566 Dr. George Senter. The Eole of Diffusion in the [Mar. 7, 



the lowest parts were near the ends the two nodes. When the 

 stationary wave was of the same length as the trough two mounds 

 arose, one near each end, and so on. 



It is suggested that the tidal ridges in estuaries and the chains of 

 sand banks under the sea are formed in this way, by stationary or 

 nearly stationary water waves, and that the sand dunes of the sea- 

 shore and of the desert, and the clouds in a " mackerel sky " may be 

 similarly the products of stationary air waves. 



" The Mdle of Diffusion in the Catalysis of Hydrogen Peroxide by 

 Colloidal Platinum." By GEORGE SENTER, Ph.D., B.Sc. 

 (Lond.). Lecturer on Chemistry at St. Mary's Hospital 

 Medical School. Communicated by Sir WILLIAM EAMSAY, 

 K.C.B., F.RS. Eeceived March 7, Read March 30, 1905. 



According to a theory of reaction-velocities in heterogeneous systems 

 recently put forward by Nernst,* the observed velocities are those 

 with which diffusion and convection renew the reacting material at 

 the boundary of the two phases, the actual chemical change at the 

 boundary being very rapid in comparison. Nernst is of opinion that 

 the same considerations apply to the catalytic decomposition of 

 hydrogen peroxide by colloidal platinum and similar substances, but 

 has not gone fully into the discussion of this particular class of 

 heterogeneous reactions. 



In a recent paper on the effect of poisons on the catalysis of 

 hydrogen peroxide by hsemase,t I have discussed the probable 

 mechanism of catalysis by colloidal particles, and have arrived at the 

 conclusion that the experimental results obtained by Bredig and his 

 pupils and by myself are best accounted for on Nernst's hypothesis. 



In an important paper on this subject, Sand,J starting from certain 

 assumptions regarding the size and nature of the particles in a 

 colloidal platinum solution, has calculated the minimum value of the 

 velocity-constant (which we may call K D ) to be expected on Nernst's 

 hypothesis, and finds that it is at least 16 times as great as the 

 velocity-constant obtained experimentally by Bredig. Hence, he 

 concludes that Nernst's hypothesis does not apply to this particular 

 reaction ; the observed velocity is really that of a chemical action. 



* ' Zeitschrift fur physikal. Chemie,' vol. 47, p. 52, 1904. 

 t * Koy. Soc. Proc.,' vol. 74, p. 201, 1904. 

 j 'Koy. Soc. Proc.,' vol. 74, p. 356 (1905). 



Bredig and Muller von Berneck, ' Zeit. fur physikal. Chemie,' vol. 31, p. 258, 

 1899 ; Bredig and K. Ikeda, ' Zeit. fur physikal. Chemie,' vol. 37, p. 1, 1901. 



