ON THE VELOCITY OF CHEMICAL REACTIONS 21 



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., and the other curved due to the moving image S^. The 

 curved line represents the reaction in that the absyssas are propor- 

 tional to the intervals of time and the ordinates represent (but are 

 not proportional to) the quantities of the substance that have reacted. 

 After the reaction is completed the plate is drawn up and lowered 

 again and the image S^ 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 hori- 

 zontal line represents the position of the image S^ 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 be- 

 tween two successive lines represents 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 verticle 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 so- 

 lution 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 escapement of the clock 

 was operated by an electro -magnet, the circuit of which was made 

 and broken by the swing of the pendulum of a standard clock. On 

 account of the escapement 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 shrinkage of the 



