239 



protuberances, as if they had just begun to melt. Pla- 

 tina hardens and stiffens all metals; one more than 

 another, lead the most, in a moderate quantity it 

 diminishes, and in a largo one destroys the toughness 

 of all the malleable metals, but communicates some 

 degree of this quality to cast iron. Tin bears much 

 the least, and gold and silver the greatest quantify 

 \rithout the loss of their malleability. A very small 

 portion of platina scarce injures the cplour of copper 

 and gold : a larger renders both pale. A far less 

 quantity has less c fleet on copper than on gold. It 

 debases and darkens in proportion to its quantity, 

 the colour of the white metals; that of silver much 

 the least, and of lead the most. It in good measure 

 preserves iron and copper from tarnishing, scarce 

 alters gold or silver in this respect, makes tin tarnish 

 soon, and lead exceeding quick. 



Copper comes next to silver in ductility. Brass 

 is an artificial metal, composed of copper fused with 

 lapis calaminaris. Jrmi is less ductile than any of 

 these, and contains more dross.* It likewise easily 

 rusts, whereas silver seldom rusts, and gold seldom 

 either rusts or cankers. Tin resembles lead+, but is 

 considerably harder and not near so heavy. Indeed 



* The spirit of vitriol being mixed with iron, after ferment- 

 ing, produces a green vitriol like the natural one. But it for 

 spirit of vitriol, you use oil of vitriol, which is the most acid part 

 of that mineral, there happens immediately a small f rmen- 

 tation, which is quickly over. That fermentation begins again in 

 a few days, under the form of a white smoke, which rises to the 

 surface, and the whole mass of iron turns ito a very white pap 

 which smells like common sulphur. When the fermentation 'is 

 over, the iron, instead of turning into green vitriol, becomes on 

 a sudden white vitriol. Mean time there is on its surface, a 

 black dust, which it has thrown up. It seems this would have 

 made it green. For if white vitriol be mingled with this dust, 

 it acquires a green colour. 



f White lead is thin plates of lead dissolved in vinegar. 



Red-lead is commonly calcined. 



Black-lead (very improperly so called) is only a talky kind of 

 earth. 



