268 THE PRINCIPLES OF SCIENCE. 



tical with capillary attraction, which is capable of inter- 

 fering with the pressure of aqueous vapour and aiding its 

 condensation?. There are many cases of so-called catalytic 

 or surface action, such as the extraordinary power of animal 

 charcoal for attracting organic matter, or of spongy pla- 

 tinum for condensing hydrogen, which can only be con- 

 sidered as exalted cases of a much more general power of 

 attraction. ' The number of substances which are decom- 

 posed 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 well expect to find that all kinds of matter are 

 more or less susceptible of change by the incidence of light 

 rays 2 . It is the opinion of Mr. Justice Grove that wherever 

 an electric current passes there is a tendency to decom- 

 position, 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 probably correct in describing electricity as chemical 

 affinity acting on masses, or rather, as Grove suggests, 

 creating a disturbance through a chain of particles a . 

 Laplace went so far as to suggest that all chemical phe- 

 nomena may be regarded as the results of the Newtonian 

 law of attraction, applied to atoms of various mass and 

 position ; but the time is probably long distant when the 

 progress of molecular philosophy and of mathematical 

 methods will enable such a generalization to be verified 

 or refuted. 



The Law of Continuity. 



Under the title Law of Continuity we may place many 

 applications of the general principle of reasoning, that 



y 'Philosophical Magazine,' 4th Series, vol. xlii. p. 451. 

 55 Grove, 'Correlation of Physical Forces,' 3rd edit. p. 118. 

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



