Part 12

Considering this close and twofold relation--namely, that without decomposition transmission of electricity does not occur, and that for a given definite quantity of electricity passed an equally definite and constant quantity of water or other matter is decomposed; considering also that the agent, which is electricity, is simply employed in overcoming electrical powers in the body subjected to its action, it seems a probable and almost a natural consequence that the quantity which passes is the equivalent of that of the particles separated; i.e., that if the electrical power which holds the elements of a grain of water in combination, or which makes a grain of oxygen and hydrogen in the right proportions unite into water when they are made to combine, could be thrown into a current, it would exactly equal the current required for the separation of that grain of water into its elements again; in other words, that the electricity which decomposes and that which is evolved by the decomposition of a certain quantity of matter are alike.

This view of the subject gives an almost overwhelming idea of the extraordinary quantity or degree of electric power which naturally belongs to the particles of matter, and the idea may be illustrated by reference to the voltaic pile.

The source of the electricity in the voltaic instrument is due almost entirely to chemical action. Substances interposed between its metals are all electrolytes, and the current cannot be transmitted without their decomposition. If, now, a voltaic trough have its extremities connected by a body capable of being decomposed, such as water, we shall have a continuous current through the apparatus, and we may regard the part where the acid is acting on the plates and the part where the current is acting upon the water as the reciprocals of each other. In both parts we have the two conditions, inseparable in such bodies as these: the passing of a current, and decomposition. In the one case we have decomposition associated with a current; in the other, a current followed by decomposition.

Let us apply this in support of my surmise respecting the enormous electric power of each particle or atom of matter.

Two wires, one of platina, and one of zinc, each one-eighteenth of an inch in diameter, placed five-sixteenths of an inch apart, and immersed to the depth of five-eighths of an inch in acid, consisting of one drop of oil of vitriol and four ounces of distilled water at a temperature of about 60° Fahrenheit, and connected at the other ends by a copper wire eighteen feet long, and one-eighteenth of an inch in thickness, yielded as much electricity in little more than three seconds of time as a Leyden battery charged by thirty turns of a very large and powerful plate electric machine in full action. This quantity, although sufficient if passed at once through the head of a rat or cat to have killed it, as by a flash of lightning, was evolved by the mutual action of so small a portion of the zinc wire and water in contact with it that the loss of weight by either would be inappreciable; and as to the water which could be decomposed by that current, it must have been insensible in quantity, for no trace of hydrogen appeared upon the surface of the platina during these three seconds. It would appear that 800,000 such charges of the Leyden battery would be necessary to decompose a single grain of water; or, if I am right, to equal the quantity of electricity which is naturally associated with the elements of that grain of water, endowing them with their mutual chemical affinity.

This theory of the definite evolution and the equivalent definite action of electricity beautifully harmonises the associated theories of definite proportions and electro-chemical affinity.

According to it, the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers, it being the electricity which determines the equivalent number, because it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalent to each other in their ordinary chemical action have equal quantities of electricity naturally associated with them. I cannot refrain from recalling here the beautiful idea put forth, I believe, by Berzelius in his development of his views of the electro-chemical theory of affinity, that the heat and light evolved during cases of powerful combination are the consequence of the electric discharge which is at the moment taking place. The idea is in perfect accordance with the view I have taken of the quantity of electricity associated with the particles of matter.

The definite production of electricity in association with its definite action proves, I think, that the current of electricity in the voltaic pile is sustained by chemical decomposition, or, rather, by chemical action, and not by contact only. But here, as elsewhere, I beg to reserve my opinion as to the real action of contact.

Admitting, however, that chemical action is the source of electricity, what an infinitely small fraction of that which is active do we obtain and employ in our voltaic batteries! Zinc and platina wires one-eighteenth of an inch in diameter and about half an inch long, dipped into dilute sulphuric acid, so weak that it is not sensibly sour to the tongue, or scarcely sensitive to our most delicate test papers, will evolve more electricity in one-twentieth of a minute than any man would willingly allow to pass through his body at once.

The chemical energy represented by the satisfaction of the chemical affinities of a grain of water and four grains of zinc can evolve electricity equal in quantity to that of a powerful thunderstorm. Nor is it merely true that the quantity is active; it can be directed--made to perform its full equivalent duty. Is there not, then, great reason to believe that, by a closer investigation of the development and action of this subtile agent, we shall be able to increase the power of our batteries, or to invent new instruments which shall a thousandfold surpass in energy those we at present possess?

III.--The Gymnotus, or Electric Eel

Wonderful as are the laws and phenomena of electricity when made evident to us in inorganic or dead matter, their interest can bear scarcely any comparison with that which attaches to the same force when connected with the nervous system and with life.

The existence of animals able to give the same concussion to the living system as the electrical machine, the voltaic battery, and the thunderstorm being made known to us by various naturalists, it became important to identify their electricity with the electricity produced by man from dead matter. In the case of the Torpedo [a fish belonging to the family of Electric Rings] this identity has been fully proved, but in the case of the Gymnotus the proof has not been quite complete, and I thought it well to obtain a specimen of the latter fish.

A gymnotus being obtained, I conducted a series of experiments. Besides the hands two kinds of collectors of electricity were used--one with a copper disc for contact with the fish, and the other with a plate of copper bent into saddle shape, so that it might enclose a certain extent of the back and sides of the fish. These conductors, being put over the fish, collected power sufficient to produce many electric effects.

SHOCK. The shock was very powerful when the hands were placed one near the head and the other near the tail, and the nearer the hands were together, within certain limits, the less powerful was the shock. The disc conductors conveyed the shock very well when the hands were wetted.

GALVANOMETER. A galvanometer was readily affected by using the saddle conductors, applied to the anterior and posterior parts of the gymnotus. A powerful discharge of the fish caused a deflection of thirty or forty degrees. The deflection was constantly in a given direction, the electric current being always from the anterior part of the animal through the galvanometer wire to the posterior parts. The former were, therefore, for the time externally positive and the latter negative.

MAKING A MAGNET. When a little helix containing twenty-two feet of silked wire wound on a quill was put into a circuit, and an annealed steel needle placed in the helix, the needle became a magnet; and the direction of its polarity in every cast indicated a current from the anterior to the posterior parts of the gymnotus.

CHEMICAL DECOMPOSITION. Polar decomposition of a solution of iodide of potassium was easily obtained.

EVOLUTION OF HEAT. Using a Harris' thermo-electrometer, we thought we were able, in one instance, to observe a feeble elevation of temperature.

SPARK. By suitable apparatus a spark was obtained four times.

Such were the general electric phenomena obtained from the gymnotus, and on several occasions many of the phenomena were obtained together. Thus, a magnet was made, a galvanometer deflected, and, perhaps, a wire heated by one single discharge of the electric force of the animal. When the shock is strong, it is like that of a large Leyden battery charged to a low degree, or that of a good voltaic battery of, perhaps, one hundred or more pairs of plates, of which the circuit is completed for a moment only.

I endeavoured by experiment to form some idea of the quantity of electricity, and came to the conclusion that a single medium discharge of the fish is at least equal to the electricity of a Leyden battery of fifteen jars, containing 3,500 square inches of glass coated on both sides, charged to its highest degree. This conclusion is in perfect accordance with the degree of deflection which the discharge can produce in a galvanometer needle, and also with the amount of chemical decomposition produced in the electrolysing experiments.

The gymnotus frequently gives a double and even a triple shock, with scarcely a sensible interval between each discharge.

As at the moment of shock the anterior parts are positive and the posterior negative, it may be concluded that there is a current from the former to the latter through every part of the water which surrounds the animal, to a considerable distance from its body. The shock which is felt, therefore, when the hands are in the most favourable position is the effect of a very small portion only of the electricity which the animal discharges at the moment, by far the largest portion passing through the surrounding water.

This enormous external current must be accompanied by some effect within the fish equivalent to a current, the direction of which is from the tail towards the head, and equal to the sum of all these external forces. Whether the process of evolving or exciting the electricity within the fish includes the production of the internal current, which is not necessarily so quick and momentary as the external one, we cannot at present say; but at the time of the shock the animal does not apparently feel the electric sensation which he causes in those around him.

The gymnotus can stun and kill fish which are in very various relations to its own body. The extent of surface which the fish that is about to be struck offers to the water conducting the electricity increases the effect of the shock, and the larger the fish, accordingly, the greater must be the shock to which it will be subjected.

The Chemical History of a Candle

"The Chemical History of a Candle" was the most famous course in the long and remarkable series of Christmas lectures, "adapted to a juvenile auditory," at the Royal Institution, and remains a rarely-approached model of what such lectures should be. They were illustrated by experiments and specimens, but did not depend upon these for coherence and interest. They were delivered in 1860-61, and have just been translated, though all but half-a-century old, into German.