426 



Mr. J. Buchanan. 



[May 27, 



(d). " Negative " solids. Friction tends to increase the specific 

 inductive capacity, 



/. AE=+« .£K. 



"Negative" solids therefore tend to become positively electrified by 

 friction. 



The conclusions under (a), (&), (c), (d) are all found to be verified 

 by experiment. 



Professor Foster has suggested to me that, in connexion with the 

 ideas expressed by equation (8), it is interesting to find the statement 

 by Beccaria* that the cause of the electrical difference set up between 

 two pieces of similar silk ribbon when rubbed together lies in the 

 unequal warming of the opposed surfaces. The oft-quoted experi- 

 ments of Faraday with a feather and piece of canvas fall obviously 

 under the same head. 



I may perhaps be allowed to cite in addition some very old experi- 

 ments with glass made by Bergman. f On rubbing two similar strips 

 of glass together, the portion of the surface of either strip which 

 received the greatest amount of friction per unit area became positive. 

 This agrees perfectly with what may be deduced from (8). For the 



two strips, -^ r = — a was the same ; hence by (8) — 



If SK^ > &K 2 we get AE X positive, as Bergman found. 



Experiments on the Electrification of Steam by Friction. 



The experimental results given below are far from complete ; but 

 I venture to publish them as affording in some measure an experi- 

 mental verification of the ideas put forward in Part I. 



The method of experimenting and the arrangement of apparatus 

 employed were essentially the same as were used by Faraday in his 

 classical experiments on this same subject. J 



It is needless for me to say how very much I am indebted to 

 Faraday's observations during the whole course of this experimental 

 enquiry. Like all that the great experimenter undertook, the record 

 of his observations in the " Researches " is a treasure-house for later 

 experimenters to draw supplies from. 



To generate steam, a small vertical copper boiler was used, which 

 was heated by gas led to the burner by india-rubber tubing. By 

 placing the boiler on small blocks of shellac the insulation was found 

 at all times to be excellent. The weather was very favourable. 



* Riess, " Reibungs-Electricitat," § 914. 

 f Ibid., § 913, fig. 175. 



X " Experimental Researches," § 2075, et seq. 



