The cyclopaedia of practical medicine : comprising treatises on the nature and treatment of diseases, materia medica and therapeutics, medical jurisprudence, etc., etc. / Edited by John Forbes, Alexander Tweedie, John Conolly.
- Date:
- 1848
Licence: Public Domain Mark
Credit: The cyclopaedia of practical medicine : comprising treatises on the nature and treatment of diseases, materia medica and therapeutics, medical jurisprudence, etc., etc. / Edited by John Forbes, Alexander Tweedie, John Conolly. Source: Wellcome Collection.
Provider: This material has been provided by the University of Massachusetts Medical School, Lamar Soutter Library, through the Medical Heritage Library. The original may be consulted at the Lamar Soutter Library at the University of Massachusetts Medical School.
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![•what we shall have to say respecting the thera- peutic applications of the electric fluid. When a glass rod is rubbed with silk, and then brought close to a little sphere of elder pith sus- pended bj- a silk thread, a combination commonly called an electric pendulum, the sphere is first at- tracted by the glass, and, upon contact, immedi- ately repelled. But upon substituting for the glass rod one of metal, no such phenomena will be ob- served. Such experiments have suggested a divi- sion of bodies into two classes, the electrics and the non-electrics, or those which admit and those which do not admit of being rendered electric by friction. Glass, sulphur, wool, silk, and all resi- nous substances are electrics. Carbon, the several metals, water, and every variety of organized sub- stance, in consequence of the water which all such include, acid and saline solutions, &c. are non-electrics. This division, however, we shall presently find to be based upon an erroneous sup- position. All bodies, in fact, are electrics ; and if there are some which, under ordinary circum- stances, do not appear to become excited by fric- tion, it is because the electricity is permitted to flow from them into the earth as fast as it is gene- rated. This brings us to another division of bodies, naving reference to their electrical relations, found- ed on a different principle, namely, that which re- solves them into conductors and non-conductors, or rather into those which arc with facility and those which are with difficulty traversed by the electric fluid ; for, strictly speaking, there is no absolute non-conductor. If a glass rod be apjilicd to an excited body, it will become electrified merely at the point of contact. But if a metallic rod, suspended upon silk, be applied in a similar man- ner, it will at the same instant exhibit symptoms of excitement at every point. This simple expe- riment illustrates sufficiently the difference of the conducting powers of the two rods. When a body is surrounded on every side by non-conductors, it is said to be insulated. Thus a ball of metal resting upon a cake of resin, or a stool with glass legs, or suspended by a silk thread, is insulated, for it is in connection with the earth only through the medium of non-conductors. If the insulation were perfect, the ball once electri- fied should continue so for an indefinite time. In practice, however, this never occurs, for the reason already assigned, namely, that there are no abso- lute non-conductors. Dry air is probably the best insulator with which we are acquainted ; but, by impregnation with aqueous vapours, it acquires conducting jwwers. The two divisions just noticed admit of being reduced into one, the electrics being non-conduc- tors, and the non-electrics conductors. It is, in fact, owing to this latter coincidence that some . bodies appear incapable of being excited. Thus, if a disc of copper, held in the hand, be rubbed with a silk handkerchief, it docs not become ex- cited ; but if, before the afiplieation of the friction. It be insulated, the usual phenomena manifested Dy electrics will present themselves. In every case of electrical excitement two dis- tinct powers are cotemporaneously developed. Thus, when glass is rubbed by silk, both become electrified, as may be proven by approaching them in succession to an electric pendulum. The elec- tricity of the glass, however, is diflerent from that of the silk, for when either is communicated to two contiguous pendulums, these repel each other. But when the electricity of the glass is communi- cated to one pendulum, and that of the silk to the other, attraction ensues. These diHorcnt powers require to be designated by dilTerent names, and hence the terms vitreous and resiuuus, which, whatever be the substances rubbed, are applied to the distinct energies developed bj- the process of friction. Such was the language of Dufay. But, in more modern times, these designations have been by some replaced, particularly in Great Bri- tain, by the terms positive and negative, intro- duced by Franklin, the former corresponding to the vitreous, the latter to the resinous electricity of Dufa}'. This difference of nomenclature de- pends upon the dilferent opinions entertained by these ])hilosophers respecting the nature of the electric influence. Dufay viewed it as a com- pound, Franklin as a simple fluid. The former referred all electrical phenomena to the resolution of the compound into its vitreous and resinous elements. The latter explained them by supposing that all bodies, in their natural or neutral state, contain a certain quantity of the electric fluid, and that the processes, which produce electrical ex- citement, operate merely by disturbing this pro- portion, and thus causing an accumulation in some and a deficiency in others. We shall not enter here into any criticism upon the respective merits of these rival hypotheses. Suffice it to say, that the theory of Dufay is that adopted by the highest authorities in science, principally, as it would seem, because of its analogy to that which is alone applicable to the kindred subject of mag- netism. In order to a consistent explanation of all known electrical facts, it is only necessary to sup- pose, with Dufay, that electricities of the same name repel, and of contrary names attract each other, and with a force varying in the inverse ratio of the square of the distance. This principle of the action of the elementary fluids at a distance is applicable to a multitude of phenomena, amongst others, for example, to the charging of the Ley- den phial. By electricians it is known under the name of induction or injluence. It may be illus- trated by the attraction already noticed as occur-* ring when we approach an excited glass tube to an electric pendulum. The vitreous electricity of the tube decomposes the natural fluid of the ball of the pendulum, attracting its resinous and re- pelling its vitreous element; the attraction, liow- ever, being more energetic than the repulsion, in- asmuch as it is exerted at a shorter distance. But, as the ball is insulated, its constituent electricities are merely separated from each other, but cannot leave it; hence it will itself be carried towards the tube, and with a momentum corresponding to the difference of the opposed forces. There is but one point more connected with the theory of electricity to which it is necessary to advert here. When a body is electrically charged, the free electric fluid is confined solely to its sur- face, and does not, as experiment proves, dip to any appreciable depth into its substance. Its dis- tribution, however, on the surface is never equable,](https://iiif.wellcomecollection.org/image/b21197040_0768.jp2/full/800%2C/0/default.jpg)
