THE EARTtt. 163 



is equal to the weight of a body of water of its 

 own bulk. Some light bodies, therefore, such 

 as cork, lose much of their weight, and therefore 

 swim ; other more ponderous bodies sink, because 

 they are heavier than their bulk of water. 



Upon this simple theorem entirely depends the 

 art of weighing metals hydrostatically. I have a 

 guinea, for instance, and desire to know whether 

 it be pure gold : I have weighed it in the usual 

 way with another guinea, and find it exactly of 

 the same weight, but still I have some suspicion, 

 from its greater bulk, that it is not pure. In 

 order to determine this, I have nothing more to 

 do than to weigh it in water with that same 

 guinea that I know to be good, and of the same 

 weight, and this will instantly show the diffe- 

 rence ; for the true ponderous metal will sink, 

 and the false bulky one will be sustained in pro- 

 portion to the greatness of its surface. Those 

 whose business it is to examine the purity of me- 

 tals, have a balance made for this purpose, by 

 which they can precisely determine which is 

 most ponderous, or, as it is expressed, which has 

 the greatest specific gravity. Seventy-one pounds 

 and a half of quicksilver is found to be equal in 

 bulk to a hundred pound weight of gold. In 

 the same proportion, sixty of lead, fifty-four of 

 silver, forty-seven of copper, forty-five of brass, 

 forty-two of iron, and thirty ^nine of tin, are each 

 equal to a hundred pounds of the same most pon^ 

 derous of all metals. 



This method of precisely determining the pu- 

 rity of gold, by weighing in water, was first dis- 



