Ponderability is a quality of bodies that relates to sensible weight. A ponderable body is one that possesses sensible weight. A great difference exists in the relative weights of different substances. Thus platinum is 23 times heavier than water; water 840 times heavier than air; and air 14 times heavier than hydrogen gas. Platinum is, therefore, 170,480 times heavier than hydrogen gas. It will readily be understood that if any substance exists that is as much lighter than hydrogen as hydrogen is lighter than platinum, the weight of such a body would be absolutely inappreciable by any of our present instruments; such bodies would, therefore, be called imponderable. In the ordinary division of simple substances at present in use by chemists, the whole are classed into ponderables and imponderables: the imponderable substances are, heat, light, and electricity. Some persons have supposed that the idea of imponderable matter is absurd, and that gravity is a universal property of bodies. However this may be, if we consider ponderability as indicating our ability to weigh the body, it will be manifest that heat, light, and electricity may, with considerable justness, be termed imponderable.