572 



NA TURE 



[Oct. ii, i 



larly some relating to telephonic conductors and conductors for 

 electric lighting on the " alternate-current " system. In a short 

 article to be included in vol. iii. of my collected papers, which I 

 hope will soon be published, I intend to describe a generaliza- 

 tion, with, as will be seen, a consequently essential modification 

 of this analogy, by which it is extended to include the mutual 



induction between conductors separated by air or other insu- 

 lators, and currents in solids of different conductivity fixed 

 together in contact. 



4. If the subject is heat, as in Fourier's original development 

 of the theory of diffusion, the "quality" is temperature. 



5. If the subject is diffusion of matter, the "quality" is, 



Diagram showing Progress of Laminar Din 1 kk>> 



density of the matter diffused, or deviation of density from some 

 mean or standard density considered. It is to Fick, thirty-three 

 years ago Demonstrator of Anatomy, and now Professor of 

 Physiology in the University of Zurich, that we owe this appli- 

 cation of Fourier's diffusional theory, so vitally important in 

 physiological chemistry and physics, and so valuable in natural 



philosophy generally. When the substance through which the 

 diffusion takes place is fluid, a very complicated but practically 

 important subject is presented if the fluid be stirred. The ex- 

 ceedingly rapid progress of the diffusion produced by vigorous 

 up-and-down-stirring, causing to be done in half a minute the 

 diffusional work which would require years or centuries if the 



