92 PROCEEDINGS OF THE AMERICAN ACADEMY. 



occasions. My own experience leads me to believe that these tempera- 

 tures were not any of them high enough to effect the expulsion of the 

 included mother liquor. The same objection applies in a smaller de- 

 gree to the work of van Dijk.2 Silver containing included mother 

 liquor does not give up this mother liquor until the temperature has 

 been raised to so high a point that the metal becomes somewhat soft- 

 ened, and then the mother liquor is set free by a series of small explo- 

 sions or decrepitations. The temperature needed is probably over 

 000°, as may be inferred from the statements made in my previous 

 article on this subject. 3 It is probably true that the current density 

 and other conditions at the time of the deposit cause variations in the 

 amount included, but I have never by any process obtained silver which 

 did not include a trace of mother liquor. That the inclusions are not 

 due to extraneous impurities in the silver nitrate, but really occur with 

 the purest salt, is conclusively shown by the recent experiments of 

 Duschak and Hulett.4 Therefore it is clear that the weight of silver 

 dried at 160° does not give the precise weight deposited by the current, 

 although the amount of included mother liquor may be so constant 

 as not to interfere with the use of the weight obtained in this way as 

 a technical measure of current strength. 



Messrs. Smith and Lowry have done good service in emphasizing 

 the importance of using really pure silver nitrate — a precaution not 

 always heeded by physicists. One detail of their argument does not 

 seem to be proved, however. They state that nitric acid causes a de- 

 crease in the amount of the deposit, — a very probable effect, which 

 might have been predicted beforehand ; but this conclusion can hardly 

 be drawn from the results which they give on page 595. When small 

 quantities of nitric acid (corresponding to about. 0.1 to 0.2 of a per 

 cent of the amount of silver nitrate present) were added, the average of 

 their four results showed not a decrease but an increase in the weight 

 of the deposit by 7 parts in 100,000 ; and when as much as 1 per cent 

 of nitric acid is present, the average deficiency was only 4 parts in 

 100,000 as an average of seven experiments showing a rather large 

 probable error. One would therefore be inclined to infer on the basis 

 of their experiments that a small amount of nitric acid has no effect — 

 or at least a much smaller effect than they are inclined to ascribe to it. 

 One finds it difficult to agree with their conclusion on page 596 : 



2 Van Dijk and Kunst, Ann. der Phys., 14, 569 (1904) ; Van Dijk, ibid., 19, 

 249 (1906). 



3 These Proceedings, 37, 435 (1902). 



4 Trans. Am. Electrochem. Soc. (1908). 



