OF THE BKITISH ASSOCIATION. 21 



of still further encroaching on the valuable time of the Section, I shall say a 

 few words on a branch of physics which not very long ago would have been 

 considered rather a branch of metaphysics. I mean the atomic theory, or, as 

 it is now called, the molecular theory of the constitution of bodies. 



Not many years ago if we had been asked in what regions of physical 

 science the advance of discovery was least apparent, we should have pointed 

 to the hopelessly distant fixed stars on the one hand, and to the inscrutable 

 delicacy of the texture of material bodies on the other. 



Indeed, if we are to regard Comte as in any degree representing the 

 scientific opinion of his time, the research into what takes place beyond our 

 own solar system seemed then to be exceedingly unpromising, if not altogether 

 illusory. 



The opinion that the bodies which we see and handle, which we can set 

 in motion or leave at rest, which we can break in pieces and destroy, are 

 composed of smaller bodies which we cannot see or handle, which are always 

 in motion, and which can neither be stopped nor broken in pieces, nor in any 

 way destroyed or deprived of the least of their properties, was known by the 

 name of the Atomic theory. It was associated with the names of Democritus, 

 Epicurus, and Lucretius, and was commonly supposed to admit the existence 

 only of atoms and void, to the exclusion of any other basis of things from the 

 universe. 



In many physical reasonings and mathematical calculations we are accustomed 

 to argue as if such substances as air, water, or metal, which appear to our 

 senses uniform and continuous, were strictly and mathematically uniform and 

 continuous. 



We know that we can divide a pint of water into many millions of 

 portions, each of which is as fully endowed with all the properties of water 

 as the whole pint was ; and it seems only natural to conclude that we might 

 go on subdividing the water for ever, just as we can never come to a limit 

 in subdividing the space in which it is contained. We have heard how Faraday 

 divided a grain of gold into an inconceivable number of separate particles, and 

 we may see Dr Tyndall produce from a mere suspicion of nitrite of butyle 

 an immense cloud, the minute visible portion of which is still cloud, and there- 

 fore must contain many molecules of nitrite of butyle. 



But evidence from different and independent sources is now crowding in upon 

 us which compels us to admit that if we could push the process of subdivision 



