Chap. II Electric Substances; With Some of the Phenomena Attending Their Excitation
Those substances by which electrical phenomena may be produced, form the subject which next demands our attention; but these are so numerous that it would be vain to attempt to specify them all. Perhaps it may be doubted, whether every material substance, with the exception only of metals, water, and charcoal, may not be considered as an electric.
Some however exhibit particular phenomena more obviously than others; and hence a number of catalogues have been formed, for shewing the effects which arise when electrics are excited with different rubbers. The specification which we esteem the most complete, was formed by the ingenious Mr. Cavallo, and we shall give it in his own words.
“In the following table (says he) may be seen what electricity will be excited in different bodies, when rubbed with different substances. Smooth glass, for instance, will be found by this table to acquire a positive electricity, when rubbed with any substance hitherto tried, except the back of a cat: (by which I mean the skin of a cat while on the animal alive:) rough glass, (viz. glass, the polish of which has been destroyed by emery or otherwise) will be found to acquire the positive electricity, when rubbed with dry oiled silk, sulphur &c. and the negative when rubbed with woolen cloth, the hand &c. and so of the rest.”
| Electrics. | Qualities. | Rubbers. |
|---|---|---|
| “The back of a cat | Positive | Every substance with which it has hitherto been tried. |
| Smooth Glass | Positive | Every substance hitherto tried, except the back of a cat. |
| Rough Glass | Positive | Dry oiled silk, sulphur, metals. |
| Negative | Woolen cloth, quills, wood, paper, sealing wax, white wax, the human hand. | |
| Tourmaline | Positive | Amber, or air blown upon it. |
| Negative | Diamond and the human hand. | |
| White silk | Positive | Black silk, metals and black cloth. |
| Negative | Paper, hand, hare’s & weasel’s skin. | |
| Black silk | Positive | Sealing wax. |
| Negative | Hare’s, weasel’s and ferret’s skin, load-stone, brass, iron, silver, hand. | |
| Weasel’s skin | Positive | Metals, silk, load-stone, leather, hand, paper, baked wood. |
| Negative | Other fine furs. | |
| Sealing wax | Positive | Metals. |
| Negative | Hare’s, ferret’s and weasel’s skin, hand, leather, woolen cloth, paper. | |
| Baked wood | Positive | Silk. |
| Negative | Flannel.” |
From the above table it appears, that the powers of electric substances vary prodigiously from one another; and that, according to the different rubbers made use of, we may sometimes produce one phenomenon and sometimes another. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit; which may be done in the following manner.
First. Those which exhibit a strong and permanent attractive and repulsive power, of which the most remarkable is silk.
Second. Those which exhibit the electric light, attraction, repulsion, and all the other phenomena of electricity in a very vigorous, though not durable manner; of these glass is eminently preferable to all others.
Third. Those which exhibit electric appearances for any length of time, and which communicate to conducting bodies, the greatest electric power.—Of these, the substances called negative electrics, such as sealing-wax, resinous substances, and resinous compounds, are the best.
Fourth. Those which readily exhibit electrical phenomena by heating and cooling.—Of these, the tourmaline9 is the principal.
The best method of disturbing the electric fluid, that is of making it pass from one body to another, is friction. This may be done either by rubbing one electric with another, or with a conductor; but the electricity is generally stronger in the latter case. Other methods for causing electrics to shew electric appearances, are, melting, or pouring a melted electric on another substance, heating and cooling, evaporating or effervescing.