356 Dr. H. J. S. Sand. The Kole of Diffusion during [Nov. 22, 



In the last column, UL, the values of V c are given, calculated by the 

 same formula, but using for P the temperature variation deduced from 

 the experiments on the upper limit. The correspondence of columns 4 

 and 5, and the divergence in column 6, are quite sufficient to show 

 that the temperature variation of the lower limit is in agreement with the 

 law of viscosity rather than with the formula obtained from the upper 

 limit The experiments are important in showing a close agreement 

 with the theoretical formula of Reynolds, which requires not only the 

 viscosity formula to hold, but the inverse-diameter law as well. The 

 agreement of the individual observations confirms our previous 

 conclusions, in showing the uncertainty in measuring critical velocity, 

 not from the want of accuracy in the measuring appliances, but from 

 the variation in the point itself. It is only by taking a mean of a 

 number of observations that anything like an accurate value can be 

 obtained. 



" The Role of Diffusion during Catalysis by Colloidal Metals and 

 Similar Substances." By HENRY J. S. SAND, Ph.D., M.Sc., 

 University College, Nottingham. Communica.ted by Professor 

 J. H. POYNTING, F.R.S. Pieceived November 22, Eead De- 

 cember 8, 1904 



In a paper on reaction-velocities in heterogeneous systems, Nernst* 

 has recently put forward the view that all chemical reactions taking 

 place on the boundary of two phases proceed to equilibrium practically 

 instantaneously, and that the velocities actually observed are simply 

 those with which diffusion and convection renew the reacting material 

 at the boundary. As a special instance of heterogeneous reactions, he 

 mentions catalytic decompositions due to finely divided particles, such 

 as colloidal metals, and he believes it probable that the kinetics of 

 these reactions can be deduced from the assumption that chemical 

 equilibrium remains permanently established on the surface of the 

 particles. 



Processes of this kind have recently undergone a considerable amount 

 of experimental investigation, the most important instances being the 

 catalytic decomposition of hydrogen peroxide due to colloidal platinumf 

 and colloidal goldj by Bredig and his pupils, and the catalytic decom- 

 position of the same substance due to hsemase by Senter. In his last 

 paper Senter discusses his reaction in the light of Nernst's hypothesis, 

 and arrives at the conclusion that the known facts do not contradict it. 



* 'Ztschft. Phys. Chem.,' vol. 47, p. 52 (1904). 



f Bredig and Miiller, v. Berneck, ' Ztechft. Phys. Chem.,' vol. 31, p. 258 (1899) ; 

 Bredig and Ikeda, Hid., vol, 37, p. } (1901). 



I Bredig and Keinders, ' Ztscbft. Phys. Chem.,' vol. 37, p. 323 (1901). 

 Ibid., vol. 44, p. 257 (1903); ' Boy. Soc. Proc.,' vol. 74, p. 201 (1904). 



