ELECTRICITY. 



821 



thread, proves that the air, as well as silk, is a non- 

 conductor ; from which circumstance bodies sur- 

 rounded by it, except on one side, and this side being 

 in contact with a non-conductor, are said to be 

 insulated. If this condition be not observed, that is, 

 if a body be in contact with conducting substances 

 which communicate with the earth, its electricity 

 will escape through them to the earth, which may 

 be regarded as the great reservoir, both for the 

 absorption and supply of this fluid. The insulating 

 power of the atmosphere depends upon its density 

 and its dryness. In proportion as the air is rarefied 

 by the removal of the superincumbent pressure, its 

 power of confining electricity diminishes, till, at 

 length, when the rarefaction is very great, it oppose 

 scarcely any resistance to the passage of electricity. 

 The presence of moisture in the air also diminishes 

 its insulating power. Water is a good conductor of 

 electricity ; accordingly, any portion of it suspended 

 in the air tends to carry off electricity from bodies 

 charged with it, and which are immersed in such an 

 atmosphere. Moisture also easily attaches itself to 

 glass and other electrics, depriving them of the 

 power of insulation. Hence we discover the reason 

 why experiments which succeed in a clear, dry day, 

 will often fail in damp weather ; and the utility of 

 drying all the instruments employed in electrical 

 experiments, in order to exclude, as much as possible, 

 the interference arising fromthe presence of condensed 

 moisture. The conducting powers of most bodies 

 are influenced by changes of temperature, and also 

 of form. Thus water, in its liquid state, is a good 

 conductor ; but when in the state of ice, at a temper- 

 ature of 13 Fahr., it is a non-conductor r and capable 

 of being excited by friction like any other electric. 

 Reducing substances to powder has an effect upon 

 their powers of conducting electricity. Snow con- 

 ducts less readily than ice at the same temperature ; 

 but glass, as well as sulphur, on the contrary, acquire 

 some conducting power by being pulverized. Vege- 

 table and animal substances lose their conducting 

 powers when made thoroughly dry. No substance 

 with which we are acquainted can be said to be 

 wholly impervious to electricity : nor, on the other 

 hand, is there any body which opposes no resistance 

 to the transmission of electricity. The following 

 table presents a view of the principal classes of 

 bodies, arranged in a series, beginning with those 

 possessed of the greatest conducting power, and 

 terminating with those that have the least. The 

 order in which they possess the power of insulating, 

 is, of course, the reverse of this : 



Animal fluids 



Water. 



Snow. 



Living vegetables. 



Living animals. 



Smoke. 



Steam. 



Rarefied air. 



Earths and stones in their 

 natural state. 



Pulverized glass. 



Flowers of sulphur. 



White sugar. 



Dry parchment. 



Cotton. 



Feather*. 



Hair, especially that of a 



living cat. 

 Silk. 



Transparent gems. 

 Diamond. 

 Glass. 

 Fat 

 Wax. 

 Sulphur. 

 Resins. 

 Amber. 

 Gum lac. 



The perfect, or least oxid- 

 able metals. 



The more oxidable metals. 



Charcoal prepared from the 

 harder woods, and recent- 

 ly ignited. 



Plumbago. 



The concentrated mineral 

 acids. 



Dilute acids. 



Solutions of metallic salts. 



Metallic ores. 



Dry metallic oxides. 



Oils. 



Vegetable ashes. 



Animal ashes. 



Ice below 13" Fahr. 



Phosphorus. 



Lime. 



Dry chalk. 



Caouchouc. 



Camphor. 



Silicious and argillaceous 



stones, ia proportion to 



their hardness. 

 Porcelain. 

 Baked wood. 

 Dry atmospheric air, and 



other gase*. 



Although the exact point in the above scale, 

 which forms the separation between conducting and 

 insulating bodies, cannot be precisely marked, yet 

 we have indicated it by a division. The laws which 

 regulate the gradual dissipation of electricity from 

 imperfectly insulated bodies, have been carefully 

 investigated by M. Coulomb. The causes which 

 operate in these circumstances, are, 1. the imperfec- 

 tion of the insulating property in the solids by which 

 they are supported ; 2. the contact of successive 

 portions of air, every particle of which carries off a 

 certain quantity of electricity ; 3. the deposition of 

 moisture upon the surface of the insulating bodies, 

 which establishes communications between their 

 opposite ends, and may be considered as virtually 

 increasing their conducting power. Still another 

 circumstance, which materially affects the dissipation 

 of electricity, is the shape of the body in which it is 

 accumulated. The form most favourable for its 

 retention is that of a sphere ; next, a cylinder termi- 

 nated at both extremities by a hemisphere. On the 

 other hand, electricity escapes most readily from 

 bodies of a pointed figure, especially if the point 

 projects to a distance from the surface. In such 

 bodies, it is scarcely possible to retain any accumula- 

 tion of the electric fluid ; whereas, pointed bodies 

 receive electricity more readily than those of any 

 other form. Electric excitation in different bodies 

 exhibits different phenomena. We have seen that 

 light substances xcited by glass repel one another, 

 and are likewise repelled by the excited glass. The 

 same thing also happens with respect to bodies 

 which have received their electricity from excited 

 sulphur, or sealing wax. But on examining the 

 action of any of the bodies of the former class upon 

 any of those belonging to the latter, we find that, 

 instead of repelling, they attract each other; and 

 what is still more remarkable, the instant these 

 bodies come in contact, provided they have both 

 been electrified in an equal degree, they cease at 

 once to exhibit any signs of electrical excitement ; 

 the electricity in the one appearing to neutralize that 

 in the other. Thus we seem to have evidence of two 

 kinds of electricity ; and as these were first noticed, 

 the one in glass and the other in resinous bodies, they 

 were named vitreous and resinous electricity. Their 

 mode of action on matter has been expressed by the 

 following general law, viz. : Bodies charged with 

 either species of electricity, repel bodies charged with 

 the same species, but attract bodies charged with the 

 other species ; and at equal distances, the attractive 

 power in the one case is exactly equal to the repulsive 

 power in the other. Accordingly, we learn the kind 

 of electricity with which a given body is charged, by 

 approaching it to an insulated pith ball, which has 

 previously been touched either with excited glass, or 

 with excited sealing wax. It is known, moreover, 

 that, when two electrics are rubbed against one 

 another, the one acquires, always, one kind of 

 electricity, the other the opposite ; and both are pro- 

 duced in equal degrees. Thus, when glass is rubbed 

 by silk or flannel, just as much resinous electricity is 

 produced in the silk or flannel, as there is vitreous elec- 

 tricity produced in the glass ; and, consequently, as 

 they are endowed with opposite electricities, there 

 should be an attraction existing between the excited 

 surfaces of the bodies rubbed. This fact is easily 

 proved by the simple and familiar experiment of the 

 ibbons. If a white and a black ribbon, of two or 

 three feet long, and perfectly dry, be applied to each 

 other by their smooth surfaces, and are then drawn 

 repeatedly between the finger and thumb, so as to rub 

 against each other, they will be found to adhere to- ^ 

 Aether, and, if pulled asunder at one end, will rush 

 gether with great quickness ; while united, they 



