MATTER 253 



brick wall, but a wave of electric oscillations can. In 

 order to describe the motion of these luminous and electric 

 waves the physicist conceives ether to penetrate all bodies 

 and to act as a medium for the transit of energy through 

 them. Matter cannot therefore be looked upon as the 

 thing which is absolutely impenetrable. 



Or, are we missing the point of what is meant, when 

 it is asserted that matter is that which is impenetrable ? 

 Are we to postulate the real existence of atoms and then 

 to suppose the individual members of the swarm impene- 

 trable ? Here again a difficulty arises. There is much 

 that tends to convince physicists that the atom cannot be 

 conceived as the simplest element of the conceptual 

 analysis of material groups. Just as a bell when struck 

 sets the air in motion and gives a note, so we conceive an 

 atom capable of being struck, and of setting not the air 

 but the ether in motion, of giving, as we might express 

 it, an ether note. These notes produce in us certain 

 optical sense -impressions — for example, the bright lines 

 of the spectrum of an attenuated gas. As without seeing 

 two bells we might, and indeed often do, distinguish them 

 by their notes,^ so the physicist distinguishes an atom of 

 hydrogen from an atom of oxygen, although he has never 

 seen either, by. the different light notes which he conceives 

 to arise from them. But as the bell to give a note must 

 be considered as vibrating — changing its shape or under- 

 going strain — so the physicist practically finds himself 

 compelled to conceive the atom as undergoing strain, or 

 changing its shape. This conception forces us to suppose 

 the atom built up of distinct parts capable of changing 

 their relative position. What are these ultimate parts of 

 the atom, by the relative motion of which we describe our 

 sense -impressions of the bright lines in the spectrum? 

 We have as yet formed no conception. Does the ether 

 or anything else penetrate between these ultimate parts 

 of the atom ? We cannot say. In the present state of 



1 The householder is generally able to distinguish the sound of his back- 

 door from that of the front-door bell, although, probably, in ninety-nine cases 

 out of a hundred he may never have examined the bells in his house. 



