_Mrs. B._ And if you continue to pour water into the goblet when it is full, the water will run over, instead of rising above its level in the tube. I shall now explain to you the meaning of the _specific gravity_ of bodies. _Caroline._ What! is there another species of gravity, with which we are not yet acquainted? _Mrs. B._ No: the specific gravity of a body, means simply its weight, compared with that of another body, of the same size. When we say, that substances, such as lead, and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of substances in nature. Would you call wood, and chalk, light or heavy bodies? _Caroline._ Some kinds of wood are heavy, certainly, as oak and mahogany; others are light, as cedar and poplar. _Emily._ I think I should call wood in general, a heavy body; for cedar and poplar, are light, only in comparison to wood of a heavier description. I am at a loss to determine whether chalk should be ranked as a heavy, or a light body; I should be inclined to say the former, if it was not that it is lighter than most other minerals. I perceive that we have but vague notions of light and heavy. I wish there was some standard of comparison, to which we could refer the weight of all other bodies. _Mrs. B._ The necessity of such a standard, has been so much felt, that a body has been fixed upon for this purpose. What substance do you think would be best calculated to answer this end? _Caroline._ It must be one generally known, and easily obtained; lead or iron, for instance. _Mrs. B._ The metals, would not answer the purpose well, for several reasons; they are not always equally compact, and they are rarely quite pure; two pieces of iron, for instance, although of the same size, might not, from the causes mentioned, weigh exactly alike. _Caroline._ But, Mrs. B., if you compare the weight, of equal quantities of different bodies, they will all be alike. You know the old saying, that a pound of feathers, is as heavy as a pound of lead? _Mrs. B._ When therefore we compare the weight of different kinds of bodies, it would be absurd to take quantities of equal _weight_, we must take quantities of equal _bulk_; pints or quarts, not ounces or pounds. _Caroline._ Very true; I perplexed myself by thinking that quantity referred to weight, rather than to measure. It is true, it would be as absurd to compare bodies of the same size, in order to ascertain which was largest, as to compare bodies of the same weight, in order to discover which was heaviest. _Mrs. B._ In estimating the specific gravity of bodies, therefore, we must compare equal bulks, and we shall find that their specific gravity, will be proportional to their weights. The body which has been adopted as a standard of reference, is distilled, or rain water.