408 SCIENTIFIC RECREATIONS. 



are made perfectly clean on the surface, and a liquid, made by dissolving 

 mercury in nitric acid (aqua-fortis), is passed over them with a brush made 

 of fine brass wire, called a " scratch-brush." The mercury immediately 

 adheres to the surface of the metal, making it look like silver ; when this is 

 done, a little of the amalgam is rubbed on, and the article evenly covered 

 with it. It is then heated in a charcoal fire till all the mercury evaporates, 

 and the brass is left with a coating of gold, which is very dull, but may be 

 burnished with a steel burnisher and made bright if necessary. In former 

 times articles were inlaid with thin plates of gold, which were placed in hollows 

 made with a graver, and melted in, a little borax being applied between. 



When a solution of " chloride of gold " is mixed with ether, the ether 

 takes the gold away from the solution, and may be poured off the top charged 

 with it. This solution, if applied to polished steel by means of a camel-hair 

 pencil, rapidly evaporates, leaving a film of gold adhering to the steel, which, 

 when burnished with any hard substance, has a very elegant appearance. 

 In this way any ornamental design in gold may be produced, but it is not 

 very durable. The gilt ornaments, scrolls, and mottoes on sword-blades, are 

 sometimes done, in this way. 



PLATINUM is the heaviest of all metals, gold being next. Platinum 

 is practically infusible, and quite indifferent to reagents. It is therefore 

 very useful in certain manufactories, and in the laboratory. It can be 

 dissolved by aqua-regia. The stills for sulphuric acid are made of platinum, 

 and the metal is used for Russian coinage, but must be very difficult to 

 work on account of its infusible property. 



In the finely-divided state it forms a gray and very porous mass, which 

 is known as spongy platinum, and possesses the remarkable property of con- 

 densing gases within its pores. Hence, when a jet of hydrogen is directed 

 upon a piece of spongy platinum, the heat caused by its 

 condensation suffices to inflame the gas. This singular power 

 has been applied to the construction of a very beautiful 

 apparatus, known as Dobereiner's lamp, which consists of a 

 glass jar, a, covered by a brass lid, e, which is furnished with a 

 suitable stop-cock, c, and in connection with a small bell jar, /, 

 in which is suspended, by means of a wire, a cylinder of 

 metallic zinc, z. When required for use, the outer jar is two- 

 thirds filled with a mixture of one part sulphuric acid and four 

 P ai "ts water, and the stop-cock opened to allow the escape of 

 lamp. atmospheric air, the spongy platinum contained in the small 

 brass cylinder, d, being covered by a piece of paper. The stop-cock is then 

 closed, and the bell jar,/, allowed to fill with hydrogen, and after it has been 

 filled and emptied several times, the paper is removed from the platinum 

 and the cock is again opened, when the gas, which escapes first, makes the 

 metal red-hot and finally inflames. This property of platinum is also used 

 in the " Davy " lamp. 



