1871.] Molecules, Ultimates, Atoms, and Waves. 171 



numerous more minute atoms, differing in their inertia, and 

 held together by a force too great to be overcome by any 

 chemical means which can be brought to bear upon them 

 for the purpose of effecting their separation. This view at 

 least possesses such an amount of probability, that it may 

 be fairly assumed as a basis of argument and of further in- 

 vestigation. 



Making this assumption, then, it will be found convenient 

 to designate the ultimate particles of the chemical elements 

 by the term " ultimates," reserving the term " atoms " for 

 their assumed constituents. We shall thus have the three 

 terms, "molecules," "ultimates," and "atoms;" the first 

 denoting the particles of chemical compounds, the second 

 those of the chemical elements, and the third the assumed 

 constituents of those ultimates, the atoms being themselves 

 incapable of further analysis. The term "particle" may 

 be applied generically to embrace all the three. 



So far as observation goes, the only way in which vibra- 

 tions can be excited in the infinite luminiferous ether is 

 by corresponding vibrations in the particles of ponderable 

 matter. On the other hand, these latter may always be 

 excited by pre-existing vibrations in the ether itself — the one 

 set of vibrations acting and reacting on the other. We are 

 ignorant, however, of the manner in which vibrations may 

 be excited in the particles of ponderable matter, apart from 

 any pre-existing vibrations in the ether, or from the conversion 

 of progressive into vibratory motion by collision. Electricity 

 is ostensibly one kind of force which may be available for 

 this purpose; but then we know not to what extent the ether 

 associated with the ponderable particles may be needful to the 

 existence of the electricity, although we do know that it can 

 neither exist in the free ether, nor pass through it without 

 the help of ponderable matter. Waiving this question, let 

 it be assumed that the vibrations in the particles of pon- 

 derable matter are excited by some adequate force — some 

 kind of motive energy. 



The amount of vibratory motion assumed by the particles 

 will depend — (1st) on the degree of motive energy applied, 

 and (2nd) on the inertia of the particles — their resistance 

 to the applied force. This inertia is of two sorts, (1st) 

 intrinsic, (2nd) adventitious. The intrinsic is that possessed 

 by each particle in itself, independently of its relations to 

 other particles with which it may be associated. The ad- 

 ventitious inertia arises out of those relations. It is the 

 immobility which any particle possesses in virtue of the 

 forces exerted on it by other neighbouring particles. The 



