HYPOTHESES. 25 



7. It is necessary, before quitting the subject of hypo- 

 theses, to guard against the appearance of reflecting upon 

 the scientific value of several branches of physical inquiry, 

 which, though only in their infancy, I hold to be strictly 

 inductive. There is a great difference between inventing 

 agencies to account for classes of phenomena, and endea- 

 vouring, in conformity with known laws, to conjecture what 

 former collocations of known agents may have given birth to 

 individual facts still in existence. The latter is the legiti- 

 mate operation of inferring from an observed effect, the 

 existence, in time past, of a cause similar to that by which 

 we know it to be produced in all cases in which we have 

 actual experience of its origin. This, for example, is the 

 scope of the inquiries of geology ; and they are no more 

 illogical or visionary than judicial inquiries, which also aim 

 at discovering a past event by inference from those of its 

 effects which still subsist. As we can ascertain whether a 

 man was murdered or died a natural death, from the indica- 

 tions exhibited by the corpse, the presence or absence of signs 

 of struggling on the ground or on the adjacent objects, the 



properties of elastic fluids ? This opinion of Dr. Whewell reduces the undula- 

 tions to a figure of speech, and the undulatory theory to the proposition which 

 all must admit, that the transmission of light takes place according to laws 

 which present a very striking and remarkable agreement with those of undula- 

 tions. If Dr. Whewell is prepared to stand by this doctrine, I have no diffe- 

 rence with him on the subject. 



Since this chapter was written, the hypothesis of the luminiferous ether has 

 acquired a great accession of apparent strength, by being adopted into the new 

 doctrine of the Conservation of Force, as affording a mechanism by which to 

 explain the mode of production not of light only, but of heat, and probably of 

 all the other so-called imponderable agencies. In the present immature stage 

 of the great speculation in question, I would not undertake to define the ulti- 

 mate relation of the hypothetical fluid to it ; but I must remark that the essen- 

 tial part of the new theory, the reciprocal convertibility and interchangeability 

 of these great cosmic agencies, is quite independent of the molecular motions 

 which have been imagined as the immediate causes of those different manifesta- 

 tions and of their substitutions for one another ; and the former doctrine by no 

 means necessarily carries the latter with it. I confess that the entire theory of 

 the vibrations of the ether, and the movements which these vibrations are sup- 

 posed to communicate to the particles of solid bodies, seems to me at present 

 the weakest part of the new system, tending rather to weigh down than to prop 

 up those of its doctrines which rest on real scientific induction. 



