382 MEETING OF THE BRITISH ASSOCIATION. 



from particle to particle of intervening matter. Not that I speak 

 of particles in the sense of the atomist. Whatever our views may 

 be of the nature of particles, we must conceive them as centres in- 

 vested with surrounding forces. We have no evidence, either 

 from our senses or otherwise, of these centres being occupied by 

 solid cores of indivisible incompressible matter essentially distinct 

 from force. Dr. Young has shown that even in so dense a body 

 as water, these nuclei, if they exist at all, must be so small in re- 

 lation to the intervening spaces, that 100 men distributed at equal 

 distances over the whole surface of England would represent their 

 relative magnitude and distance. What then must be these rela- 

 tive dimensions in highly rarefied matter ? But why encumber 

 our conceptions of material forces by this unnecessary imagining 

 of a central molecule ? If we retain the forces and reject the mole- 

 cule, we shall still have every property we can recognize in matter 

 by the use of our senses or by the aid of our reason. Viewed in 

 this light, matter is not merely a thing subject to force, but is 

 itself composed and constituted of force. 



DYNAMICAL THEORY OP HEAT. 



The dynamical theory of heat is probably the most important 

 discovery of the present century. We now know that each 

 Fahrenheit degree of temperature in a pound of water is equiva- 

 lent to a weight of 772 lbs. lifted one foot high, and that these 

 amounts of heat and power are reciprocally convertible into one 

 another. This theory of heat, with its numerical computation, is 

 chiefly due to the labours of Mayer and Joule, though many other 

 names, including those of Thomson and Rankine, are deservedly 

 associated with its development. I speak of this discovery as one 

 of the present age because it has been established in our time ; 

 but if we search back for earlier conception of the identity of 

 heat and motion, we shall find (as we always do in such cases) that 

 similar ideas have been held before, though in a clouded and un- 

 demonstrated form. In the writings of Lord Bacon we find it 

 stated that heat is to be regarded as motion, and nothing else. In 

 dilating upon this subject, that extraordinary man shows that he 

 had grasped the true theory of heat to the utmost extent that was 

 compatible with the state of knowledge existing in his time. Even 

 Aristotle seems to have entertained the idea that motion was to be 

 considered as the foundation not only of heat, but of all manifes- 

 tations of matter ; and, for aught we know, still earlier thinkers 

 may have held similar views. The science of gunnery, to which 



