We have already mentioned the difference existing between two equal volumes of different substances, and their weight, which proves that they may contain a different number of atoms in the same space. We also know, from the principle of Archimedes, that _if a body be immersed in a fluid, a portion of its weight will be sustained by the fluid equal to the weight of the fluid displaced_. [Illustration: Fig. 70.—Deception jugs of old pattern.] [This theorem is easily proved by filling a bucket with water, and moving it about in water, when it will be easy to lift; and likewise the human body may be easily sustained in water by a finger under the chin.] The manner in which Archimedes discovered the displacement of liquids is well known, but is always interesting. King Hiero, of Syracuse, ordered a crown of gold to be made, and when it had been completed and delivered to His Majesty, he had his doubts about the honesty of the goldsmith, and called to Archimedes to tell him whether or not the crown was of gold, pure and simple. Archimedes was puzzled, and went home deep in thought. Still considering the problem he went to the bath, and in his abstraction filled it to the brim. Stepping in he spilt a considerable quantity of water, and at once the idea struck him that any body put into water would displace its own weight of the liquid. He did not wait to dress, but ran half-naked to the palace, crying out, “Eureka, Eureka! I have found it, I have found it! “What had he found?—He had solved the problem. [Illustration: Fig. 71.—Section of jug.] 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. 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. [Illustration: Fig. 72.—Weighing metal in water.] [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.]