produced by Collisions of Atoms or Molecules. 319 



§ 7. We now meet the question : — Is ether incompressible ? 

 We should be compelled to answer — Yes, it is incompressible, 

 if it is subject to the law of universal gravitation. But, 

 presently, when we 'try to account for motion produced in 

 ether, by ponderable or electrical atoms moving through it, 

 we shall feel ourselves persuaded that ether is compressible*. 

 Believing this, we are forced to believe that it is non- 

 gravitational. Thus we find ourselves settled in the con- 

 viction that ether is compressible, and that ether experiences 

 no gravitational forces between its parts. 



§ 8. Suppose now that an atom, whether ponderable or 

 electrical, disturbs ether solely by attracting it or repelling it 

 with a force varying according to distance ; and that, with 

 no other mutual influence than this, the atom and the ether 

 jointly occupy the same space. If ether were incompressible, 

 this attraction or repulsion would be utterly ineffective. The 

 atom would move through the space occupied by the ether, 

 without giving any motion to the ether, and without itself 

 experiencing any influence of force due to the ether. Hence, 

 in order that atoms may take energy from motions of ether, 

 and that ether may take energy from motions of matter, we 

 must suppose the ether to be compressible and dilatable ; and 

 to be compressed, or to be dilated, or compressed at some 

 distances, and dilated at other distances, in virtue of the force 

 exerted on it by the atom. 



§ 9. While assuming ether to be compressible, we suppose 

 its resistance to compression (positive or negative) to be so 

 very great that the velocity of condensational-raref actional 

 waves in pure ether is practically infinite, and that the energy 

 of whatever of such waves may be produced by collisions of 

 atoms or electrions is practically nil in comparison with the 

 energy of the equi-voluminal waves, constituting radiant heat 

 and light, which are actually produced by these collisions. 

 It is only under the enormous forces of attraction or repulsion 

 exerted by atoms on ether that augmentation or diminution 

 of its density is practically influential. 



§ 10. By purely d}mamical reasoning, it may be proved to 



follow from the hypotheses of §§ 4, 6, 8, and 9, that an atom, 



(supposed for a moment to be infinitely small,) kept moving 



through ether at any velocity, q, greater than v, the velocity 



of light, produces no disturbance in the ether in front of a 



cone having its vertex at the atom and semi-vertical angle 



v 

 equal to sm -1 -t; but that the moving atom produces, in its 



* * Baltimore Lectures/ Appendix A ; Appendix B ; § 3. 

 f 'Baltimore Lectures/ Appendix B, §§ 6, 7. 

 Z2 



