334 PROCEEDINGS OF THE AMERICAN ACADEMY. 



oxalic acid on bromine is not equal to the sum of the speed of water on 

 bromine plus the speed of the bromine-oxalic acid reaction. The assump- 

 tion of such an equality would at all events be hazardous. Since bromine 

 has no appreciable action on water in the dark at ordinary temperatures, 

 this objection has no important weight when offered against the main 

 portion of the work outlined in this paper. 



It becomes now a matter of importance to determine the cause of this 

 remarkable specific action of the hydrobromic acid. That the relation is 

 a complex one goes without saying. As has already been pointed out, the 

 smallness of the effect caused by hydrochloric acid indicates that the de- 

 pression of the dissociation of the oxalic acid is not the only cause at 

 work. On comparing the effect of different concentrations of hydro- 

 bromic acid in the presence of like concentrations of the other substances, 

 it is seen that the retarding effect grows faster than the concentration of 

 the hydrobromic acid. Thus when 88 milligrams of bromine were present, 

 65 milligrams of hydrobromic acid allowed a rate of 8.3, while 134 

 depressed the rate to 1.3 (IV, 31, and V, 123). A doubling of the 

 concentration of the hydrobromic acid more than halved the rate. 



If this effect is catalytic, and peculiar to this reaction, we should expect 

 to find different effects in other cases. On the other hand, it is possi- 

 ble that the hydrobromic acid may really enter into combination with 

 some of the bromine, and thus withdraw it from the reaction. Tf hydro- 

 bromic acid diminishes the reacting concentration of bromine in this 

 case, it should also produce this effect in all other cases, both chemical 

 and physical. It should, for example, diminish the bromine-vapor 

 tension of bromine water. This conclusion was easily tested, and found 

 to be amply supported by fact. It forms another link in the chain of 

 evidence indicating the existence of polybromides in solution. 



When hydrobromic acid is added to an aqueous solution of bromine 

 the color of the latter is lost to a great degree, the solution becoming 

 much paler. Such a color change indicates chemical change. That the 

 vapor tension is much depressed was clearly shown by the following 

 experiment: — 



Six gas-washing bottles were arranged in series, and into the first (A) 

 was put a solution of bromine in water. The second and third bottles 

 contained a solution of potassium iodide. Into the fourth (D) a bromine 

 solution containing hydrobromic acid was introduced, and the last two 

 bottles were filled with the iodine solution. The arrangement was as 

 shown below, — 



