a.NT3 THE NATURE OF FORCE. 503 



It must be remembered that a pair of atoms thus connected may 

 be in a constant state of minute vibration, so to speak, even in still 

 ether, since the ether pressure really consists of rapid impulses. 



All the different degrees of chemical affinity may be accounted for 

 by differences between the masses of the atoms, the extents of the 

 faces which can possibly come in contact, the number of faces, and 

 the different degrees of elasticity of the intermediate ether. 



It may be objected that the vibrations of the ether are transversal, 

 and those of common air longitudinal, so that the two cannot be 

 analogous. 



The undulations of common air consist both of longitudinal and 

 transversal components ; so do those of the ether. The ear is so 

 constructed as to be sensible only to longitudinal vibrations, and the 

 eys so that the transverse vibrations alone affect it. An objection of 

 much greater force is the following. Experience shows that radiant 

 heat is capable of evincing all the phenomena of polarization, which 

 can only be explained by the transverse vibrations. How is it then 

 that the thermo-electric pile is affected in the same way as the eye. 

 Similarity of construction would scarcely to taken as a satisfactory 

 explanation in this case. 



Prof. Challis, in his " Principles of Mathematics and Physics," 

 shows, on the hypothesis of an ether constituted as above, that in 

 certain cases of polarization the direct vibrations become very much 

 reduced in intensity compared with the transverse vibrations. If 

 this should be found to be the case, wherever the polarization of 

 heat corresponds with that of light, the objection would be answered. 



One great difficulty which the action by contact theory has yet to 

 solve, is the attraction of gravitation. Astronomical observations, 

 so far, have not shown that gravity has a sensible aberration like 

 light. The inference is, that the velocity of the gravity impulses 

 must be extremely great compared with the velocity of the earth, 

 and much greater than that of light. What is there in the nature 

 of a universe, constructed as we have supposed, to render it self- 

 contained 1 We must answer, Nothing. We are obliged on the contact 

 theory either to suppose matter to be coextensive with space, or else, 

 if we consider only our own portion of the universe, we may suppose 

 its boundaries to be indefinitely distant, in which case any changes 

 going on at the limits of the universe would be inappreciable to us. 

 We have thus noticed some of the chief objections to the contact 



