4 66 THE POPULAR SCIENCE MONTHLY. 



rial particles of each find a habitation in the spaces of the other. More- 

 over, this experiment is but one of a large class, which all alike pre- 

 sent the appearance of two or more bodies existing at the same time 

 in the same place. And this phenomenon is a symbol which, trans- 

 lated, 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 enor- 

 mously must their distances asunder exceed the diameters of the mole- 

 cules 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 ex- 

 periment 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 fol- 

 lowing elegant description of an experiment on the diffusion of 

 vapors : 



" We have here a glass globe, provided with the necessary mount- 

 ings 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 

 * l]y J. P. Cooke, Jr., " International Scientific Series," D. Appleton & Co. 



