42 



ELECTRICITY. 



conductor with which he may be in com- 

 munication. 



We have already seen, indeed, ( 78, 

 79, 80,) that it is only a very small part 

 of an electrified body, namely, the mere 

 surface, that is in an active state, either 

 of positive or negative electricity, and 

 that the rest of the substance of the body 

 is in a state of perfect neutrality. 



(150.) It also appears that the unin- 

 terrupted passage of any quantity of 

 electricity through a perfect conductor, 

 such as a rod of metal which is of suffi- 

 cient thickness to convey it, occasions 

 no perceptible alteration in the mecha- 

 nical properties of the conducting body. 



(151.) On the contrary, very consi- 

 derable effects are produced when a 

 powerful charge is sent through a wire, 

 which from the smallness of its size will 

 not admit of the whole quantity to pass 

 with perfect freedom ; or through a sub- 

 stance which, although large, is deficient 

 in conducting power ; or, in other words, 

 which opposes a degree of resistance to 

 the passage of electricity. Thus, an iron 

 conductor will carry off the whole elec- 

 tricity of a thunder-cloud in safety and 

 in silence, while a beam of wood, or a 

 tree, struck by lightning, is shivered into 

 a thousand fragments. 



(152.) When electricity thus changes 

 the physical properties of bodies, its 

 operation may, in general, be referred to 

 that of separating their particles in the 

 line of its course. This separation is 

 effected with more or less violence, ac- 

 cording to the intensityand quantity of the 

 charge, and is frequently attended by the 

 evolution of heat and light. The mecha- 

 nical effects of electricity resemble those 

 which would be produced by a material 

 agent driven with great velocity and force 

 through the substance of the body. Some 

 of these effects, on the other hand, seem 

 to be the consequences of the expansion 

 produced by heat; but many of the 

 changes induced by electricity are of a 

 chemical nature, and such as mechani- 

 cal agencies alone are insufficient to ex- 

 plain. We proceed to describe these 

 several effects more particularly. 



1. Mechanical Effects of Electricity. 



(153.) The cohesion of the particles of 

 solid bodies may be conceived to oppose 

 some resistance to the tendency of elec- 

 tricity to separate these particles from 

 one another ; for we find that fluids are 

 more violently acted upon than solids, by 

 the passage of the electric discharge. If 

 the stem of a capillary tube, such as is 



employed for making thermometers, be 

 filled with mercury, and placed so that 

 the filament of this metal forms part of 

 the circuit ; on the discharge being 

 made, the glass tube will be burst, and 

 its fragments, together with the mer- 

 cury, will be completely dispersed. If a 

 fluid of inferior conducting power, such 

 as water, be contained in a tube of 

 larger diameter than in the preceding 

 experiment, the passage, even of a mo- 

 derate charge, will be sufficient to break 

 the tube, and scatter its contents. Oil, 

 alcohol, and ether, oppose still greater 

 resistance than water to the ^.passage of 

 electricity, and they are expanded and 

 scattered with still greater violence by a 

 discharge being made to pass through 

 them. 



(154.) Beccaria introduced two wires 

 through holes in the opposite sides of a 

 perforated ball of solid glass of two in- 

 ches diameter, the ends of the wires 

 being separated by a drop of water, 

 which occupied the centre of the perfo- 

 foration. On passing a shock through 

 the wires and intervening drop, the ball 

 was shattered with great violence. By 

 a similar arrangement, Mr. Morgan 

 succeeded in breaking green glass bot- 

 tles filled with water, when the distance 

 of the wires between which the explosion 

 passed exceeded two inches. In this 

 way, also, glass tubes, half an inch thick, 

 with a bore of the same diameter, were 

 burst with a very moderate charge, in 

 Mr. Singer's experiments. If a cup-like 

 cavity be turned in a piece of ivory, ca- 

 pable of receiving the half of a" light 

 wooden ball, with a small conical cell at 

 the bottom of the cavity, and two wires 

 be inserted into it through the sides of 

 the ivory ; on putting a drop of water, 

 alcohol, or ether between the wires, and 

 placing the ball over ^them in its cavity, 

 and sending a charge through the drop 

 of fluid, part of it will be suddenly con- 

 verted into vapour, and the ball will be 

 propelled with great violence/ Even a 

 common drinking glass, filled with water, 

 may be broken by the explosive force 

 with which vapour is formed at the 

 point where the electricity passes. Bec- 

 caria constructed a small mortar with a 

 ball, behind which a drop of water was 

 placed, so as to be between the two 

 wires that passed through the sides of 

 the mortar. The charge being sent 

 through the two wires, the drop of water 

 was expanded with such force, as to- 

 drive out the ball with great velocity 

 Mr. Lullin, of Geneva, found that, by 



