GAL VANO-PL ASTICS. 691 



A somewhat large coin (a crown piece or a florin) is well 

 cleaned with a brush and soap and water. Being placed upon a 

 table, a strip of paper, 2 cm wide and 20 cm long, is very tightly 

 wrapped round the edge of the coin and gummed at the end. Into 

 this little vessel some stearine which has been melted in a ladle is 

 poured so as to form a layer above the coin 5 or 6 mm thick. If the 

 paper was not drawn very tight round the edge, some stearine will 

 flow through the interstices ; as this can be seen at once, the trouble 

 of having to make a new paper rim may be avoided by allowing the 

 first small portion poured in to become solid, it will then close up 

 all interstices, and the remainder of the stearine may after a little 

 while be poured in safely. After an hour or so the paper rim is 

 torn away, and the cast will mostly come off at the- same time ; if 

 not it must be taken off with care, so as neither to break it nor to 

 produce scratches. 



Stearine is a non-conductor ; it must therefore be covered with a 

 conducting substance before it can be used as cathode. This is done 

 by ' black-leading ' it. Black-lead is an inferior sort of graphite, 

 one of the forms in which carbon occurs in nature. Graphite is a 

 soft variety of carbon, generally of a laminated structure, which is 

 found in mines ; the better kinds are used for pencils, the inferior 

 kinds for giving a black polish to iron articles, such as stoves, etc., 

 and as it is nearly incombustible, crucibles are also made of it, 

 which stand the strongest heat without burning. A little black- 

 lead is rubbed down in a mortar to as fine a powder as possible 

 until it no longer feels gritty. To make it still finer it should be 

 sifted through a piece of fine linen or cotton. The powder is shaken 

 from the mortar upon a square piece of the fabric, 10 or 12 cm long 

 each way ; the corners are then taken up, joined, and tied with 

 thread, and the little bag formed in this way is knocked against a 

 flat horizontal board until a sufficient quantity has passed through 

 the meshes of the fabric. The black-lead is first rubbed upon the 

 edge of the stearine cast with the 

 point of the finger, and is then applied 

 to the surface which represents the 

 hollow copy of the coin by means of a 

 soft camel 's-hair brush. 



A thin copper wire, made very soft 



in the flame and then rubbed bright 



., , -T , j, i FIG. 349 (anproj.; real size\ 



again, must be laid round the edge of 



the cast and fastened by cautiously twisting it, as in fig. 349. The 

 end of this wire may now be connected with the negative pole of a 

 weak battery, and the cast be immersed in a saturated solution of 



