ELECTRICITY. 



433 



Pfcriptive its conducting power, and became an electric. Above 

 Electricity. ti,j s temperature, the ice begins to conduct. Tliis fa- 

 V "~Y~"^ cultv increases with the temperature, and becomes very 

 great near the boiling point. M. Achard employed, 

 in his experiments, ice perfectly transparent, and free 

 from air-bubbles. To obtain this, he poured distilled 

 water into a vessel, and placed it, in frosty weather, 

 on the window of a room tolerably warm, when com- 

 pared with the external air. Hence the water froze on 

 one side when it was fluid on the other ; and the air, 

 as it was extricated, passed into the fluid part, and thus 

 left the ice perfectly transparent, and free from air- 

 bubbles. He then formed the ice into a spheroid, and, 

 by rubbing it when in motion about its axis, he was 

 able to produce electricity in great quantities. The fol- 

 lowing Table contains a list of bodies that are non- 

 conductors, arranged in the order of the resistance 

 which they oppose to the passage of the electric fluid. 



bleof 

 i -con- 

 tors. 



Li,-./ o/' Xun-conduclori. 



Shell lac. 



Amber. 



Resins. 



Sulphur. 



Wax. 



Jet. 



Vitrifications of all kinds. 



Talc. 



Diamond. 



Transparent gems. 



Raw silk. 



Bleached silk. 



Dyed silk. 



WooL 



I lair. 



Feathers. 



Dry paper. 



Parchment. 



leather. 



Air. 



All dry gasw. 



Baked wood. 



Dry vegetable bodie. 



Porcelain. 



Dry marble. 



Some siliceous stones. 



Some argillaceous stones. 



Camphor. 



Caoutchouc. 



I.ycopodium. 



Native carbonate of barytes. 



Dry chalk. 



Lime. 



Phosphorus. 



Ice at 13= of Fahrenheit. 



Many transparent crystals when perfectly dry. 



Ashes of animal bodies. 



Ashes of vegetable bodies. 



Oils, the heaviest l>eing the best. 



Dry metallic oxides. 



arks on The remarkable property possessed by shell lac of 

 able, transmitting the electric fluid with more difficulty than 

 any other body, was discovered by M. Coulomb, and 

 hag already been noticed in our History of Electricity. 



When any of the substances in the preceding table 

 receive the least accession of moisture, their non-con- 



VOL. VIII. PART II. 



ducting power is diminished in proportion to the quan- Descriptive' 

 tity of moisture which they have absorbed. Electricity. 



The slightest examination of the table is sufficient *"" 

 to convince us, that there is no relation between any 

 of the chemical and physical properties of the bodies 

 which it contains, and their power of resisting the 

 passage of the electric fluid. The early electricians 

 imagined that metals and waters were the great con- 

 ducting principles ; but this property is obviously pos- 

 sessed by bodies which, so far as we know, contain 

 neither of these ingredients. 



In our History of Electricity, p. 416, we have already Distance 

 given some details respecting the length to which elec- ^""ff'i 

 tricity has been conducted along metallic wires, and the J^L *,"' 

 rapidity of its motion. These details are sufficient to been con . 

 shew, that no measureable interval of time elapses du- ducted, 

 ring the passage of the electric fluid along a space of 

 four miles. 



Mr Singers, however, has expressed a doubt respect- 

 ing the accuracy of these experiments. " They were 

 made," says he, " at a very early period, and have not, 

 I believe, been repeated since the improved state of the 

 science has afforded the means of effecting such experi- 

 ments with precision. Metals, although the most per- 

 fect conductors we have, oppose some resistance to the 

 motion of electricity, and a charge will even prefer a 

 short passage through air to a circuit of 20 or 30 feet 

 through thin wire. It is therefore rather uncertain that 

 the charge of a small phial has ever passed through an 

 interval of four miles." Elements of Electricity, p. 142. 

 note. 



The subject of electric atmospheres belongs properly Electric at- 

 to the present Section. The term atmosphere is here mosphem. 

 employed to denote the space round any electrical body 

 through which its electrical influence extends. Mr Can- 

 ton was the first electrician who examined this curious 

 subject, and he found that all bodies immerged in the 

 electric atmosphere of a positively electrified body, were 

 negatively electrified; while those which were placed in 

 the atmosphere of a negatively electrified body acquired 

 positive electricity. This may be proved by the very 

 simple experiment of holding a pair of pith balls at 

 some distance from an excited glass tube, and they will 

 be found to diverge with negative electricity. In like 

 manner, they will diverge with positive electricity when 

 placed within the sphere of action of a stick of excited 

 sealing-wax negatively electrified. 



SECT. IV. On Electrical Attraction and Repulsion. 



THE attraction exhibited by electrified bodies was Q D e lectri- 

 one of the first phenomena that was observed by philoso- C al attrac- 

 phers ; and we have already had occasion to notice an tion and re- 

 example of repulsion, in the divergency of two pith Fusion. 

 balls. These simple phenomena may be perceived with 

 sufficient distinctness, by employing merely a tube of 

 glass excited by a piece of silk ; but, in order to exhibit 

 the more interesting phenomena, of the approach and 

 recession of light substances to an electrified body, we 

 must be able to produce a higher degree of electricity 

 than can be effected by a simple tube. 



In order to obtain this, a cylinder of glass is mounted Electrical 

 in a frame, so that it can be turned rapidly round its machine, 

 axis by means of a winch or handle. On one side of 

 it is placed a small cushion covered with silk, against 

 which the glass cylinder is rubbed during its rotatory 

 motion ; and on the other side a brass or metal tube, 

 resting upon a stand of glass, for the purpose of collect- 

 3 I 



