Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

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.

In order to obtain the specific gravity of a body which is lighter than water, you must attach to it a heavy one, whose specific gravity is known, and immerse them together; the specific gravity of the lighter body, may then be easily calculated from observing the loss of weight it produces, in the heavy body.