The water of the Dead Sea is said to contain about twenty-six per cent. of saline matter, which chiefly consists of common salt. It is perfectly clear and bright, and in consequence of the great density, a person may easily float on its surface, like the egg on the brine, so that if a ship could be heavily laden whilst floating on the water of the Dead Sea, it would most likely sink if transported to the Thames. This illustration of specific gravity is also shown by a model ship, which being first floated on the brine, will afterwards sink if conveyed to another vessel containing water. One of the tin model ships sold as a magnetic [Page 50] toy answers nicely for this experiment, but it must be weighted or adjusted so that it just floats in the brine, A; then it will sink, when placed, in another vessel containing only water. [Illustration: Fig. 67. A. Vessel containing brine, upon which the little model floats. B. Vessel containing water, in which the ship sinks.] Another amusing illustration of the same kind is displayed with goldfish, which swim easily in water, floating on brine, but cannot dive to the bottom of the vessel, owing to the density of the saturated solution of salt. If the fish are taken out immediately after the experiment, and placed in fresh water, they will not be hurt by contact with the strong salt water. These examples of the relative weights of equal bulks, enable the youthful mind to grasp the more difficult problem of ascertaining the specific gravity of any solid or liquid substance; and here the strict meaning of terms should not be passed by. _Specific_ weight must not be confounded with _Absolute_ weight; the latter means the entire amount of ponderable matter in any body: thus, twenty-four cubic feet of sand weigh about one ton, whilst specific weight means the _relation_ that subsists between the _absolute weight_ and the _volume_ or _space_ which that _weight_ occupies. Thus a cubic foot of water weighs sixty-two and a half pounds, or 1000 ounces avoirdupois, but changed to gold, the cubic foot weighs more than half a ton, and would be equal to about 19,300 ounces--hence the relation between the cubic foot of water and that of [Page 51] gold is nearly as 1 to 19.3; the latter is therefore called the specific gravity of gold. Such a mode of taking the specific gravity of different substances--viz., by the weight of equal bulks, whether cubic feet or inches, could not be employed in consequence of the difficulty of procuring exact cubic inches or feet of the various substances which by their peculiar properties of brittleness or hardness would present insuperable obstacles to any attempt to fashion or shape them into exact volumes. It is therefore necessary to adopt the method first devised by Archimedes, 600 B.C., when he discovered the admixture of another metal with the gold of King Hiero's crown.