236 Mr. E. H. Cook on the Existence of 



With these considerations before us, what need is there to 

 assume the existence of an all-pervading ether ? The particles 

 of ordinary matter are small enough, and can, nay do, vibrate 

 at the requisite speed ; why, then, are these particles not able 

 to transmit the waves of light ? Substituting for the lumini- 

 ferous- ether theory this molecular theory, let us now see if the 

 explanation of some of the difficulties of the former theory are 

 satisfactorily accounted for. Before doing so, however, I will 

 endeavour to answer two objections which it seems to me may 

 be made against this theory. First, it may be asked, If light 

 travels through bodies by the vibrations of its molecules, why 

 is not the velocity of light through the body the same as the 

 velocity of sound ? In answering this we must bear in mind 

 the differences between sound and light. The shortest wave 

 of sound would be produced by 38000 vibrations per second, 

 and would have a wave-length of about 9 millimetres, or 11250 

 times as great as the length of the longest wave of light. We 

 have also to remember that the particles of a sound-wave os- 

 cillate in the direction of propagation, whilst those in a light- 

 wave oscillate in planes perpendicular to that of propagation. 

 Is it unreasonable to suppose, then, that so vast a commotion 

 as that produced by sound in the direction of propagation should 

 be retarded more than the minute disturbance produced by 

 light in planes at right angles to this direction ? Another ob- 

 jection which may be urged is, How do we account for the 

 motion reaching us from the sun ? We may do this in two 

 ways : first, we may fill the space between the sun and earth 

 with the luminiferous ether, and give this ether the property 

 of non-rniscibility with the atmosphere ; or, secondly, we may 

 assume the unlimited extent of our atmosphere. Either of 

 these assumptions would be sufficient to account for the phe- 

 nomena ; and both have before been made. 



Let us now see if these difficulties we have mentioned in 

 regard to the conception of the ether are lessened if we con- 

 sider the particles of matter to vibrate. It is evident that a 

 condensation of matter does occur around the celestial bodies, 

 and also that a gradual increase of the refractive power occurs 

 as we approach large masses. We have here no difficulty in 

 conceiving the cause of the difference in the refractive powers 

 of bodies ; it is simply due to the different density of the bodies 

 and to the mode of grouping of the molecules interfering with 

 perfect freedom of motion of these molecules. A glance at 

 what may be termed the experimental difficulties given above 

 will suffice to ■ show the ability of this theory to satisfactorily 

 account for these experiments. 



Numerous experimental facts support this assumption in an 



