ELECTRICITY. 341 



occupy a small space, and, on account of their 

 thinness, hold a very good charge. But when a 

 large battery is required, then these phials cannot 

 be used, for they break very easily ; and for that 

 purpose cylindrical glass jars of about fifteen inches 

 high, and four or five inches in diameter, are the 

 most convenient. 



"When glass plates, or jars, having a sufficiently 

 large opening, are to be coated, the best method is, 

 to coat them with tin-foil on both sides, which may 

 be fixed upon the glass with paste ; but in case the 

 jars have not an aperture large enough to admit 

 the tin-foil, then brass-filings, such as are sold by 

 the pin-makers, may be advantageously used; and 

 they may be stuck on with gum-water. Care 

 must be taken that the coatings do not come very 

 near the mouth of the jar, for that will cause the 

 jar to discharge itself. If the coating is about 

 two inches below the top, it will in general do very 

 well ; but there are some kinds of glass, especially 

 tinged glass, that, when coated and charged, have 

 the property of discharging themselves more easily 

 than others, even when the coating is five or six 

 inches below the edge. There is another sort of 

 glass, like that of which Florence flasks are made, 

 which, on account of some unverified particles in 

 its substance, is not capable of holding the least 

 charge ; on these accounts, therefore, whenever a 

 great number of jars are to be chosen for a large 

 battery, it is advisable to try some of them first, so 

 that their quality and power may be ascertained. 



Plate 20. fig. 1. is a battery composed of twelve 

 jars, coated in the inside and outside with tin-foil, 

 which altogether contain about twelve feet of 

 coated glass. About the middle of each of these 

 jars is a cork that sustains a wire, which at the top 



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