VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. QqQ 



congelation is promoted by exposure to air, by which white fumes rise, and 

 moisture may be absorbed, or by any other mode of slight dilution with water. 



17. Dilution of this compound acid, with more or less water, alters consider- 

 ably its properties, with regard to its action on metals. Thus it has been ob- 

 served, that in its concentrated state it does not act on iron; but by adding 

 water, it acquires a power of acting on that metal, and with different effect ac- 

 cording to the proportion of the water added. Thus, by adding to 2 measures of 

 the compound acid 1 measure of water, the liquor is rendered capable of calcining 

 iron, and forming with it a "white powder, but without effervescence. With an 

 equal measure of water effervescence was produced. With a larger proportion of 

 water the iron gave also a brown colour to the liquor, such as phlogisticated 

 nitrous acid acquires from iron, or communicates to a solution of martial vitriol 

 in water. 



18. Dilution with water renders this compound acid capable of dissolving 

 copper and zinc, and probably those other metals which are subject to the 

 action of the dilute vitriolic or nitrous acids. 



^ 2. An account of a new process for separating silver from copper. — IQ. The 

 properties of this liquor, in dissolving silver easily, without acting on copper, 

 have rendered it capable of a very useful application in the arts. Among the 

 manufactures at Birmingham, that of making vessels of silver plated on copper 

 is 3 very considerable one. In cutting out the rolled plated metal into pieces of 

 the required forms and sizes, there are many shreds, or scraps as they are 

 called, unfit for any purpose but the recovery of the metals by separating them 

 from each other. The easiest and most economical method of parting these 2 

 metals, so as not to lose either of them, is an object of some consequence to 

 the manufacturers. For this purpose 2 modes were practised, 1, by melting the 

 whole of the mixed metals with lead, and separating them by eliquation and 

 testing; and the 2d, by dissolving both metals in oil of vitriol, with the help of 

 heat, and by separating the vitriol of copper, by dissolving it in water, from the 

 vitriol of silver, which is afterwards to be reduced and purified. In the first of 

 these methods, there is a considerable waste of lead and copper; and in the 2d, 

 the quantity of vitriolic acid employed is very great, as much more is dissipated 

 in the form of volatile vitriolic, or sulphureous acid, than remains in the com- 

 position of the 2 vitriols. 



Some years ago I communicated to an artist the method of effecting the sepa- 

 ration of silver and copper by means of the above-mentioned compound of vitriolic 

 acid and nitre; and, as I am informed, that it is now commonly practised by 

 the manufacturers in Birmingham, I have no doubt but it is much more economi- 

 cal, and it is certainly much more easily executed, than any of the other methods: 

 for nothing more is required than to put the pieces of plated metal into an 



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