100 Dr. Wollaston on a Method of 



other metals are rendered by the various processes of ordinary 

 metallurgy; and, if now poured over, and allowed to subside 

 in a clean basin, a uniform mud or pulp will be obtained, 

 ready for the further process of casting. 



The mould which I have used for casting is a brass barrel, 

 6J inches long, turned rather taper within, with a view to faci- 

 litate the extraction of the ingot to be formed, being 1.12 inches 

 in diameter at top, and 1.23 inches at a quarter of an inch 

 from the bottom, and plugged at its larger extremity with a 

 stopper of steel, that enters the barrel to the depth of a quarter 

 of an inch. The inside of the mould being now well greased 

 with a little lard, and the stopper being fitted tight into the 

 barrel by surrounding it with blotting-paper, (for the paper 

 facilitates the extraction of the stopper, and allows the escape 

 of water during compression,) the barrel is to be set upright 

 in a jug of water, and is itself to be filled with that fluid. It 

 is next to be filled quite full with the mud of platina ; which, 

 subsiding to the bottom of the water, is sure to fill the barrel 

 without cavities, and with uniformity, — a uniformity to be ren- 

 dered perfect by subsequent pressure. In order, however, to 

 guard effectually against cavities, the barrel may be weighed 

 after filling it, and the actual weight of its contents being thus 

 ascertained, may be compared with that weight of platina and 

 water which it is known by estimate that the barrel ought to 

 contain*. A circular piece of soft paper first, and then of 

 woollen cloth, being laid upon the surface, allow the water to 

 pass, during partial compression by the force of the hand with 

 a wooden plug. A circular plate of copper is then placed 

 upon the top, and thus sufficient consistency is given to the 



* From the mean weight of the ingots obtained in previous operations, 



it is known that the barrel described in the text ought to contain 16 



ounces troy of dry platina powder. The weight of the contents of the 



, , ,. sp. grav. of platina — 1 ,, 



barrel = 16 ounces x -±-~^ h—TT- + the weight of a cubic 



sp. grav. of platina ° 



inch of water x capacity of the barrel in cubic inches = 1 6 ounces x — '■ — 



21.25 

 + .526 ounces x 7.05 = 18.9575 ounces troy. Should the contents of 

 the barrel weigh materially less than this estimated weight, there must 

 be a want of uniformity in the disposition of the powder within the 

 barrel. 



