John Dalton 17 



fixes the limits of creation, as regards the possible diversity 

 of matter. But all such scruples must be set aside, for 

 the atom of Dalton is only a stepping-stone to a higher 

 level of knowledge. The chemist knows what he means 

 by an atom, and when he is building up his compounds 

 with them, he is not concerned with the question of their 

 ultimate constitution; just as the builder who constructs a 

 house with bricks need not trouble to enquire whether the 

 substance of the bricks is continuous or made of up of mole- 

 cules. The merit of Dalton 's atomic theory, like that of the 

 law of gravitation, is that it sets certain boundaries beyond 

 which our imagination need not wander for the moment ; 

 it defines a limited problem and for the time solves it. 



Speculations on the nature of light could not fail to 

 attract the attention of the old philosophers; but, for our 

 present purpose, we need not go farther back than to the 

 rival theories of Newton and Huygens. The former led, 

 no doubt, by his predilection for an accurately definable 

 starting point from which he could proceed to develop the 

 consequence of a theory with mathematical precision- 

 adopted the view (to be found already in the writings of 

 Democritus), that light consists of small corpuscles emitted 

 by the luminous body. The rectilinear propagation of light, 

 and its bending as it passes from one transparent body 

 to another, could easily be explained on this theory, and 

 though it was incapable of dealing with the more complex 

 properties of light, it received general support until the 

 middle of last century. 



It was apparently Hooke who first suggested that light 

 was an undulatory motion in an all-pervading medium, but 

 Huygens has the merit of showing how this hypothesis could 

 explain luminous phenomena with a precision at least equaJ 

 to that of the corpuscular theory. There being at that 

 time no crucial test to decide between the rival theories, 

 the cleavage of scientific opinion took place along the line 

 of separation between metaphysical tendencies. Those who 

 disliked the idea of a vacuum and action at a distance 

 inclined towards Huygens, others towards Newton. Com- 

 promises have never been favoured by men of science, and 

 as the theory of gravitation starts from an assumption 



B 



