please for as often as you renew the contact, so often will you feel the shock.

Here is a different apparatus (Plate VIII. Fig. 21.) In these three glasses (and I might use twenty instead of three) is a solution of salt and water. Into each, except the two outer ones, is plunged a small plate of zinc, and another of silver. These plates are made to communicate with each other, by means of a thin wire, fastened so that the silver of the first glass is connected with the zinc of the second; the silver of the second with the zinc of the third, and so on: now if you dip one hand into the first glass, and the other into the last, the shock is felt.

Charles. Will any kind of glasses answer for this experiment?

Tutor. Yes, they will: wine-glasses, or goblets, or finger-glasses; and so will China cups.

A third kind of battery, which is the most powerful, and the one that is most generally used, is this. It consists of a trough of baked wood, three inches deep, and about as broad. In the sides of this trough are grooves opposite to each other, and about a quarter of an inch asunder. Into each pair of these grooves is put a plate of zinc, and another of silver, and they are to be cemented in such a manner as to prevent any communication between the different cells. The cells are now filled with a solution of salt and water. The battery is com

plete; with your hands make a communication between the two end cells.

Charles. I felt a strong shock.

Tutor. Wet your hands, and join your left with James's right, then put your right hand into one end cell, and let James put his left into the opposite one.

James. We both felt the shock like an electric shock, but not so severe.

Tutor. Several persons may receive the shock together, by joining hands, if their hands are well moistened with water. The strength of the shock is much diminished by passing through so long a circuit. The shock from a battery, consisting of fifty or sixty pairs of zinc and silver, or zinc and copper, may be felt as high as the elbows. And if five or six such batteries be united with metal cramps, the combined force of the shock would be such that few would willingly take it a second time.

Charles. What are the wires for at each end of the trough?

Tutor. With these, a variety of experiments may be made upon combustible bodies. I will show you one with gunpowder, but I must have recourse to four troughs, united by cramps, or to one much larger than this.

Towards the ends of the wires are two pieces of glass tubes, these are for the operator to hold by, while he directs the wires. Suppose now four or more troughs united, and the wire

to be at the two extremities, I put some gunpowder on a piece of flat glass, and then holding the wires by the glass tubes, I bring the ends of them to the gunpowder, and just before they touch, the gunpowder will be inflamed.

Instead of gunpowder, gold and silver leaf may be burnt in this way: ether, spirits of wine, and other inflammable substances, are easily fired by the Voltaic battery; it will consume even small metallic wires.

Copper or brass leaf, commonly called Dutch gold, burns with a beautiful green light, silver with a pale blue light, and gold with a yellowish green light.

James. Will the battery continue to act any great length of time?

Tutor. The action of all these kinds of batteries is the strongest, when they are first filled with the fluid; and it declines in proportion as the metals are oxidated, or the fluid loses its power. Of course, after a certain time, the fluid must be changed and the metals cleaned, either with sand, or by immersing them a short time in diluted muriatic acid. The best fluid for filling the cells with, is water mixed with one-tenth of nitrous acid. Care must always be taken to wipe quite dry the edges of the plates, to prevent a communication between the cells and it will be found, that the energy of the battery is in proportion to the rapidity with which the zinc is oxidated.

CONVERSATION XLV.

Voltaic Conductors-Circles-Tables-Experiments.

Tutor. You know that conductors of the electric fluid differ from each other in their conducting power.

Charles. Yes, the metals were the most perfect conductors, then charcoal, afterwards water. and other fluids. See p. 157.

Tutor. In Voltaism we call the former dry and perfect conductors, these are the first class: the latter, or second class, imperfect conductors: and in rendering the Voltaic power sensible, the combination must consist of three conductors of the different classes.

James. Do you mean two of the first class, and one of the second?

Tutor. When two of these bodies are of the first class, and one is of the second, the combination is said to be of the first order.

Charles. The large battery which you used yesterday was of the first order then, because there were two metals, viz. zinc and silver, and one fluid.

Tutor. This is called a simple Voltaic circle, the two metals touched each other in some points,

and at other points they were connected by the fluid which was of the different class.

James. Will you give us an example of the second order?

Tutor. When a person drinks porter from a pewter mug, the moisture of his under lip is one conductor of the second class, the porter is the other, and the metal is the third body, or conductor of the first class.

The discoloration of a silver spoon, in the act of eating eggs, is a Voltaic operation. A spoon merely immersed in the egg undergoes no discoloration, it is the act of eating that produces the change. This is a Voltaic combination of the second order, the fluid egg, and the saliva, are substances of the second class of conductors, and the silver of the first class.

Charles. Which are the most powerful Voltaic circles?

Tutor. They are those of the first order, where two solids of different degrees of oxidability are combined with a fluid capable of oxidating, at least, one of the solids. Thus gold, silver, and water, do not form an active Voltaic circle, but it will become active if a little nitric acid, or any fluid decomposible by silver, be mixed with the water. An active Voltaic circle is formed of zinc, silver, and water, because the zinc is oxidated by water. But a little nitric acid, added to the water, renders the combination still more active, as the acid acts upon the silver and the zinc.