If the experiment is repeated with a suspended stick of sealing-wax rubbed with flannel, the glass and the sealing-wax attract each other, but the two sticks of wax repel one another. Both glass and sealing-wax are electrified, as may be seen by bringing them near the pith ball, but there must be some difference between them as we get attraction in one case and repulsion in the other. The explanation is that the electric charges on the silk-rubbed glass and on the flannel-rubbed sealing-wax are of different kinds, the former being called positive, and the latter negative. Bodies with similar charges, such as the two glass rods, repel one another; while bodies with unlike charges, such as the glass and the sealing-wax, attract each other. We can now see why the pith ball was first attracted and then repelled. To start with, the ball was not electrified, and was attracted when the rubbed glass or sealing-wax was brought near it. When however the ball touched the electrified body it received a share of the latter’s electricity, and as similar charges repel one another, the ball was driven away. The kind of electricity produced depends not only on the substance rubbed, but also on the material used as the rubber. For instance, we can give glass a negative charge by rubbing it with flannel, and sealing-wax becomes positively charged when rubbed with silk. The important point to remember is that there are only two kinds of electricity, and that every substance electrified by rubbing is charged either positively, like the silk-rubbed glass, or negatively, like the flannel-rubbed sealing-wax. If we try to electrify a metal rod by holding it in the hand and rubbing it, we get no result, but if we fasten to the metal a handle of glass, and hold it by this while rubbing, we find that it becomes electrified in the same way as the glass rod or the sealing-wax. Substances such as glass do not allow electricity to pass along them, so that in rubbing a glass rod the part rubbed becomes charged, and the electricity stays there, being unable to spread to the other parts of the rod. Substances such as metals allow electricity to pass easily, so that when a metal rod is rubbed electricity is produced, but it immediately spreads over the whole rod, reaches the hand, and escapes. If we wish the metal to retain its charge we must provide it with a handle of glass or of some other material which does not allow electricity to pass. Dr. Gilbert did not know this, and so he came to the conclusion that metals were non-electrics, or could not be electrified.