ELECTRICITY. 



57 



The Indian Ink should be of the best 

 quality, care should be taken that it be 

 uniformly good throughout the stick. 



An inferior kind, hard, grilty,and posses- 

 sed of little colouring power, is sometimes 

 imposed upon the purchaser, by the de- 

 ceptive practice of attaching a little bit of 

 good ink at each end of the stick. A good 

 mode of examination, is to break the stick 

 in two; this defeats the above trick, and 

 exposes the fracture; that of good ink is 

 -brilliantj^angular and clean; the bad, dull 

 and earthy. 



De 



ELECTRICITY. 



Extract from a treatise upon Electricity by J. A 

 Luc, Esqr., F. R. S Phil. Journ. No.ll26. 



1. The electric' Jluid\ resides on all 

 terestrial bodies, every particle air includ- 

 ed ; being retained upom them by a 

 mutual attraction which however, differs 

 in degree, according to the bodies ; some 

 attract the electric fluid when it comes in 

 contact with them ; but then it adheres 

 strongly to the parts which receive it, 

 moving very slowly along the surface of 

 these bodies, which are therefore non- 

 eonducters: others receive it at more or 

 less distance, and it is propagated more or 

 less rapidly along their surfaces. Glass 

 though absolutely impenetrable to the 

 electric matter permits the electric fluid 

 (i. e. the electric matter united with vec- 

 tor) to move with a sensible progress along 

 its surface. 



2. Friction excited between two bodies 

 has no other effect than that of disturbing 

 the natural equilibrium of the electric 



\. fluid which tends always to produce 



\\ among all bodies, according to its actual 

 (but local in a certain extent) quantity on 

 them, and in the ambient air. If both 



. the bodies which Gyi^rcx^c frict ioji on each 

 other are good conducters, this distur- 

 bance, the equilibrium being constantly 

 restored, is not perceived ; but if one has 

 more disposition than the other to attract 

 the electric fluid thus agitated, with the 

 faculty of transmitting it to its remote 

 parts, when they are separated, either 

 suddenly, or in general_before the equili- 

 brium of the fluid can be restored between 

 them — one is ionnd positive as having ac- 

 quired a proportional quantity of electric 

 fluid greater than the ambient air, and 



■. the other negative, as having lost that 



quantity: both being supposed to have pre- 

 viously possessed the same electric state 

 as the a?nbie?it a\v, 



3. The general effect, therefore, oi fric- 

 tion between two bodies is, that one never 

 becomes positive without the other being 

 made negative (or vice-versa.) 



This evident proof, that all electric 

 phenomena whicli we are hitherto able to 

 produce at will, namely, by friction., pro- 

 ceed from the disturbance of the equili- 

 brium of only one fluid, will be afford- 

 ed by the experiments which I shall here 

 relate with another remark. The surest 

 mode of following the course of the elec- 

 tric fluid, when in motion is by means of 

 electroscopes ; and that made of gold leaf 

 being judged the most proper for this pur- 

 pose ; but as this would be torn in pieces 

 by the smallest machine in common use, 

 it became necessary for this ingenious 

 philosopher, to invent an apparatus for 

 himself. 



The following brief description answers 

 to the result of his ingenuity. The ap- 

 paratus consists of a wooden base, sup- 

 porting two upright wooden pillars fitted 

 at their upper extremities to receive dif- 

 ferent spindles, which are separately in- 

 serted, and each turned by a winch handle. 

 The pritne conductor is composed of a 

 brass tube, stopped at each end, support- 

 ed by ^ glass pillar, covered with insulat- 

 ing varnish and fixed in the base. 



One end of the conductor communicates 

 with the spindles by means of a very thin 

 slip of whale-bone covered with gilt paper, 

 fixed to a piece of brass that screws into 

 the conductor, so as to admit of being ad- 

 justed to the proper bearing on the spindle :- 

 'this adjustment must be acquired by ex- 

 periment. The other extremity commu- 

 nicates with a gold leaf electroscope. 



The rubbers are of difierent kinds, but 

 all fixed to the apparatus in the same man- 

 ner: they principally consist of a very 

 elastic hrass lamina, and only differ in that 

 end which presses on the spindle. The 

 different kinds of rubbers must press 

 ao-ainst the spindles with different degrees 

 of force, and always yield readily to the 

 inequalities on the surfaces of the spind- 

 les. 



For these purposes, the rubbers are fix- 

 ed horizontally into a proper pillar, and 

 part of the pressure occasioned by the ac- 



