therefore _lost_ instead of _gaining_ weight whilst immersed in the liquid. The rule may now be worked out: Ounces. Weight of crown in air 17½ Ditto in water 15½ --- Less in water 2 17½ / 2 = 8¾ The quotient 8¾ demonstrates that the crown is manufactured of copper, because it would have been about 19¼ if made of pure gold. [Illustration: Fig. 68. A. Ordinary pair of scales. B. Scale-pan, containing 17½ ounces, being the weight of the crown in air. C. Pan, with hook and crown attached, which is sunk in the water contained in the vessel D; this pan contains the two ounces, which must be placed there to make the crown sink and exactly balance B.] [Page 53] _Table of the Specific Gravities of the Metals in common use._ Platinum 20.98 Gold 19.26 to 19.3 and 19.64 Mercury 13.57 Lead 11.35 Silver 10.47 to 10.5 Bismuth 9.82 Copper 8.89 Iron 7.79 Tin 7.29 Zinc 6.5 to 7.4 The simple rule already explained may be applied to all metals of any size or weight, and when the mass is of an irregular shape, having various cavities on the surface, there may be some difficulty in taking the specific gravity, in consequence of the adhesion of _air-bubbles_; but this may be obviated either by brushing them away with a feather, or, what is frequently much better, by dipping the metal or mineral first into alcohol, and then into water, before placing it in the vessel of water, by which the actual specific gravity is to be taken.