RALPH S. LILLIE 



119 



feature of the behavior has an important general significance, as will 

 be seen below. 



The same type of behavior was found in all the solutions used, 

 which included nitric acid of the following concentrations (in volumes 

 per cent of 1.42 HNO3): 55, 57.5, 60, 65, 70, 75, 80, 85, 90. The 

 time required for complete recovery of transmissivity increases rap- 

 idly with increasing concentration of acid. Table II gives the typi- 

 cal recovery times for different solutions at 20° . In the most 

 concentrated solutions, 85 and 90 per cent, especially the latter, the 

 recovery process may exhibit irregularities whose explanation is not 

 entirely clear; and at times transmissivity remains incomplete, even 



TABLE II. 



after hours or days in these solutions. Apparently very strong acid 

 renders the surface film resistant to alteration under certain condi- 

 tions. Frequently, however, there is complete recovery in 90 per 

 cent or even stronger acid; but the time required is usually 20 minutes 

 or more. In the lower concentrations complete transmissivity always 

 returns within the time indicated in Table II . 



In their general character the measurements for the different solu- 

 tions resemble those given in Table I, so that it is unnecessary to 

 reproduce them in detail; Table III, however, gives the averages 

 obtained for the different solutions from 57.5 per cent on.^^ In these 



^^ Exact measurements with 55 per cent acid are difficult to make because the 

 wire at the end of the reaction (which lasts 5 or 6 seconds) is so covered with oxide 

 that the distance traveled by a later activation wave is not distinctly marked. 

 Recovery is more rapid and the whole behavior is less regular than in the stronger 

 solutions. 



