22 INTRODUCTION. 



Molecular theory. The expression molecule is derived from the 

 Latin word molecula a little mass, and means the smallest particle 

 of matter that can exist by itself, or into which matter is capable of 

 being subdivided by physical actions. To explain more fully what is 

 meant by the expression molecule, we will return to the conversion of 

 water into steam. 



When water boils at the ordinary atmospheric pressure it expands 

 about 1800 times, or one cubic inch of water yields about 1800 cubic 

 inches, equal to about one cubic foot of steam. In explaining this 

 fact we have either to assume that the water, as well as the steam, is 

 homogeneous matter (Fig. 1), or that the water consisted of small 

 particles of a given size, which now exist in the steam again as such, 

 with the only difference that they are more widely separated from 

 each other (Fig. 2). 



Of the many proofs which we have of the fact that the latter 

 assumption is correct, I will mention but one, viz., that the quantities 

 of vapor formed by volatile liquids at any certain temperature above 

 the boiling-point, in close vessels of the same size, are the same, no 

 matter whether the vessel was entirely empty or contains the vapors 

 of one, two, or more other substances. For instance : If we place 

 one cubic inch of water in a flask holding one cubic foot, from which 

 flask the air has been previously removed, and then heat the flask to 

 the boiling-point, the cubic inch of water will evaporate, filling the 

 vessel with steam. Upon now introducing into the flask a second and 

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

 of these liquids exactly the same quantity will evaporate which would 



