Duane — Velocity of Chemical Reactions. 351 



sents fifteen minutes. This distance was determined on a 

 separate plate by lighting a magnesium burner for an instant 

 every hour at some distance in front of the horizontal slit, and 

 by measuring on the dividing engine the distance between the 

 lines thus formed. A heavy vertical line to the extreme right 

 of the plate (not seen in the copy) is a magnesium flash-light 

 line and represents the instant at which the hydrochloric acid 

 and sugar solution were mixed. 



The photographic plate was fastened in a frame hung on a 

 fine iron wire that was wrapped around a cylinder on the axle 

 of one of the wheels in the works of a clock. The escape- 

 ment of the clock was operated by an electro-magnet the cir- 

 cuit of which was made and broken by the swing of the 

 pendulum of a standard clock. On account of the escape- 

 ment the downward motion of the plate was by jerks, but as 

 each jerk carried the plate only about ^-th of a mm. this is 

 not apparent from the photograph. 



It is evident from the way in which the fifteen-minute 

 lines are drawn that no correction need be made for the shrink- 

 age of the gelatine films, for practically the same shrinkage 

 takes place on one plate as on another. The shrinkage too 

 reduces all the ordinates of the curve in the same proportion, 

 and if the relative positions of the images are always deter- 

 mined by measurements on a photographic plate no correction 

 need be applied to the ordinates. 



The object of having two wedge-shaped compartments is to 

 reduce the dispersion of the light passing through them as much 

 as possible, indeed white light from an incandescent lamp can 

 be used. The solution in the wedge a b d is, the same as that 

 in a c d except that in it the reaction has already taken place. 

 Under these circumstances the indices of refraction in the two 

 wedges are very nearly equal and a slow change of a few 

 degrees in temperature does not displace the image S x since 

 such a change affects the index of refraction in both solutions 

 practically to the same extent. 



In the above described experiment the dimensions of the 

 apparatus were as follows : SL (Fig. 1) = 150 cm ; LS 2 = 250 cm ; 

 ab = l cm ; ad = 5*3 cm ; Lb = 15 cm , approximately. The breadth 

 of the slit in front of the figure was *2 mm , and its distance from 

 the figure about '3 cm . 



That the distance of the image S x at any time from its final 

 position after the reaction has been completed is very approxi- 

 mately proportional to the amount of cane sugar left in the 

 solution at that time, may be seen from the following reason- 

 ing. The proof is based upon the assumptions that the density 



n 2 — 1 1 

 d and the specific index of refraction . -r of the solution 



