Duane — Velocity of Chemical Reactions. 349 



Art. XXYIII. — On the Velocity of Chemical Reactions ; by 

 William Duane. 



In investigating the laws of physical chemistry it is expe- 

 dient for the physicist to devise and develop the methods of 

 measurement, and for the chemist to apply them. In the fol- 

 lowing pages are described two methods of measuring the 

 velocity of chemical reactions. The velocity of a chemical 

 reaction is the rate at which a chemical compound appears or 

 disappears during that reaction. The changes in the quantity 

 of this compound present during successive intervals of time 

 are measured by the changes in some property of the chemical 

 system during the intervals. This property is usually either 

 a chemical or a physical one. 



l 



In the first of the following methods the basis of the 

 measurement is the change in the index of refraction of the 

 system, and in the second the change in its volume. 



The first method is applicable to those chemical systems 

 only that are transparent. It is substantially the following. 

 Rays of light from an illuminated slit S (Fig. 1) passing through 

 a long focus lens L and the tube abed form a distant image 

 Sj of S. The slit S is perpendicular to the plane of the 

 diagram which represents a horizontal section of the appa- 

 ratus. The tube aocd has plane glass ends a b and c d / 

 and a plane glass plate a c divides it into two wedge-shaped 

 compartments. The ends ah and c d are not quite parallel 

 to each other, so that if the two compartments are filled with 

 liquids having the same index of refraction there will be a 

 slight resultant refraction of the light rays that pass through 

 the tube. The rays of light that pass outside of the tube 

 therefore will form an image S 2 a little to one side of S x . It 

 is evident that if the liquid in one compartment (the wedge 

 a c d for instance) is undergoing a chemical change its index 

 of refraction in general will vary and the image S x will move 

 sideways. The distance that S a has moved will be a measure 

 of the change that has taken place in the index of refraction 

 and, therefore, of the amount of substance in a c d, that has 

 reacted. The displacements of S, can be determined by com- 

 paring its distances from S 3 which remains stationary. 



Am. Jour. Sol— Fourth Series, Yol. XI, No. 65.— May, 1901. 

 23 



