ADDRESS. XCllI 



Committee on Electric Measurement ; and it is at present premature to specu- 

 late as to the closeness of the agreement between that Telocity and the 

 velocity of light. This leads me to remark how much science, even in its 

 most lofty speculations, gains in return for benefits conferred by its applica- 

 tion to promote the social and material welfare of man. Those who perilled 

 and lost their money in the original Atlantic Telegraph were impelled and 

 supported by a sense of the grandeur of their enterprise, and of the world- 

 wide benefits whicli must flow from its success ; they were at the same time 

 not unmoved by the beauty of the scientific problem directly, presented to 

 them ; but they little thought that it was to be immediately, through their 

 work, that the scientific world was to be instructed in a long-neglected and 

 discredited fundamental electric discovery of Faraday's, or that, again, when 

 the assistance of the British Association was invoked to supply their elec- 

 tricians with methods for absolute measurement (which tliey found necessary 

 to secure the best economical return for their expenditure, and to obviate 

 and detect those faults in their electric material which had led to disaster), 

 they were laying the foundation for accurate electric measurement in every 

 scientific laboratory in the world, and initiating a train of investigation whicli 

 now sends up branches into the loftiest regions and subtlest ether of natural 

 philosophy. Long may the British Association continue a bond of union, 

 and a medium for the interchange of good offices between science and the 

 world ! 



The greatest achievement yet made in molecular theory of the proper- 

 ties of matter is the Kinetic theory of Gases, shadowed forth by Lucretius, 

 definitely stated by Daniel Bernoulli, largely developed by Herapath, made 

 a reality by Joule, and worked out to its present advanced state by Clausius 

 and Maxwell. Joule, from his dynamical equivalent of heat, and his expe- 

 riments upon the heat produced by the condensation of gas, was able to 

 estimate the average velocity of the ultimate molecules or atoms composing 

 it. His estimate for hydrogen was 6225 feet per second at temperature 60° 

 Fahr., and 6055 feet per second at the freezing-point. Clausius took fully 

 into account the impacts of molecules on one another, and the kinetic energy 

 of relative motions of the matter constituting an individual atom. He in- 

 vesfigated the relation between their diameters, the number in a given 

 space, and the mean length of path from impact to impact, and so gave the 

 foundation for estimates of the absolute dimensions of atoms, to which I shall 

 refer later. He explained the slowness of gaseous diffusion by the mutual 

 impacts of the atoms, and laid a secure foundation for a complete theory of 

 the diffusion of fluids, previously a most refractory enigma. The deeply 

 penetrating genius of MaxweU brought in viscosity and thermal conductivity, 

 and thus completed the dynamical explanation of aU the known properties 

 of gases, except their electric resistance and brittleness to electric force. 



No such comprehensive molecular theory had ever been even imagined 

 before the nineteenth century. Definite and complete in its area as it 

 is, it is but a well-drawn part of a great chart, in which aU physical 

 science wiU be represented with every property of matter shown in dyna- 

 mical relation to the whole. The prospect we now have of an early 

 completion of this chart is based on the assumption of atoms. But there 

 can be no permanent satisfaction to the mind in explaining heat, light, elas- 

 ticity, diffusion, electricity and magnetism, in gases, liquids, and solids, and 

 describing precisely the relations of these different states of matter to one 

 another by statistics of gxeat numbers of atoms, when the properties of the 

 atom itself are simply assumed. When the theory, of which we have the first 



