338 MINT 



thimbles, into which the assay-pieces are placed for treatment with acid, instead of 

 into the ordinary glass vessels over gaff-burners. The advantages of this invention 

 are too manifest to require elaboration ; but saving of acid, gas, labour, and risk of 

 error are amongst the chief of them ; and, besides, the plan has been in successful 

 operation in their own assay offices in Hatton Garden, as well as in other impor- 

 tant assay offices, for several years past. The first cost would seem to be the 

 principal objection to this plan of Messrs. Johnson, Matthey, and Co. ; but this is 

 really a small matter, for the apparatus becomes stock-in-trade, and its cost should 

 be viewed simply as so much capital,, whose interest is paid by the saving effected in 

 glass, but more especially by the smaller amount of acid actually employed, while 

 the platinum can at all times be sold, for nearly its original cost, as old metal. 



The mode of operation is as follows : A stand of slate is so arranged that a means 

 of heating is made to rest on its base. For this purpose a jet of gas is preferred ; 

 where, however, gas cannot be obtained, an ordinary oil lamp or a charcoal fire may 

 bo used. On a shelf over the source of heat are placed two or three receptacles of 

 platinum, each communicating with a vessel made of porcelain, provided with three 

 necks and an overflow-pipe. There is a kind of sieve or tray of platinum, so arranged 

 as to carry from 16 to 100 thimbles of platinum, and provided with a handle, so that 

 this tray, with its charge, can be manipulated at pleasure. The thimbles vary in 

 size according to taste, but each one is slit or cut at the bottom, so that the solu- 

 tion of silver as it is formed may by its density fall out, and allow the clean acid to 

 take its place. 



When an operation is to be performed, the tray filled with the charged thimbles 

 that is, containing the assay-pieces is placed in one of the platinum receptacles or 

 boilers, and heated to a fitting temperature ; when the desired effect is produced, the 

 tray is lifted into another receptacle, and again heated ; this may be carried to three 

 times if necessary. The products of decomposition of the acid go, with the acid which 

 evaporates, into the porcelain vessel, where the free acid falls through the overflow- 

 pipe into a proper chamber, while the acid fumes pass into the flue through the third 

 neck before spoken of. The parted assays having been washed by several immersions 

 in boiling distilled water, without removal from their thimbles, have now to be dried 

 and annealed in a platinum muffle, so formed as to fit into an ordinary muffle, and, 

 after annealing, to be weighed in the usual manner, having saved at least 75 per cent, 

 of the usual trouble. 



It is almost needless to add that the system of proofs must be used with this pro- 

 cess. 



The assay for silver is not so tedious, as it is finished at the point where it leaves 

 the muffle on the cupel ; but up to this point it passes through precisely the same 

 process as the gold. 



The Master of the Mint, on receipt of the assay reports, determines if the metal has 

 been found within the limits, or remedy allowed for error ; and, if ho be satisfied, 

 signs the reports, and thus enables the bars to be forwarded for coining. And from 

 this point gold and silver undergo nearly the same operations ; to follow, then, the 

 practice used for sovereign-bars will suffice for all. 



The sovereign-bars having been weighed by the officer, and given by him to his 

 men, are wrought in sets of twenty ; each set is called a batch, and each bar in the 

 batch undergoes precisely the same process. The bar is passed into an opening of 

 the breaking-down mill, fig. 1524, where it receives a considerable compression, for 

 the rollers A, seizing its end, drag it forward, while they roll back and retard the pro- 

 gress of that part of the bar which is not between them. The result is that the bar 

 is lengthened, but not widened materially, so that length is gained at the expense of 

 the thickness, which is regulated by the distance between the rollers. The rollers 

 are driven by shafts, and adjusting-couplings, which are themselves driven by the 

 geared wheels. The distance between the rollers is determined by the action of the 

 lever o, which, by the endless wheels on its axle fitting into geared wheels, givi 

 motion to powerful screws shown at F, which terminate in cups on the upper part o: 

 the upper brasses of the rollers A, as may be seen at c. The upper brasses are kept 

 always against the ends of the screws by weights, D, which are beneath the mill, 

 but from which levers and rods, B, terminate at the lower part of the upper brasses, 

 at about the position indicated by dotted lines, so that the upper roller has motion 

 either upwards or downwards at pleasure, but the motion upwards is arrested by 

 the powerful screw v, and this point once determined by the reading of a scale, is 

 fixed by the clamp near G. The thickness, therefore, of eacli bar in a batch is 

 determined within certain limits ; and, when each bar in the batch has been rolled, 

 the mill is altered, refixed, and again the rolling goes on till each bar has passed 

 several times, at varying pinches, through these rollers. Owing to the wear of the 

 moulds in which the bars are exist and which is largely due to the presence of 







