DIVISIBILITY. 21 



Molecular theory. The expression molecule, is derived from 

 the Latin word molecida 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 



FIG. i. 



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 sub- 

 stances. For instance : If w r e place one cubic inch of water 

 into 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 



