A manual of chemistry : on the basis of Dr. Turner's Elements of chemistry : containing, in a condensed form, all the most important facts and principles of the science designed for a text book in colleges and other seminaries of learning / by John Johnston.

  • Johnston, John, 1806-1879.
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: A manual of chemistry : on the basis of Dr. Turner's Elements of chemistry : containing, in a condensed form, all the most important facts and principles of the science designed for a text book in colleges and other seminaries of learning / by John Johnston. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

    77/488
    positive. A silk thread in a known electric state, thus indicates the kind of electricity possessed by other substances : a conve- nient mode of doing this is to draw a thread of white silk rapidly through a fold of coarse brown paper, previously warmed, by which means its whole length will be rendered positive. 173. When two substances are rubbed together so as to elec- trify one of them, the other, if in a state to retain electricity, will be excited also, one being always negative and the other positive. It is easy to be satisfied of this by very simple ex- periments. Rub a stick of sealing-wax on warm coarse brown paper, and the paper will be found to repel a positively excited thread of silk, while the wax will attract it; if a warm wine- glass be rubbed on the brown paper, the glass will be positive, as shown by its repelling the positive thread, while the same thread will be attracted by the negative paper; friction of seal- ing-wax on a silk riband renders the wax negative and the riband positive, but with glass the riband is negative. If two silk ribands, one white and the other black, be made quite warm, placed in contact, and then drawn quickly through the closed fingers, they will be found on separation to be highly attractive to each other, the white being positive and the black negative. The back of a cat is positive to all substances with which it has been tried, and smooth glass is positive to all ex- cept the back of a cat. Sealing-wax is negative to all the sub- stances just enumerated, but becomes positive by friction with most of the metals. The reader will perceive from these facts that the same substance may acquire both kinds of electricity, becom- ing positive by friction with one body, and negative with another. THEORIES OF ELECTRICITY. 174. The nature of electricity is at present involved in ob- scurity. If really materia], it is so light, subtile, and diffusive, that it has hitherto been found impossible to recognize in it the ordi- nary characteristics of matter ; and, therefore, electric pheno- mena might be referred, not to the agency of a specific substance, but to some property or state of common matter, just as sound is produced by a vibrating medium. But the effects of electricity are so similar to those of a mechanical agent, that its claim to be considered a material substance, has generally been admitted. 175 Dufay's Theory of Two Fluids.—This theory, though usually spoken of as Dufay's, originated partly with him and partly with Symmer. It supposes there are two electric fluids which are equally subtile, elastic, and universally diffused, and each highly repulsive to its own particles, but as highly at- tractive to those of the opposite kind ; these attractive and repulsive forces being exactly equal at the same distances. When a body is in its natural state, that is, when it exhibits no sio-ns of electricity, it is supposed these two fluids exist in it in a state of combination so as to neutralize each other; but