xxvn.] GENERALISATION. 615 



ordinary power of animal charcoal for attracting organic 

 matter, or of spongy platinum for condensing hydrogen, 

 which can only be considered as exalted cases of a more 

 general power of attraction. The number of substances 

 which are decomposed by light in a striking manner is very 

 limited; but many other substances, such as vegetable 

 colours, are affected by long exposure ; on the principle of 

 continuity we might expect to find that all kinds of matter 

 are more or less susceptible of change by the incidence of 

 light rays. 1 It is the opinion of Grove that wherever an 

 electric current passes there is a tendency to decomposition, 

 a strain on the molecules, which when sufficiently intense 

 leads to disruption. Even a metallic conducting wire may 

 be regarded as tending to decomposition. Davy was pro- 

 bably correct in describing electricity as chemical affinity 

 acting on masses, or rather, as Grove suggests, creating a 

 disturbance through a chain of particles. 2 Laplace went so 

 far as to suggest that all chemical phenomena may be results 

 of the Newtonian law of attraction, applied to atoms of 

 various mass and position ; but the time is probably far 

 distant when the progress of molecular philosophy and of 

 mathematical methods will enable such a generalisation to 

 be verified or refuted. 



The Law of Continuity. 



Under the title of the Law of Continuity we may place 

 many applications of the general principle of reasoning, 

 that what is true of one case will be true of similar cases, 

 and probably true of what are probably similar. When- 

 ever we find that a law or similarity is rigorously fulfilled 

 up to a certain point in time or space, we expect with a 

 high degree of probability that it will continue to be 

 fulfilled at least a little further. If we see part only of a 

 circle, we naturally expect that the circular form will be 

 continued in the part hidden from us. If a body has moved 

 uniformly over a certain space, we expect that it will 

 continue to move uniformly. The ground of such inferences 

 is doubtless identical with that of other inductive inferences. 



1 Grove, Correlation of Physical Forces, 3rd edit. p. 115. 

 ? Ibid. pp. 1 66, 199, &c. 



