30 CHEMICAL PHYSICS. 



a third liquid for instance, alcohol and ether we find that of each 

 of these liquids exactly the same quantity will evaporate which would 

 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 



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. Other facts which lead us to accept the theory of the 

 molecular condition cf matter are : The passage of gases through 

 solids : for example, of carbon dioxide gas through red-hot iron ; of 

 water under pressure through gold ; the decrease in a volume of water 

 when a salt is dissolved in it ; the extreme divisibility of matter as 

 shown by solution, etc. 



Our conception of molecules (though individually by far too small to make 

 any impression whatever upon our senses) 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 CTnn&rnnnj 

 of one inch, and that one cubic inch of a gas under ordinary conditions con- 

 tains about one hundred thousand million million milions of molecules. 



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

 ception, but in order to give some idea of the size of these molecules it may be 

 mentioned that if a mass of water as large as a pea were to be magnified to the 

 size of our earth, each molecule being magnified in the same proportion, these 

 molecules would represent balls of about two inches in diameter. 



While molecules consequently are exceedingly small particles, yet they are 

 not entirely immeasurable ; they are, as Sir W. Thomson says, pieces of matter 

 of measurable dimensions, with shape, motion, and laws of action, intelligible 

 subjects of scientific investigation. 



Intimately connected with the molecular theory is the Law (more 

 correctly, the hypothesis) of Avogadro, which may be stated as follows : 

 All gases or vapors, without exception, contain, in the same volume, the 

 same number of molecules, provided temperature and pressure are the 

 same. Or, in other words : Equal volumes of different gases contain, 

 under equal circumstances, the same number of molecules. The correct- 

 ness of this law has good mathematical support deduced from the law 

 of Boyle, many other facts and considerations leading to the same 



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. 



