BEVIS' IMPROVEMENTS ON THE LEYDEN JAR. 555 



Meanwhile Dr. Bevis, who had advised Watson to coat 

 the outside of the jar with sheet lead instead of holding it 

 in his hand, again tells him of another and capital improve- 

 ment. Bevis had coated both sides of a thin pane of glass 

 about a foot square with leaf silver, and had found that, 

 after charging the glass in the usual way, a person touch- 

 ing both silver coatings received a shock as strong as from 

 a half-pint vial of water. Watson had hitherto supposed 

 that the strength of the discharge of the jar was due solely 

 to the u great quantity of non-electric (conducting) matter' ' 

 contained in it; but here only about six grains of silver 

 .had been used to cover the glass, so that the quantity was 

 exceedingly small and thus that hypothesis fell. But the 

 Leyden jar, in the shape in which it is still commonly 

 known, resulted. Watson coated a cylindrical jar of thin 

 glass with leaf silver inside and out, and obtained an ex- 

 plosion equal in strength to that of his three lead-covered 

 vials in parallel; and evolved a new theory, which ascribed 

 the effect u not so much to the quantity of non-electrical 

 matter contained in the glass, as to the number of points 

 of non-electrical contact within the glass and the density 

 of the matter constituting those points, provided this matter 

 be in its own nature a ready conductor of electricity." 



The more powerful discharges which still larger jars 

 gave him and the ease with which they traversed non-in- 

 sulated conductors, encouraged Watson to make another 

 attempt to find out the velocity of electricity by bringing 

 both ends of a long circuit wire to a single observer; but, 

 although the circuit measured 12,276 feet in length, he was 

 again obliged to record the fact that the passage of the 

 "commotion" cannot be regarded as other than instan- 

 taneous. 



Watson's account of these latest experiments was pub- 

 lished in book form in the fall of 1748, and the diligent 

 Collinson duly dispatched it to Franklin. The avidity 



