Chap. 35.] fof Voucbftone. 231 



leaft effect on it in its metallic ftate. Upon this depend* 

 the trial of gold by the lapis lydius, or touchftone, 

 which is of a dark colour and pretty fine grain, but 

 when polifhed has fufficient roughnefs, fo that when 

 the metal is rubbed over it a mark is left, which 

 mark will be affected by any of the common acids, 

 aqua fortis for inftance, in proportion to the impurities 

 the metal contains, but not at all if the gold is perfectly 

 pure. 



Goldfmiths ufe likewife two fets of needles, one for 

 filver and the other for gold, when they want to exa- 

 mine the finenefs of different parcels. The fets of 

 needles for gold are alloyed with different proportions 

 of copper ; one is made with twenty-three parts of 

 gold to one of copper, another with twenty-two parts 

 of gold to one of copper> and fo in proportion. When 

 they have a piece of gold to be tried, they firft exa- 

 mine its colour to determine what quantity of alloy it 

 contains* To know this the better, they mark the 

 touchftone with it, and then make another mark with 

 the needle, which they think to be of nearly fimilar 

 purity. Thus they compare them - y and after this, to 

 be certain that the metal is gold, they apply to the 

 mark on the ftone a drop of aqua fortis. If it is 

 copper tinged with zinc, or any other imitation of 

 gold, the aqua fortis immediately diffolves it. If ic 

 contains gold and fome other metal, it difiblves the 

 other metal and leaves the gold. 



The only faline fluid which difiblves gold in its 

 metallic ftate is aqua regia, or a mixture of the nitrous 

 and muriatic acids. It is prepared in various wdys : 

 i ft, By mixing the two acids in their pure ftate. 

 idly, By adding common fait, or fal ammoniac, to 

 aqua fortis, and then diftilling the mixture. A pare 

 of the nitrous acid decornpofes the muriatic fait, and 



detaches 



