He got a lump of gold the same weight as the crown, and immersed it in water. He found it weighed nineteen times as much as its own bulk of water. But when he weighed the kings crown he found it displaced more water than the pure gold had done, and consequently it had been adulterated by a lighter metal. He assumed that the alloy was silver, and by immersing lumps of silver and gold of equal weight with the crown, and weighing the water that overflowed from each dip, he was able to tell the king how far he had been cheated by the goldsmith.
Fig. 72.—Weighing metal in water.
It is by this method now that we can ascertain the specific gravity of bodies. One cubic inch of water weighs about half an ounce (or to be exact, 252½ grains). Take a piece of lead and weigh it in air; it weighs, say, eleven ounces. Then weigh it in a vase of water, and it will be only ten ounces in weight. So lead is eleven times heavier than water, or eleven ounces of lead occupy the same space as one ounce of water.
Fig. 73.—Hydrometer.
[The heavier a fluid is, or the greater its density, the greater will be the weight it will support. Therefore we can ascertain the purity or otherwise of certain liquids by using hydrometers, etc., which will float higher or lower in different liquids, and being gauged at the standard of purity, we can ascertain (for instance) how much water is in the milk when supplied from the dairy.]
But to return to Specific Gravity, which means the “Comparative density of any substance relatively to water,” or as Professor Huxley says, “The weight of a volume of any liquid or solid in proportion to the weight of the same volume of water, at a known temperature and pressure.”
Water, therefore, is taken as the unit; so anything whose equal volume under the same circumstances is twice as heavy as the water, is declared to have its specific gravity 2; if three-and-a-half times it is 3·5, and so on. We append a few examples; so we see that things which possess a higher specific gravity than water sink, which comes to the same thing as saying they are heavier than water, and vice versâ.
To find the specific gravity of any solid body proceed as above, in the experiment of the lead. By weighing the substance in and out of water we find the weight of the water displaced; that is, the first weight less the second. Divide the weight in air by the remainder, and we shall find the specific gravity of the substance.