34 PHYSICS 



Diffusion 



Diffusion was formally introduced into physics by Graham (1850). 

 Fick (1855), appreciating the analogy of diffusion and heat conduc- 

 tion, placed the phenomenon on a satisfactory theoretical basis, and 

 Pick's law has since been rigorously tested, in particular by H. F. 

 Weber (1879). 



The development of diffusion from a physical point of view fol- 

 lowed Pfeffer's discovery (1877) of osmotic pressure, soon after 

 to be interpreted by van 't Hoff (1887) in terms of Boyle's and 

 Avogadro's laws. A molecular theory of diffusion was thereupon 

 given by Nernst (1887). 



Dynamics 



In pure dynamics the nineteenth century inherited from the 

 eighteenth that unrivaled feat of reasoning called by Lagrange 

 the Mecanique analytique (1788), and the great master was present 

 as far as 1813 to point out its resources and to watch over the legit- 

 imacy of its applications. Throughout the whole century each new 

 advance has but vindicated the preeminent power and safety of 

 its methods. It triumphed with Maxwell (1864), when he deduced 

 the concealed kinetics of the electromagnetic field, and with Gibbs 

 (1876-78), when he adapted it to the equilibrium of chemical sys- 

 tems. It will triumph again in the electromagnetic dynamics of 

 the future. 



Naturally there were reactions against the tyranny of the method 

 of "liaisons." The most outspoken of these, propounded under the 

 protection of Laplace himself, was the celebrated Mecanique phy- 

 sique of Poisson (1828), an accentuation of Boscovich's (1758) 

 dynamics, which permeates the work of Navier, Cauchy, de St. 

 Venant, Boussinesq, even Fresnel, Ampere, and a host of others. 

 Cauchy in particular spent much time to reconcile the molecular 

 method with the Lagrangian abstractions. But Poisson's method, 

 though sustained by such splendid genius, has, nevertheless, on 

 more than one occasion in capillarity, in elastics shown itself 

 to be untrustworthy. It was rudely shaken when, with the rise of 

 modern electricity, the influence of the medium was more and more 

 pushed to the front. 



Another complete reconstruction of dynamics is due to Thomson 

 and Tait (1867), in their endeavor to gain clearness and uniformity 

 of design, by referring the whole subject logically back to Newton. 

 This great work is the first to make systematic use of the doctrine 

 of the conservation of energy. 



Finally, Hertz (1894), imbued with the general trend of con- 

 temporaneous thought, made a powerful effort to exclude force 



