THE MOLECULAR THEORY. 467 



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 quan- 

 tity of any liquid on which we desire to experiment without other- 

 wise 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 in- 

 terfere 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 in- 

 to 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 oc- 

 cupied 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 col- 

 umn. Their relative weights would lead us to expect them to main- 

 tain these positions ; but the well-known properties of these gases 

 enable us to learn that they do not. Neither one alone is explosive ; 

 their mixtixre 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 



