350 



Duane — Velocity of Chemical Reactions. 



In order to obtain a complete record of the position of S x a 

 photographic plate S a S 2 is placed in a vertical position at the 

 images S, and S 2 and just in front of it a screen. A nar- 

 row horizontal slit cut in the screen allows a small part of the 

 light only to pass through. At any instant of time, there- 

 fore, there will be two small spots of light on the plate at the 

 intersections of the two images S, and S 2 with the projection 

 of the slit on the plate. A system of cog-wheels allows the 

 photographic plate to fall slowly during the reaction so that 

 two lines are drawn on it, one of them straight due to the 

 fixed image S 2 and the other curved due to the moving image 

 Sj. The curved line represents the reaction in that the absyssas 

 are proportional to the intervals of time and the ordinates rep- 

 resent (but are not proportional to) the quantities of the sub- 

 stance that have reacted. After the reaction is completed the 

 plate is drawn up and lowered again and the image Sj traces a 

 third line that is practically straight. This line may be taken 

 as the zero line and the distances between it and the curved 

 one are (at least in some cases) proportional to the amounts of 

 the original compounds left in the solution. 



Fig. 2 is a reduced copy of a photograph representing the in- 

 version of a 25 per cent solution of cane sugar, the inversion being 

 accelerated by the addition of hydrochloric acid. The middle 

 horizontal line represents the position of the image S 5 twenty- 

 four hours after the reaction had started. The vertical lines 

 were drawn with a dividing engine after the plate had been 

 developed. The distance between two successive lines repre- 



