Page:Popular Science Monthly Volume 15.djvu/482

 particles of each find a habitation in the spaces of the other. Moreover, this experiment is but one of a large class, which all alike present the appearance of two or more bodies existing at the same time in the same place. And this phenomenon is a symbol which, translated, declares the existence of intervening spaces between the ultimate parts of which bodies of matter are composed. In regard to expansion and change of form, one of the most familiar and universal effects is the expansion of bodies by heat, and the most obvious classification of material objects is into three physical forms—the solid, the liquid, and the gaseous. We have only to admit the existence of molecules and of molecular spaces, and expansion can be defined at once to be the enlargement of these spaces under the influence of a force which drives the molecules asunder. Moreover, since distance is known to control the influence of attraction, it is plain that the melting of a solid and the vaporization of a liquid would be the necessary consequences of increasing the molecular distances, until cohesion is, in the one case nearly, and in the second case altogether, overcome. The existence of matter in three physical forms, and its changes from one to another under the influence of varying temperature, here find a most happy explanation.

But there follows a most important inference. If the gaseous form of matter is due to the separation of its molecules, then how enormously must their distances asunder exceed the diameters of the molecules themselves! For example, a cubic inch of water becomes about seventeen hundred cubic inches of steam. If this increase of volume is due to the enlargement of molecular spaces, how small a fraction of the vapor volume can consist of the material molecules! Can any experiment be brought to our relief, and furnish any solid ground on which we may stand and check the theory by testing the truth of this consequence? In "The New Chemistry," its author gives the following elegant description of an experiment on the diffusion of vapors:

"We have here a glass globe, provided with the necessary mountings—a stopcock, a pressure-gauge, and a thermometer, and which we will assume has a capacity of one cubic foot.

"Into this globe we will first pour one cubic inch of water, and in order to reduce the conditions to the simplest possible, we will connect the globe with our air-pumps and exhaust the air, although, as it will soon appear, this is not necessary for the success of our experiment. Exposing next the globe to the temperature of boiling water, the liquid will evaporate, and we shall have our vessel filled with ordinary steam. If, now, that cubic foot of space is really packed close with the material which we call water—if there is no break in the continuity of the aqueous mass, we should expect that the vapor would fill the space to the exclusion of everything else, or at least would fill it with a certain