268 THE PRINCIPLES OF SCIENCE. 



tical with capillary attraction, which is capable of inter 

 fering with the pressure of aqueous vapour and aiding its 

 condensationy. 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 z . 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 

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

 creating a disturbance through a chain of particles 1 . 

 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. 



TJie 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. 

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

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



