Cooper. — An Experiment on the Wave Theory of Light. 487 



Diconic acid was prepared by Hergt by heating citric acid 

 and strong hydrochloric acid in sealed tubes to 190°, and 

 according to him melted at 199-200° C. 



There can be little doubt of the identity of the two sub- 

 stances, more especially as they agree in the following quali- 

 tative reactions : (1.) A solution of the ammonium salt of the 

 acid gave, with ferric chloride, an orange precipitate which 

 was not dissolved on boiling. (2.) Barium-chloride gave no 

 precipitate either cold or hot. (3.) Calcium-chloride gave no 

 precipitate either cold or hot. 



Notwithstanding that the yield of diconic acid by this 

 method is disappointingly small, the observation that it does 

 result by the action of sulphuric acid at comparatively low 

 temperatures is not only worthy of note, but suggests that by 

 suitable variations of the conditions of the reaction the sub- 

 stance may be prepared in large quantities by some easy 

 method, instead of the exceedingly tedious process described 

 by Hergt. 



During the investigation certain hitherto unnoticed pecu- 

 liarities in the silver and barium salts of aconitic acid were 

 observed : (1.) Silver-aconitate, on being heated, decomposes 

 with explosive violence. (2.) Ammonium-aconitate : On add- 

 ing a solution of barium-chloride to a solution of ammonium- 

 aconitate no precipitate is formed, but on boiling the solution 

 a crystallice precipitate results. The precipitate does not 

 dissolve on cooling, but becomes gelatinous. It is insoluble 

 in a solution of ammonium-benzoate, but soluble in a solu- 

 tion of ammonium-acetate. 



The reaction is very characteristic, taking place even in 

 dilute solution. 



Art. XLV. — A71 Experiment bearing on the Wave Theory of 



Light. 



By J. S. S. Cooper, M.A., B.Sc. 

 [Read before the Otago Institute, 9th June, 1903.] 

 1. Theor"^. 

 Consider a parallel plate of some transparent medium whose 

 refractive index varies as the distance of the point from an 

 initial plane at right angles to the face of the plate. Let 

 light from a distant source (a plane wave) be incident nor- 

 mally on this plate. The wave theory indicates that there 

 will be a deviation. 



