degree of energy which must he overcome before any other vapor could be forced in. Now what is the case? The stopcock of the globe is so arranged that we can introduce into it an additional quantity of any liquid on which we desire to experiment without otherwise opening the vessel. If, then, by this means, we add more water, the additional quantity will not evaporate, provided the temperature be kept at the boiling-point. Let us next, however, add a quantity of alcohol, and what do we find? Why, not only that this immediately evaporates, but we find that just as much alcohol-vapor will be formed as if no steam were present. The presence of the steam does not interfere in the least degree with the expansion of liquid alcohol into alcohol-vapor. The only difference which we observe is that the alcohol expands more slowly into the aqueous vapor than it would into a vacuum. If, now that the globe is filled with aqueous vapor and alcohol-vapor at the same time, each acting in all respects as if it occupied the space alone, we add a quantity of ether, we shall have the same phenomena repeated. The ether will expand and fill the space with its vapor, and the globe will hold just as much ether-vapor as if neither of the other two were present; and so we might go on, as far as we know, indefinitely. There is not here a chemical union between the several vapors, and we can not in any sense regard the space as filled with a compound of the three. It contains all three at the same time, each acting as if it were the sole occupant of the space."
Now these experimental results find an explanation nowhere else but in the inference, previously made, that molecular spaces do exist, and that they are so relatively large that the molecules of each gas find, in the spaces between the particles of all the others, abundant room to manifest all their characters.
If, now, we turn from vapors to the examination of permanent gases, we find a kindred action. Moreover, it is an action which not only confirms our evidence of the existence of molecules and molecular spaces, but, as we shall see in the sequel, in addition thereto suggests an answer to this important question in the history of molecules—are they in motion or at rest?
Hydrogen gas is sixteen times lighter than oxygen. Let us bring the open mouth of an inverted jar filled with hydrogen down upon the open mouth of a similar jar filled with oxygen. By this means we obtain a single cylinder of gas, the lower half of which consists of the heavier oxygen, and the upper half of the lighter hydrogen, the two gases being in contact only at their surfaces in the middle of the column. Their relative weights would lead us to expect them to maintain these positions; but the well-known properties of these gases enable us to learn that they do not. Neither one alone is explosive; their mixture is. Now, after a time, if we separate the jars and bring a flame to the mouth of the lower one, and then to the mouth of the upper one, two successive explosions occur, declaring that both jars
