action of the air by one or two coats of some 

 quick-drying varnish. 



The mirrors thus formed are very beautiful, for they 

 show the splendid red colour of copper, and are more 

 perfect in reflecting surface than the most highly polished 

 metal. They are, moreover, if properly protected from the 

 air, absolutely permanent. It is interesting to note that 

 the copper is in the monovalent or cuprous state in which 

 it is analogous to silver when it shows a similar capability 

 of being deposited upon glass. 



The surface on which the metal is deposited undoubtedly 

 plays an important part in the process, since both silver 

 and copper are deposited much more easily upon surfaces 

 which have not been exposed for any length of time to 

 the action of air or of water, and upon blown than upon 

 polished glass. 



It seems probable that the glass surface itself acts as a 

 catalyser, and locally accelerates the reducing action. 



F. D. CHATTAWAY. 



NO. IQ99. VOL. 77I 



GEOLOGICM. SIRVEY OF CANADA. 



tTEN'EN reports just received from the Geological Survey 

 ^ of Canada afford evidence of the valuable work that 

 is being done in investigating the mineral resources of the 

 Dominion. In Report No. 949 Mr. D. B. Dowling 

 describes the Cascade coal basin, Alberta. He gives an 

 outline of the geology and topography of the coalfield, 

 and a detailed account of the character of the coal, thick- 

 ness of seams, and extent of the measures. The report 

 is accompanied by eight folding maps. The area illus- 

 trated on the map sheets lies within and to the east of the 

 summit of the Rocky Mountains, the formations exposed 

 giving a continuous section from the highest remaining 

 beds of the Cretaceous down to the bottom of the 

 Carboniferous. The coal is of Cretaceous age. In the 

 hills south of the Bow River ten or eleven seams, more 

 than 4 feet thick, have been found, while north of Bank- 

 head, on the slope of Cascade Mountain, fourteen possibly 

 workable seams occur. At the Bankhead colliery the coal 

 is an anthracite, admirably suited for domestic purposes. 

 A screening plant handling 1000 tons a day has been 

 erected. In Report \o. 953 Mr. H. S. Poole describes the 

 barytes deposits of Lake Ainslie and North Cheticamp, 

 Nova Scotia, and gives notes on the production, manu- 

 facture, and uses of barytes in Canada. Report No. 958 

 is devoted to Dr. O. C. Hoffmann's review- of the work 

 done in the laboratory of the survey during the year. It 

 covers seventy-one pages, and contains a large amount of 

 material of chemical and mineralogical interest. In 

 Report No. 968 Mr. D. D. Cairnes gives an account of 

 the geology of the Moose Mountain area of the disturbed 

 belt of southern .Mberta. Coal has been found in several 

 places within this district, and natural gas has been found 

 to the north, south, and east of this area in the same 

 formations as those within it. In Report No. 977 Mr. 

 R. W. Ells gives an account of the geology and natural 

 resources of the area included in N.W. Quarter- 

 sheet No. 122 of the Ontario and Quebec series, com- 

 prising portions of the counties of Pontiac, Carleton, and 

 Renfrew. A lengthy list of fossils from the Chazy, 

 Black River, Trenton, and Pleistocene formations com- 

 prised within the area, compiled by Dr. H. M. Ami, is 

 appended. The minerals of economic value met with 

 include iron ore, of which there is a workable deposit at 

 I Bristol mines, galena and zinc blende, mica, asbestos. 



