ELECTRICITY. 



ductors. Conductors, on the other 

 hand, are non-electrics. The two qua- 

 lities of a capability of excitation, and a 

 power of conducting electricity, appear 

 to be incompatible with each other 

 for the one is always found to diminish 

 in proportion as the other increases. 

 The permanence of electricity in metallic 

 bodies, which are suspended in the 

 air by silk threads, shows that the air 

 as well as the silk is a non-conductor. 

 Bodies which are in this way surrounded 

 on all sides by non-conductors are said 

 to be insulated. When this condition 

 is not observed, that is, when the body 

 is in contact with conducting bodies 

 which communicate with the earth, its 

 electricity will escape by the channel 

 which is thus opened for it, and will be 

 lost by diffusion in the mass of the 

 earth, which is formed of conducting 

 materials, and which may be regarded as 

 the great reservoir both for the absorp- 

 tion and supply of electricity. Hence 

 we see why it is not possible to accu- 

 mulate electricity in a conducting body 

 while it is held in the hand, and why 

 electrics alone are capable of permanent 

 excitation. 



(21.) The insulating power of atmos- 

 pheric air depends principally upon 

 two circumstances, its density and its 

 diyness. Air with the density which it 

 has under the ordinary pressure of the 

 atmosphere, if perfectly dry, is a remark- 

 ably good insulator, even although it 

 be rapidly renewed on the surface of 

 the electrified body. This is shown by 

 an experiment of Franklin's, in which 

 he whirled an electrified ball round his 

 head, by means of a silk line, with great 

 rapidity, so as to make it perform many 

 hundred revolutions, without being 

 able to perceive that it had thereby lost 

 any sensible portion of its electricity. 

 Neither an increase, nor a diminution 

 of temperature, appears to lessen its in- 

 sulating power. But in proportion as 

 the air is rarefied by the removal of the 

 superincumbent pressure, its power of 

 confining electricity diminishes, till, at 

 last, when the rarefaction is very great, 

 it opposes scarcely any resistance to the 

 passage even of very feeble electricity ; 

 and it may be then classed among con- 

 ductors. 'This is the case with the im- 

 perfect vacuum produced by the air- 

 pump, from which it is almost impos- 

 sible to exclude minute quantities of 

 air. Even in the space left, in the upper 

 part of the tube of a barometer by the 

 descent of the mercury, or the Torri- 



cellian vacuum, as it is called, there is 

 in general present a minute portion of 

 air, as well as of mercurial vapour, 

 which are sufficient to conduct electri- 

 city. Small globules of air usually 

 adhere to the mercury, and to the sides 

 of the tube ; and these, upon the re- 

 moval of pressure, expand into an 

 atmosphere. It has been asserted by 

 Mr. Morgan that if great care be taken 

 to remove every source of error, by the 

 employment of very pure mercury, and 

 by boiling it for a long time in the tube, 

 a perfect vacuum may be obtained, 

 which does not conduct electricity. His 

 experiment, however, was made long 

 ago, and requires careful repetition, 

 before the result can be confided in. 



(22.) The circumstance which chiefly 

 determines the conducting power of 

 common air is that of its containing a 

 greater or less quantity of moisture. 

 Water is a very good conductor of elec- 

 tricity ; and that portion which is sus- 

 pended in the air tends powerfully to 

 carry off electricity from the bodies 

 which are charged with it, and which 

 are surrounded by air. Moisture also 

 easily attaches itself to glass and other 

 electrics, and deprives them of the 

 power of insulation. Hence, the same 

 experiments which succeed in a clear 

 dry day, will often fail when tried in 

 damp weather : and hence we see the 

 utility of previously drying every part 

 of the apparatus, in order to exclude as 

 much as possible the interference of 

 moisture. 



(23.) The conducting powers of most 

 bodies are influenced by changes of tem- 

 perature, and also of form. Thus, al- 

 though water in its liquid state is a good 

 conductor, yet, when congealed, in the 

 form of ice, its conducting powers are 

 much impaired ; and at a very low tem- 

 perature, namely, at 13of Fahrenheit's 

 scale, ceases altogether. Mr. Achard, 

 who observed this fact, formed ice of 

 this temperature into a spheroid, and 

 mounting it upon an axis, was able to 

 excite it by friction as any other electric. 

 On the other hand, by raising the tem- 

 perature of water, its conducting powers 

 are increased. Charcoal is also found 

 to transmit electricity with more facility 

 when hot, than when cold. Glass, which 

 is a non-conductor when cold, becomes 

 a tolerably good conductor when heated 

 to redness ; and a similar change takes 

 place in sulphur and in resinous bodies 

 when melted ; and also in baked wood 

 when heated. Reducing substances to 



