60 



ELECTRICITY. 



purpose should be constructed. They 

 should be formed of metallic rods, 

 pointed at the upper extremity, and 

 placed so as to project a few feet above 

 the highest part of the building they are 

 intended to secure ; they should be con- 

 tinued without interruption till they de- 

 scend into the ground, below the foun- 

 dation of the house. Copper is prefer- 

 able to iron as the material for their 

 construction, being less liable to destruc- 

 tion by rust, or by fusion, and possessing 

 also a greater conducting power. The 

 size of the rods should be from half an 

 inch to an inch in diameter, and the 

 point should be gilt, or made of platina, 

 that it may be more effectually preserv- 

 ed from corrosion. An important con- 

 dition in the protecting conductor is, that 

 no interruption should exist in its con- 

 tinuity from top to bottom : and advan- 

 tage will result from connecting together 

 by strips of metal all the leaden water 

 pipes, or other considerable masses of 

 metal in or about the building, so as to 

 form one continuous system of conduc- 

 tors, for carrying the electricity by dif- 

 ferent channels to the ground. The 

 lower end of the conductors should be 

 carried down into the earth till it reaches 

 either water, or at least a moist stratum. 



For the protection of ships, chains 

 made of a series of iron rods linked 

 together, are, by their flexibility, most 

 conveniently adapted. They should ex- 

 tend from the highest point of the mast 

 some way into the sea, and the lower 

 part should be removed to some dis- 

 tance from the side of the ship, by a 

 wooden spar or outrigger. 



The air of close rooms, vitiated by 

 respiration, is found to be negatively 

 electrified. 



CHAPTER XVI. 

 Theoretical Views of the Nature of 



Electricity. 



(222.) THE preceding history of the 

 phenomena relating to electricity, may 

 prepare us for the discussion of some in- 

 teresting inquiries concerning the real 

 nature of this powerful and mysterious 

 agent, and the theory of its operation. 



The first question that presents itself 

 is with respect to its materiality. Besides 

 the well-known mechanical forces which 

 belong to ordinary ponderable matter, 

 the phenomena of nature exhibit to our 

 view another class of powers, the pre- 

 sence of which, although sufficiently 

 characterised by certain "effects, is not 

 attended with any appreciable change in 

 the weight of the bodies with which they 



are connected. To this class belong 

 heat, light, electricity, and magnetism : 

 each of which, respectively, produces 

 certain changes on material bodies, either 

 of a mechanical or chemical nature, 

 which it is natural to regard as the 

 effects of motion communicated by the 

 impulse of material agents, of so subtile 

 and attenuated a kind, as to elude all 

 detection when we apply to them the 

 tests of gravity or inertia. If we admit 

 heat and light to be material, analogy 

 will lead us to ascribe the same charac- 

 ter to electricity and to magnetism, not- 

 withstanding their being imponderable. 



(223.) But the materiality of electri- 

 city has also been maintained on other 

 grounds. The pungent sensation of the 

 electric spark, the smart blow which 

 accompanies the shock, the vivid line of 

 light which marks its course, the varied 

 sounds which attend its passage through 

 the air, and the irresistible fury with 

 which it bursts asunder the densest 

 textures, all seem to denote the mecha- 

 nical effects of sudden and powerful im- 

 pulse ; all seem to imply the rushing of 

 a stream of fluid possessed of momentum 

 adequate to produce these energetic mo- 

 tions. Can we refuse to ascribe the cha- 

 racter of materiality to that which we 

 not only see and hear, but feel also ? 



(224.) This argument has been en- 

 deavoured to be strengthened by a va- 

 riety of experiments, from which the 

 communication of impulse in a particu- 

 lar direction with respect to the species 

 of electricity has been inferred. The 

 stream of air, which proceeds from a 

 pointed conductor when electricity is 

 issuing from it, appears as if the air 

 were carried forward along with the 

 electric fluid. The direction of its mo- 

 tion is still more decidedly indicated by 

 the different luminous appearances which 

 accompany the escape of the fluid from, 

 or its reception by, a pointed conductor. 

 (See/g-. 23.) We have already had oc- 

 casion to notice the manner in which 

 this curious fact appears to support 

 the hypothesis of Franklin, implying the 

 singleness of the electric fluid, ( 98.) 



(225.) The following experiment has 

 also been adduced by Cavallo and by 

 Singer, in support of the same opinion. 

 Place on the table of the universal dis- 

 charger a card bent lengthwise over a 

 round ruler, so as to form a hollow cy- 

 lindrical groove ; or, what is still better, 

 two straight sticks of sealing-wax, laid 

 parallel to each other, so that the junc- 

 tion of their rounded edges may form a 

 groove. In this groove place a pith- ball 



