DIVISIBILITY. 23 



have evaporated if these liquids had been introduced into the empty 

 flask. 1 This fact is evidence that there must be small particles of 

 steam which are not in close contact, that there are spaces between 

 these particles which may be occupied by the particles of a second, 

 third, or more substances. To these particles of matter we give the 

 name molecules, and the spaces between them we call intermolecular 



FIG. 2. 



4 



We have thus demonstrated the correctness, or, at least, the likeli- 

 hood of the so-called molecular theory, but the proof given is but one 

 of many. Of these molecules (though individually by far too small 

 to make any impression whatever upon our senses), our conception is 

 so perfect that we have formed an idea of the actual size of these 

 minute particles of matter. Very good reasons lead us to believe that 

 the diameter of a molecule is equal to about ^o-^rio'FoT ^ one i ncn ? 

 and that one cubic inch of a gas under ordinary conditions contains 

 about one hundred thousand million million millions of molecules. 



These figures at first glance appear to be beyond the limit of human 

 conception, but in order to give some idea of the size of these mole- 

 cules it may be mentioned tha/if a mass of water as large as a pea. 

 were to be magnified to the size_of_ojir earth, each molecule being 

 magnified in the same proportion, these molecules would represent 

 balls_of about twojnches in diameter.) 



Whilst molecules consequently are exceedingly small particles, yet 

 they are not entirely immeasurable ; they are, as Sir W. Thomson 



1 As each gas, in consequence of its tension, exerts a certain pressure, the pressure in the 

 flask rises with the introduction of every additional gas. 



