472 Prof. Challis on the Fundamental Ideas of 



attraction of gravity. The other physical forces, electricity, gal- 

 vanism, and magnetism, are referable, not to vibrations, but to 

 steadi/ motions of the sether; and many of the more obvious phe- 

 nomena are at once explained by hydrodynamical theorems. It 

 remains for me to meet certain objections which have been made 

 to the views T have been maintaining, and to notice some ob- 

 stacles which stand in the way of their being received. 



In the Philosophical Magazine for July 1865, in a note at the 

 bottom of page 64, I have remarked that the expression u finite 

 atoms ! " occurs. A note of admiration is no argument, but it 

 expresses an opinion, and may express a prejudice. After the 

 argument respecting atoms contained in this communication, I 

 think it needless to do more than state a counter opinion. The 

 author of this note of admiration, in upholding the law of " con- 

 servation of energy," has found it necessary to account for waste 

 of energy ; and in attempting to do this a difficulty is met with 

 which has led to the making and making again of hypotheses 

 destitute of all verisimilitude. This difficulty arises from the 

 adoption of a system of philosophy that rejects the finite atom. 



The Number of ( Good Words ' for July 1865 contains, in 

 p. 500, the following argument : — " Under no conception but 

 that of a solid can an elastic and expansible medium be self-con- 

 tained. If free to expand in all directions, it would require a 

 bounding envelope of sufficient strength to resist its outward 

 pressure. And to evade this by supposing it infinite in extent, 

 is to solve a difficulty by words without ideas — to take refuge 

 from it in the simple negation of that which constitutes the 

 difficulty." And yet the hypothesis that the sether is an elastic 

 fluid is an intelligible one, and may be made the basis of reason- 

 ing. If the reasoning explains phenomena so satisfactorily as 

 to prove the truth of the hypothesis, we shall be compelled to 

 admit the existence of an elastic fluid indefinitely extended. 

 Suppose that we are placed by the argument under this necessity, 

 I do not see that the case with respect to matter will then be 

 different from that with respect to time or space, which we are 

 equally compelled to think of as extended ad libitum. 



In the Proceedings of the Royal Society (vol. xiii. p. 534), an 

 argument, which I am unable to understand, is advanced pur- 

 porting to show that the' tc explanation of the force of gravitation 

 is not to be looked for in the action of a surrounding medium." 

 As I do not find this assertion in the paper the abstract of which 

 I quote from, there is no occasion to allude to it further. 



I have reason to say that a main obstacle to the adoption of 

 the method of philosophy I am arguing for is the favour with 

 which the conception of an isotropic constitution of the aether has 

 been received. This idea seems to have been suggested by the 

 crystalline structure of certain substances that may be seen and 



