255 



to about 3*5 square feet. The author, although doubting this state- 

 ment in all its generality, still considered an investigation of it, more 

 especially coming from such men as Brooke and Cuthbertson, desi- 

 rable, and as being calculated to throw further light on the pheno- 

 mena of the Leyden jar. 



A few preliminary experiments seemed to accord with Mr. 

 Brooke's view ; the exploding distance of the two jars with a given 

 charge did not appear extremely different. The accumulated elec- 

 tricity spread upon the glass between the strips of metal, and thus 

 enabled the partially coated jar to receive a larger accumulation, upon 

 the principle stated by Cavendish, than was really due to its extent 

 of actual coating. Mr. Brooke, in the then state of practical electri- 

 city, might have been therefore easily led to imagine that a partial 

 coating such as he describes was sufficient. It is, however, shown 

 in this paper that the cases of the two jars are widely different. As 

 the spread of the electricity becomes satisfied, a less charge is re- 

 quired for explosion, and the tension of a given quantity increases. 

 The following are the results of experiments with 100 measures 

 similar to the preceding : 



Full coating. 



Exploding distance *15 Intensity 100 at 1 inch. 



Residual measures 2-45 Therm, electric effect 8. 



Partial coating. 



Mean exploding distance .. *25 Intensity 160. 



Residual measures 4- 97 Therm, electric effect 3'5. 



It is evident the two forms of coating are not equally efficient, the 

 heating effect of discharge not being half as great in the partially 

 coated jar, whilst the residual charge is twice as great. The experiment 

 so far shows the spread of electricity on the uncoated glass to be a 

 source of absorption of charge to a greater or less extent, and goes 

 far to confirm the views of Mr. Cavendish, relative to the spreading 

 of electricity on glass. 



The phenomena of metal-coated jars having been so far examined, 

 a similar course of experiment is followed with jars coated with less 

 perfect conductors, commencing with water coatings. For this pur- 

 pose a jar exposing nearly 5 square feet of coating was prepared with 

 metal coating, and the results of a charge of 100 measures determined 



