THE LUMINIFEROUS ^THER. 115 



commonly think of as a vacuum, and it is only the phenomena of lif^ht 

 that led us iu the first instance to think ot it as filled with some kind 

 of material. The question naturally presents itself to the mind — is this 

 R'ther absolutely infinite like space? This is a question to which 

 science can give no answer. Though we can not help thinking of space 

 as infinite, yet when we turn our thoughts to some material existing in 

 space perhaps we more readily think of it as finite than infinite. But 

 if the icther, however vast the portion of space over which it extends, 

 be really limited, we can hardly fail to speculate what there may be 

 outside its limits. Space there might be wholly vacuous, or possibly 

 outside altogether this vast system of stars and aether there may be 

 another system subject to the same laws, or subject to different laws, 

 as the case may be, ecpially vast in extent; and if there be, then so far 

 as we can gather from such phenomena as are open to our investiga- 

 tion, there can be no communieation between that vast ])ortion of 

 space in part of which we live and an ideal system altogether outside 

 the pether of which we have been speaking. 



But the properties of the aether are no less remarkable than its vast 

 or even possibly limitless extent. Matter of which our senses give us 

 any cognisance is heavy, that is to say, it gravitates toward other 

 matter which agrees with it in so far as being accessible to our senses. 

 The question presents itself to the mind, does the jether gravitate 

 toward what we call ponderable matter? This is a question to which 

 wciare not able to give any positive scientific answer. If the u'ther be 

 in some way or other connected with the cause of gravitation it would 

 seem more likely that it itself does not gravitate toward ponderable 

 matter. 



Again, we have very strong reason for believing that ponderable 

 matter consists of ultimate molecules. First, that supposition accords 

 in the simplest way with the laws of crystallogra])hy. Chemical laws 

 afford still stronger confirmation of the hypothesis, through the atomic 

 theory of Dalton, now universally accepted. Comparatively recently, 

 the deduction of the fundamental property of gases from the kinetic 

 theory, as it is called, affords strong additional confirmation of that 

 view of the constitution of matter. Still more recently, the explana- 

 tion which has been afforded by that theory of that most remarkable 

 iiistrumentthe "radiometer" of Crookes has lent further confirmation in 

 the same direction. None of these evidences apply to the a'ther, and 

 accordingly we are left in doubt whether it too consists of ultimate 

 molecules, or whether on the other hand it is continuous, as we can not 

 help conceiving space to be. 



The undulatory theory of light was greatly j)romoted m the first 

 instance by the know n idienomena of sound, and the explanation which 

 they received from the hydro-dynamical theory. Accordingly, since 

 sound, as we know, consists of an undulatory movement propagated 

 through the air (or it may be through other media), and depending 



