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
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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.)

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    184. Contact.—Another reputed source of electricity is contact of dif ferent substances, especially of metals; a source originally suggested by Volta, who founded on it a theory of galvanism. Numerous tacts have been adduced by later experimenters in proof of the position that mere con- tact is sufficient to produce electric excitement. Thus, when zinc is brought in contact with copper or silver, and again separated by means of an insu- lating handle, the zinc is found positive, and the copper or silver, if it has been insulated, negative. If iron, or manganese, or even plumbago, be sub stituted for the zinc plate, the result will be the same. Other metals may also be substituted for these with the same effect, though in many cases the excitement is exceedingly weak. 185. When oxalic, succinic, benzoic, or boracic acid, either in powder or in crystals, is touched with a plate of copper insulated with a glass handle, the copper becomes positive, and the acids negative. So also phosphoric acid, when applied to copper, becomes negative, and the metal positive. When metallic plates are made to touch dry lime, strontia, baryta, magne- sia, or soda, these alkaline bodies become positive and the metal negative; and when sulphur is applied to polished lead or other metal, it becomes positive, the metal taking the opposite state. These facts rest on too firm a foundation to be shaken; but it is believed that the real cause of the phenomena has been mistaken ; and the opinion is daily gaining ground that mere contact is incapable of causing electrical excitation. The effects which have been ascribed by some to this cause are referred by others to chemical action or to friction. The copper and zinc plates, it is found, show no signs of electricity when brought into contact in an atmosphere of nitrogen or hydrogen ; and the excitation that is produced in the open air must therefore depend upon the slight oxydation that is then constantly going on even in the driest atmosphere. This opinion is farther confirmed by the fact that the electrical effects are much increased by introducing acid fumes into the atmosphere, or using other means to increase the chemical action. In other cases the electrical effects observed may be produced by the friction of the metals or other substances used. 186. Change of Form.—The changes of form caused in a sub- stance by variation of temperature, such as liquefaction and solidification, the formation and condensation of vapor, consti- tute another reputed source of electricity. On liquefying sul- phur in a glass vessel, and removing the cake after cooling, the sulphur is found to be negative and the glass positive; and on pouring water into a hot iron vessel or on a hot coal com- municating with a delicate electrometer, the rapid evaporation of the water is attended with decisive indications of electrical excitement. To processes of this nature, continually taking place in the atmosphere, the electricity of the clouds is gene- rally ascribed. This opinion, however, has been questioned, and it has been satisfactorily shown that in most of the experiments adduced in its favor, chemical actions ensue at the same time, and that much of the effect is due to such changes. If, for example, eva- poration be accompanied by chemical decomposition, as when saline solutions are evaporated, the water being separated from