712 THE MOLECULAR STRUCTURE OF MATTER. 



stances lying (to all appearances) inert on tbe earth's surface is not so 

 obvious. In the case of gases, indeed, it is almost self-evident that 

 they are composed of particles so minute as to be invisible, — in a condi- 

 tion of great individual freedom. The rapid penetration of odors to 

 great distances, the ready absorption of vapors and other gases, and 

 the phenomena connected with diffusion, compression, and expansion 

 seem to demonstrate this. One gas will rapid I\' penetrate another and 

 blend evenly with it, even if the specific gravities be very different. 

 The particles of gases are (as compared with their own diameters) sep- 

 arated widely from each other; there is plenty of room for additional 

 particles ; hence any gas which would, by virtue of its molecular motion, 

 soon diffuse itself uniformly through a vacuum will also diffuse itself 

 through one or more other gases, and once so diffused, it will never 

 separate again. A notable example of this is the i)ermanence of the 

 constitution of the atmosphere, which is a mere mixture of gases. The 

 oxygen and the nitrogen, as determined b^^ the examination of sami)les 

 collected all over the world, maintain sensibly the same relative pro- 

 portions, and even the carbonic acid, thougb liable to slight local ac- 

 cumulations, preserves, on the whole, a constant ratio, and yet the 

 densities of these gases differ very greatly. 



Liquids (though to a much less degree than gases) are also composed 

 of particles separated to a considerable relative distance from each other, 

 and capable of unlimited motion where no opposing force — such as grav- 

 ity — interferes ; for under such circumstances their energy of motion is 

 not sufficient to overcome the downward attractions of the earth 5 hence 

 they are constrained to maintain a level surface. The occlusion of gases 

 without sensible comparative increase of volume shows that the com- 

 ponent particles are widely separated. Water (for example) at the freez- 

 ing iioint occludes above one and three quarter times its own volume of 

 carbonic oxide, and about 480 times its volume of hydrochloric acid, 

 with an increase of volume in the latter case of only one-third. The 

 quantity of gas occluded increases directly as the pressure, which seems 

 to indicate that the particles of the occluded gas are as free in their 

 movements among the particles of the liquid as they would be in 

 an otherwise empty containing vessel. Liquids therefore are porous 

 bodies whose constituent particles have great freedom of motion. It 

 is no w^onder consequently that two dissimilar liquids, ijlaced in con- 

 tact with each other, should interpenetrate one another completely, if 

 time enough be allowed; and this time, as might be expected, is con. 

 siderably greater than that required for the blending of gases, because 

 of the vastly greater mobility of the particles of the latter. The diffu- 

 sion of liquids takes place not only when they are in actual contact, 

 but even when they are separated by partitions of a porous nature, 

 such as plaster of Paris, unglazed earthenware, vegetable or animal 

 membranes, and colloidal substances, all of which may be perfectly 

 water-tight, in the ordinary sense of the term, but powerless to prevent 



