444 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



the article to heat, the mercury will evaporate, and the gold or 

 silver will be left. Articles maj' be washed with gold or silver 

 by simply immersing them in some acid, holding the gold or sil- 

 ver in solution. But all these methods are adapted merely to 

 give an ornamental surface ; for they furnish little protection to 

 the metal which is covered. For protection, a common method 

 is to dip a solid metal into another in a melted condition ; as in 

 the coating of sheet iron with tin or zinc. A great difficulty in 

 this process is in causing the melted material to lie evenly upon 

 the other. If a screw, for instance, is dipped into melted tin, the 

 thread will be almost invariably filled, thus destroj'ing its value. 

 The most successful method of coating screws with tin, had been 

 to agitate them violently before the tin had time to set, at the 

 same time pouring water upon them to cool them. But this is a 

 bungling way. 



The Chairman, (Mr. Johnson.) — Centrifugal force may be so 

 applied as to accomplish it. 



Mr. Stetson. — That is worth trying. Another question firises, 

 whether we can plate an article a sixteenth of an inch thick, or 

 more, and do it smoothly, so as to be able to cover the bearings 

 lof heavy shafts. The inside of the air-pump of marine engines is 

 lined with brass in this way. A brass tube is made nearly to fill 

 the inside of the iron tube, and is then hammered to expand it 

 until it fits tightlj'. We thus liave the etFect of a brass pump 

 with the strength of iron. The air-pump rod is made of iron and 

 melted brass is poured around it. Another method is to take a 

 rod of iron coated with tin, and pass it when hot within a tube 

 of brass also tinned and heated, thus soldering the two together. 

 This will do very well, if the rod is not afterwards to be heated. 

 Electro plating is another process analagous to tinning ; but we 

 cannot deposite evenly more than a very moderate thickness. 



Mr. Rowell. — In tinning tacks they are thrown against a wall 

 with a great deal offeree, so as to knock off the useless tin. 



Mr. Churchill stated that almost all metals, with pressure and 

 heat, may be made to weld together far below their melting point, 

 when air is kept from contact with them. He also gave details 

 of improvements in tinning and a recipe for a soldering liquid. 



Mr. Seely suggested that in applying the centrifugal force in 

 tinning screws, it would be necessary that the screws should be 

 kept hot ; and that would cause the tin to penetrate and injure 

 the iron. Dip a thin iron plate in melted tin and it is soon dis- 



