tSii HISTORY OF 



quicksilver in one a\fn, will at last press it up in the othei 

 arm ; and will continue to press it upwards, until the fluid 

 in both arms be upon a par. So that here we actually see 

 quicksilver, the heaviest substance in the world, except gold 

 and platiiia, tloating upon water, which is but ti very light sub- 

 stance. 



When we see water thus capable of sustaining quicksilver, we 

 need not be surprised that it is capable of floating much lighter 

 substances, ships, animals, or timber. When any thing floats 

 upon water, we always see that a part of it sinks in the same. 

 A cork, a ship, a buoy, each burits itself in a bed on the surface 

 of the water ; this bed may be considered as so much v/ater dis- 

 placed ; the water will, therefore, lose so much of its own weight, 

 as is equal to the weight of that bed of water which it displaces. 

 If the body be heavier than a similar bulk of water, it will sink ; 

 if lighter, it will swim. Universally, therefore, a body plunged 

 in water, loses as much of its weight as 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 weigh- 

 ing 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 tind 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 

 tliis will instantly show the diff"erence ; for the true ponderous 

 metal wiU sink, and the false bulky one will be sustained in 

 proportion to the greatness of its surface. Those whose busi- 

 ness it is to examine the purity of metals, 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 pound 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- 

 eeven of copper, forty-five of brass, forty two of iron, and thirty- 



