Precisely; the common electrical machine, when excited by the friction of the rubber, gives out both the positive and negative electricities.-- (PLATE VI. Fig. 3.) The positive, by the rotation of the glass cylinder, is conveyed into the conductor, whilst the negative goes into the rubber. But unless there is a communication made between the rubber and the ground, but a very inconsiderable quantity of electricity can be excited; for the rubber, like the plates of the battery, has too small a capacity to admit of an accumulation of electricity. Unless therefore the electricity can pass out of the rubber, it will not continue to go into it, and consequently no additional accumulation will take place. Now as one kind of electricity cannot be given out without the other, the developement of the positive electricity is stopped as well as that of the negative, and the conductor therefore cannot receive a succession of charges. CAROLINE. But does not the conductor, as well as the rubber, require a communication with the earth, in order to get rid of its electricity? MRS. B. No; for it is susceptible of receiving and containing a considerable quantity of electricity, as it is much larger than the rubber, and therefore has a greater capacity; and this continued accumulation of electricity in the conductor is what is called a charge. EMILY. But when an electrical machine is furnished with two conductors to receive the two electricities, I suppose no communication with the earth is required? MRS. B. Certainly not, until the two are fully charged; for the two conductors will receive equal quantities of electricity. CAROLINE. I thought the use of the chain had been to convey the electricity _from_ the ground into the machine? MRS. B. That was the idea of Dr. Franklin, who supposed that there was but one kind of electricity, and who, by the terms positive and negative (which he first introduced), meant only different quantities of the same kind of electricity. The chain was in that case supposed to convey electricity _from_ the ground through the rubber into the conductor. But as we have adopted the hypothesis of two electricities, we must consider the chain as a vehicle to conduct the negative electricity into the earth. EMILY. And are both kinds of electricity produced whenever electricity is excited? MRS. B. Yes, invariably. If you rub a tube of glass with a woollen cloth, the glass becomes positive, and the cloth negative. If, on the contrary, you excite a stick of sealing-wax by the same means, it is the rubber which becomes positive, and the wax negative. But with regard to the Voltaic battery, in order that the acid may act freely on the zinc, and the two electricities be given out without interruption, some method must be devised, by which the plates may part with their electricities as fast as they receive them. --Can you think of any means by which this might be effected? EMILY.