If, however, we take hold of the glass stem, and rub the metal rod, we may, perhaps, produce the same property in the metal, but it will spread over the whole, not confining itself to the part rubbed. Thus we perceive that metal is a conductor, while glass is an insulator, or non-conductor, of electricity. [Illustration: Fig. 5.] 81. We would next observe that _this influence is of two kinds_. To prove this, let us perform the following experiment. Let us suspend a small pith ball by a very slender silk thread, as in Fig. 5. Next, let us rub a stick of warm, dry glass with a piece of warm silk, and with this excited stick touch the pith ball. The pith ball, after being touched, will be repelled by the excited glass. Let us next excite, in a similar manner, a stick of dry sealing-wax with a piece of warm, dry flannel, and on approaching this stick to the pith ball it will attract it, although the ball, in its present state, is repelled by the excited glass. Thus a pith ball, touched by excited glass, is repelled by excited glass, but attracted by excited sealing-wax. In like manner, it might be shown that a pith ball, touched by excited sealing-wax, will be afterwards repelled by excited sealing-wax, but attracted by excited glass. Now, what the excited glass did to the pith ball, was to communicate to it part of its own influence, after which the ball was repelled by the glass; or, in other words, _bodies charged with similar electricities repel one another_. Again, since the pith ball, when charged with the electricity from glass, was attracted to the electrified sealing-wax, we conclude that _bodies charged with unlike electricities attract one another_. The electricity from glass is sometimes called _vitreous_, and that from sealing-wax _resinous_, electricity, but more frequently the former is known as _positive_, and the latter as _negative_, electricity--it being understood that these words do not imply the possession of a positive nature by the one influence, or of a negative nature by the other, but are merely terms employed to express the apparent antagonism which exists between the two kinds of electricity. 82. The next point worthy of notice is that _whenever one electricity is produced, just as much is produced of an opposite description_. Thus, in the case of glass excited by silk, we have positive electricity developed upon the glass, while we have also negative electricity developed upon the silk to precisely the same extent. And, again, when sealing-wax is rubbed with flannel, we have negative electricity developed upon the sealing-wax, and just as much positive upon the flannel.