THE MOLECULAR THEORY. 463 



in the subdivision of matter. He claims that the line which may be 

 mentally thrust through the minutest particle of matter is not a needle 

 nor a knife that it is the mathematician's line as immaterial as space 

 and that the thrusting of such a line can accomplish no division. 

 He claims that, applicable as it may be to space, such logic need not 

 bind the physicist who studies the constitution of bodies which inhabit 

 space. 



Therefore, unconcerned about the abstract question of divisibility, 

 the physicist anxiously inquires whether the phenomena of the mate- 

 rial world can afford any testimony in regard to the ultimate constitu- 

 tion of bodies. And the molecular theory may be regarded as an in- 

 duction from a multitude of observed facts a generalization reached 

 by a careful comparison of many established principles. The object 

 of the present paper is to present the theory in this light. To exhaust 

 the evidence, however, by this method would be to present the science 

 of modern physics complete, a task impossible at a single sitting. 



The three fundamental conceptions embodied in the molecular 

 theory are : the existence of molecules, the existence of molecular 

 spaces, and the existence of molecular motions. 



Now, there may be phenomena which declare the existence of mol- 

 ecules without touching the question of molecular space or of molecu- 

 lar motion, and proofs both of the existence of molecules and of molec- 

 ular spaces may be altogether silent on the question of rest or motion. 

 But notice : whatever evidence we have of the existence of intervals 

 between the ultimate parts of a body is equally evidence of the exist- 

 ence of molecules, and whatever phenomena indicate the existence of 

 motion among these interior parts of a body do equally affirm the ex- 

 istence both of molecules and of molecular sp aces. If, then, the three 

 classes of phenomena be presented as bearing first upon the molecule, 

 second upon molecular spaces, and third upon molecular motion, the 

 testimony will be continuous and cumulative. 



First, then, as to the existence of molecules. We will confine our 

 attention to two sources of evidence the phenomena of divisibility 

 and of chemical synthesis. 



Is it possible, by continued subdivision, to reach a particle the 

 division of which would put an end to the existence of the substance 

 not in the sense of annihilation, but, in other words, to reach a particle 

 whose division compels a substance to suffer death by yielding up 

 those identifying properties by which alone we distinguish it from 

 other kinds of matter ? 



A piece of marble may be crushed and reduced to an almost im- 

 palpable powder, and yet, on examination, each little grain is found to 

 be an angular block of stone, lacking no property of the original 

 block except its size and form. The same may be said of other solids. 

 Even ice, keep its temperature low enough, may be reduced to micro- 

 scopic fineness, and each little particle, notwithstanding its minuteness, 



