320 Mr. E. H. Hall on the new Action of 



again dried and weighed. In addition to this the solution of 

 silver was filtered and treated with hydrochloric acid. The 

 precipitate was filtered off, and the silver reduced by burning 

 with the filter-paper. The amount of silver on the glass was 

 thus estimated in two ways. According to the weight lost 

 by the plate the amount of silver appeared to be 4*3 mgrs.; 

 while the amount obtained by the chemical process was only 

 about 2*5 mgrs. There are good reasons for thinking the 

 former value too great, and some reasons for thinking the 

 latter too small. Giving the latter double weight in taking 



the mean, we get ~ =3*1 mgrs. for the amount of 



o 



silver in the film. The area covered by this on the glass was 



about 20 square centims. Taking the specific gravity of silver 



to be 10*5, we get for the thickness of the film, 



*= on' 00 fn* ='0000148 centim. 

 20x10*5 



This value is more than 3*6 times as large as that obtained 

 by the resistance method. In order to make perfect accord 

 between the results obtained with the two kinds of silver 

 plates, the thickness would need to be rather more than four 

 times as great as that obtained by the resistance method; but, 

 considering all the difficulties of the case, it seems to me that 

 the large discrepancy still existing is within the limits of ex- 

 perimental error. In presenting the results of all the experi- 

 ments in tabular form further on, I shall give the results 

 obtained with these silver films as calculated on the basis of 

 the larger value (i. e. '0000148 centim.), found for the thick- 

 ness. 



Mention is made above of a fourth plate of silver. This 

 was also of a very thin film; but the silver was fastened to the 

 glass with shellac instead of being deposited from a solution. 

 The silver was in the same state as that of the thickest plate ; 

 and the results of measurements with it accord sufficiently well 

 with those obtained with that plate. As the resistance method 

 was employed in estimating the thickness, it does not seem 

 worth while to publish the results obtained. 



Iron. 



Measurements have been made with three separate plates of 

 iron. The first two plates were made early in the research ; 

 and the quantitative results, like all others obtained at that 

 time, are hardly reliable enough to be worth publishing. 



The dimensions of the third strip were as follows : — 



