in Causal Research. 359 



ferent in different directions. The medium is further supposed 

 to possess three rectangular planes of symmetry &c." 



We see that the same arbitrary speculations about u force" 

 are applied here. Alluding further on to a particular result 

 deduced by Cauchy, Prof. Stokes observes (page 259) : — 

 " This result is, however, in the present case only attained by 

 the aid of two sets of forced relations ; that is, relations which 

 there is nothing a priori to indicate, and which are not the 

 expression of any simple physical idea, but are obtained by 

 forcing the theory, which in its original state is of a highly 

 plastic nature, from the number of arbitrary constants which 

 it contains." (The number of these arbitrary constants is 

 further on stated to have been twenty-one !) Probably this 

 may be sufficient to illustrate the waste of the highest mathe- 

 matical skill on this subject. It is said that these specula- 

 tions were inserted in the Exercices de Mathematiques ; so 

 that possibly as an exclusively mathematical study (apart from 

 any physical applications) the labour may not have been 

 altogether lost. 



Prof. Stokes makes the remark (on page 262) : — " The argu- 

 ments in favour of the existence of ultimate molecules in the 

 case of ponderable matter appear to rest chiefly on the che- 

 mical law of definite proportions, and on the laws of crystal- 

 lography, neither of which of course can be assumed to apply 

 to the mysterious aether." 



I would venture to put the question here, whether the best 

 argument for the molecular constitution of the aether (as of 

 gross matter) is not to be found in the fact that the aether is 

 observed to have certain properties, and that these properties 

 would be totally inexplicable unless the aether were assumed to 

 be molecular. This was the argument of Lucretius, adopted 

 and expounded by Newton in regard to gross matter (and 

 applies equally to the aether). We can only have recourse to 

 arbitrary assumptions about "force," or to dogmatic affirma- 

 tions, which exclude all possibility of an explanation, unless 

 we regard the aether as molecular. In a paper a On the Dyna- 

 mical Theory of Gases " (Phil. Trans. 1867, page 49), Prof. 

 Maxwell has made a remark which I have before had occasion 

 to quote, viz.: — " The properties of a body supposed to be a 

 uniform plenum [i. e. not molecular] may be affirmed dogmati- 

 cally, but cannot be explained mathematically." It would 

 follow from this that, unless the aether be inferred to be mole- 

 cular, its properties can only be affirmed dogmatically, or no 

 rational account can be given of properties that can exist 

 solely in virtue of the explanation that underlies them. 



In fact, if the spiritualistic notion about " force " had never 



