PRESIDENTIAL ADDRESS, B.A. 1871. 163 



constituting an individual atom. He investigated 

 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 slow- 

 ness 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 Maxwell brought in viscosity and thermal 

 conductivity, and thus completed the dynamical 

 explanation of all 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 all physical science will be represented with 

 every property of matter shown in dynamical 

 relation to the whole. The prospect we now have 

 of an early completion of this chart is based on 



M 2 



