You must observe, that when you weigh a body in water, in order to ascertain its specific gravity, you must not sink the dish of the balance in the water; but either suspend the body to a hook at the bottom of the dish, or else take off the dish, and suspend to the arm of the balance a weight to counterbalance the other dish, and to this attach the solid to be weighed, (fig. 7.) Now suppose that a cubic inch of gold, weighed 19 ounces out of water, and lost one ounce of its weight by being weighed in water, what would be its specific gravity? _Caroline._ The cubic inch of water it displaced, must weigh that one ounce; and as a cubic inch of gold, weighs 19 ounces, gold is 19 times, as heavy as water. _Emily._ I recollect having seen a table of the comparative weights of bodies, in which gold appeared to me to be estimated at 19 thousand times, the weight of water. _Mrs. B._ You misunderstood the meaning of the table. In the estimation you allude to, the weight of water was reckoned at 1000. You must observe, that the weight of a substance when not compared to that of any other, is perfectly arbitrary; and when water is adopted as a standard, we may denominate its weight by any number we please; but then the weight of all bodies tried by this standard, must be signified by proportional numbers. _Caroline._ We may call the weight of water, for example, one, and then that of gold, would be nineteen; or if we choose to call the weight of water 1000, that of gold would be 19,000. In short, specific gravity, means how many times more a body weighs, than an equal bulk of water. _Mrs. B._ It is rather the weight of a body compared with a portion of water equal to it in bulk; for the specific gravity of many substances, is less than that of water. _Caroline._ Then you cannot ascertain the specific gravity of such substances, in the same manner as that of gold; for a body that is lighter than water, will float on its surface, without displacing any of it. _Mrs. B._ If a body were absolutely without weight, it is true that it would not displace a drop of water, but the bodies we are treating of, have all some weight, however small; and will, therefore, displace some quantity. If the body be lighter than water, it will not sink to a level with its surface, and therefore it will not displace so much water as is equal to its bulk; but only so much, as is equal to its weight. A ship, you must have observed, sinks to some depth in water, and the heavier it is laden, the deeper it sinks, as it always displaces a quantity of water, equal to its own weight. _Caroline._ But you said just now, that in the immersion of gold, the bulk, and not the weight of body, was to be considered. _Mrs. B._ That is the case with all substances which are heavier than water; but since those which are lighter, do not displace so much as their own bulk, the quantity they displace is not a test of their specific gravity.