« PredošláPokračovať »
frog were convulsed ; and having ascertained the fact by a number of experiments, he in the course of his enquiries found, that the convulsions or contractions were produced only when dissimilar metals were employed. It was now inferred that electricity is not only produced by the friction of bodies, but even by the mere contact of certain substances. At the same time it was admitted, that these substances must have some chemical agency or action upon each other, and that the effect produced seems to be proportionate to the degree of cliemical action. The following well known facts were now supposed to be explained by this science. Porter taken from a pewter' pot has always been beld' by connoisseurs in that liquor to be better than when taken from china or glass : this was now said to arise froin a certain decomposition effected by means of the liquor in the vessel, the porter, and the saliva on the under lip coming in contact with the metal. Pure mercury retains its metallic splendour a long time, but its amalgam with tin, &c. is almost immediately oxydated or tarnished. Inscriptions of very ancient date, on pure lead, have been found in a perfect state, while others of modern times, made on compound metals, are corroded and scarcely legible. Works of metal, whose parts are soldered together by means of other metallic substances, soon tarnish, or are oxydated about the places in which the different metals are joined. So likewise is the copper on ships, which is fastened on by means of iron nails. Zinc also may be kept a long time under water, with scarcely any change ; but if a piece of silver touch the zinc while under water, there will be very soon a sensible oxydation. 'Take a piece of zine and place ir under the tongue, and lay a piece of silver as big as half a crown on the tongue, and no particular taste will be observed; but bring the outer edges of the metals together, and a very disagreeable taste will be perceived, which is said to arise from the decomposition of the saliva, a watery fluid. The same thing 'may be hoticed with a guinea and a piece of charcoal. 'These
facts have been thus explained, and the theory generally admitted :--The conductors of electricity, however they may differ from each other in their conducting powers, may be divided into two classes. The first class, which are denominated the dry and more perfect conductors, consist of metallic substances and charcoal ; the second class, called also imperfect conductors, are water and other oxydating fluids. From these, or some of them, all Galvanic Circles, as they are named, are formed,
Hitherto this influence or agent had been chiefly investigated with reference to its operation on animal substances. Hence its popular name was for a long time, animal electricity: but it being soon found that its agency was more extensive, that it possessed powers not indicated by this denomination, and that of course the retention of this name would lead to error, the word Galvanism was adopted in its stead. This extension of the Galvanic principle was connected with new discoveries, and improvements from various quarters ; these, however, for a considerable time, were generally small, and unimportant in their nature. Bat among all the recent discoveries in Galvanism, that made by Professor Volta, in. 1800, is most remarkable in its nature, and most interesting in its relations. Volta set out with the idea, contrary to that of Galvani, that the electricity did not belong to the animal, but to the different metals employed,
Galvani was not likely to produce any greater effect than what could be obtained by two pieces of metal, because he believed the electricity to be in the animal. Volta was led to the discovery of the battery, by combining a number of pieces of metal together, because he was persuaded that the electricity was in the metals or Auids employed. These repeated combinations obtained the name of Galvanic, or more properly, Voltaic batteries: and the science itself is usually denominated, from the discoveries resulting from these batteries, Voltaism.
The simplest galvanic apparatus consists of a set of tumblers, containing water slightly mixed with nitric or
kulphuric acid, which are connected by bent wires with a piece of zinc at one end, and a piece of copper at the other'; connect the tumblers by placing these in them all in the same order-sone metal in the first and last, and both metals in each intermediate one, and connecting the first copper and the last zinc with the fingers, or two wires, will occasion a shock. • The pile is made thus ; take twenty or thirty pieces of zinc, of the size of a penny piece. Get as many penny pieces, and as many pieces of paper, or cloth cut in the same shape, and which are to be dipped in a 80lution of salt and water. In building the pile place zinc, paper, zinc, paper, copper, &c. until the whole be finished. The sides of the pile may bé, supported with rods of glass, or varnished wood, fixed in the board on which it is built. The following experiments may then be performed;
Having wet both hands, touch the lower part of the pile with one hand, and the upper part with the other ; constant little shocks of electricity will be felt until one hand be removed. If the hand be brought back, a similar repetition of shocks will be felt. Put a basin of water near the pile, and put the left hand into it, holding a wire, the one end of which touches the top of the battery or pile ; then put the end of a silver spoon between the lip and the gum, and with the other end of the spoon touch the lower part of the pile; a strong shock is felt in the gum and in the hand. Take the left hand from the water, but still keep hold of the wire, and then perform the last experiment in the same manner, and a shock will be felt in the gum only. Hold a silver spoon in one hand, and touch with it the battery at the lower part, then touch the upper part with the tongue,' the bitter taste is extreme. In performing the above experiments, if, instead of the two ends of the pile, the one end and the middle of it be touched, the sensations will not be nearly so strong.
The most powerful apparatus is the trough, which is composed of zinc and copper plates placed in pairs, so that all those of one metal lie toward the same end of
the trough. The end plates have connecting wires; and when the trough is filled with water, impregnated with nitric or muriatic acid, and the points of the wires brought together, the action is remarkably powerful; any number of troughs may be united and made to act
In this way substances have been decomposed on which the strongest fires had no effect.
True Liberty was Christian, sanctified,