4 Professor Hare on the Electric Fluid, 



Were this supposition to avail in the case of an electrome- 

 ter with two leaves, it cannot apply in the case of an instrument 

 lately contrived by me, in which, uninfluenced by the idea 

 that repulsion is the cause of electrometrical indications, I 

 suspend onl}' a single leaf. A brass ball, one-fourth of an 

 inch in diameter, is so situated that it may be made to touch 

 the leaf, or retire from it to the distance of an inch, by means 

 of a screw which supports it. (See Plate I. fig. 1.) This in- 

 strument is evidently more simple, and is far more sensitive, 

 tlian any instrument with two leaves heretofore contrived *. 



It will be admitted, I presume, that the contact between 

 the ball and the leaf must result from attraction, whether the 

 leaf be minus or plus; and that this would not cease to be 

 true, although a second leaf were, as usual, suspended beside 

 the first. 



In a common electrometer, it is usual to have pieces of tin 

 foil pasted on the glass-case opposite the gold leaves. If at- 

 traction be exercised between the leaves and coatings, when 

 moveable, it must also be exercised by the fixed coatings thus 

 pasted on the glass. It is therefore established, that when 

 coatings, whether moveable or fixed, are employed, the diver- 

 gence is not caused by repulsion. It cannot, then, be reason- 

 able to ascribe it to repulsion, though no coatings should be 

 present, as when the leaves are suspended where nothing can 

 attract them unless the surrounding air; especialh' as the air 

 may be shown competent to perform the same office as the 

 coatings, though not so well, on account of its presenting less 

 matter within the same space. The lightness and mobility of 

 the air is no obstacle to this conclusion. When equally acted 

 upon in all directions, as it must be in the case in point, air 

 resists like an arch, or an elastic solid. The electric attraction 

 may have a tendency to condense it about the sphere of ex- 

 citement, but cannot move one portion more than another. 

 This opinion of the agency of the air is supported by the fact, 

 that, in proportion as an exliausted receiver is larger, so will 

 the difficulty of producing a divergency in the electrometrical 

 leaves, situated within it, be increased. It would be difficult 

 to procure a receiver so large, that gold leaves might not be 

 made to diverge electrically in it, when exhausted ; but leaves 

 of light paper, which will easily be made divergent, in pleno or 

 in vacuo, in a small vessel, will cease to be affected by a like 

 influence, if suspended in an exhausted receiver sufficiently 

 large. I am aware that the air prevents the electric fluid from 



* By means of an instrument with a single leaf, since constructed, I am 

 enabled to detect the electricity produced, by one contact between a cop- 

 per and a zinc disk, each six inches in diameter. 



escaping. 



