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BOOK THE FIRST.
ON GALVANIC BATTERIES.
Electricity; various kinds, 1-8. Voltaic Batteries; circumstances advantageous
Proximate cause of Galvanism, 14-18. Different forms of Batteries, Couronne des Adhesion of the hydrogen to the negative Daniell's Battery, 37-44. Grove's Comparison between the three,
or disadvantageous to, 5-13.
(1.) As chemists have arranged an extensive series of effects. under the general term of heat, so they have named another series light, and a third, they have called Electricity. Of the first cause of all these, as of that of vitality, we are ignorant, for we can only examine them by their effects, and even their nature is equally obscure. We find, if we examine organized bodies, that all these principles are capable of being produced through the medium of life; for nearly all animals have the power of evolving heat; many insects, moreover, can voluntarily emit light, and the property of producing electricity is well evinced in the terrible shock of the electric eel, as well as in that of some other creatures.
(2.) Their weight is inappreciable by the most delicate balances, and hence they have been termed the imponderable agents; a property in which they all agree, and in numerous cases appear to be singularly and intimately connected with each other.
(3.) Electricity is the only one which we have particularly to treat of in this work, and this subject is subdivided into several departments, as, electricity of tension, or frictional electricity, where the effects of electricity derived from the electrifying machine are considered; thermo or stereo-electricity, where it is derived from solid bodies through the agency of heat; animal electricity, from organized bodies; magnetic electricity, from the natural or artificial magnet; and voltaic or galvanic, where it is obtained from the voltaic pile.
(4.) Although these names, from their multiplicity, may tend to confuse, be it remembered, there is but one electricity which thus manifests itself in such different ways, either under varying circumstances, or from differences from whence it is derived. Our enquiry will not extend into all these details, but simply into its effects when obtained from the voltaic battery.
(5.) The phenomena, to which the name of voltaic or galvanic electricity has been given, are those which arise from the voltaic or galvanic battery, so named from its discoverers, Volta and Galvani. They found that two pieces of metal, possessing different facilities for combination with oxygen, produced, when properly united, singular convulsions in a dead frog; and following out this experiment, they constructed the battery, which has now, from the improvements of later discoverers, become so powerful and valuable an instrument.
(6.) Without pursuing in detail, the interesting experiments of subsequent authors, it must always be borne in mind, that to make a galvanic battery, with advantage, two conducting substances must be employed with a compound conducting fluid intervening, capable of being decomposed; and the first substance should have the strongest possible affinity for one element of the fluid, and the second substance the least possible affinity. Thus, in a simple circuit, composed of zinc, silver, and water (the water being rendered a good conductor
by the addition of acid) zinc has a very strong attraction for the oxygen of the fluid, whilst silver has a very slight attraction; and therefore a powerful current is generated.
(7.) With regard to the relative conducting powers of bodies, the metals, and all the varieties of carbon, excepting the diamond, hold the foremost rank among solids. The fluids are generally imperfect conductors; none more so than pure water; though in combination with the acids, pure alkalies, or any of the salts, it forms a good conductor. Fused chlorides and iodides are also good conductors. The metals are conductors in the following order; silver, copper, lead, gold, brass, zinc, tin, platinum, palladium, and iron.
(8.) If we except the earthy and alkaline metals, as potassium, sodium, &c., zinc has by far the strongest affinity for oxygen; and on this account is invariably used as the electropositive metal (the term applied to the metal which is acted upon by the solution, or which in reality acts on the fluid.) All other metals, in any acid solution, are electro-negative to them; the term used to imply the opposite state to electropositive. The following table shows the state of electricity in which the metals stand, with regard to each other, in acid solutions, where every metal is positive to all below it, and negative to all above it. This series relates only to acid solutions, for it varies with almost every solution used:
This order appears to me to require to be again made the subject of experiment; I would suggest that for this investigation, every metal should be used in a finely divided state, similar to the finely divided platinum of my battery.
(9.) When a metal which acts slightly upon a fluid (as for instance, copper) is brought into contact with another metal, which has a stronger affinity for the oxygen of the fluid, the
latter, or electro-positive, is dissolved, and gives a negative tendency to the former, which in that state does not act at all upon the fluid, but is preserved by the latter. Of this singular property Sir H. Davy took advantage, for the protection of the copper sheathing of vessels, which was effectually preserved from decay by pieces of zinc or iron placed in contact with it under the water; but then unfortunately the copper, ceasing to be deleterious, did not prevent the adhesion of marine animals and vegetables, which accumulated to such an extent, as materially to impede the ships' progress through the water. In this way, zinc protects all the less oxidable metals, when pure; but if the electro-negative metals be contaminated with charcoal, or with a metal having less affinity for oxygen, they will still be acted upon.
(10.) The converse of this observation applies to the electropositive metal, as the zinc; for when pure it is not acted upon by the sulphuric acid, till contact be made with some other metal, having less affinity for oxygen: If it contain any electro-negative metal, however, it will not only be acted upon by the fluid for the generation of the galvanic current, but independently of this a great waste and expense will be incurred. This additional wasting is termed local action, and should be avoided in every possible way.
(11.) Local action, arising as it does from either the zinc or the negative metal, being contaminated with some other metal, is to be considered as an infinity of small batteries, the action of which is quite independent of the great battery; where the hydrogen is entirely transferred to the negative plate, and where consequently no apparent action is visible at the positive plate.
(12.) It is for this reason that the pure metals are exceedingly difficult to dissolve, particularly if the acids be also pure; as for instance, pure silver in pure diluted nitric acid, or pure zinc in dilute sulphuric acid; because there is no local battery of different metals established to favour the solution.
(13.) A battery, in an acid solution, when put into action, exhibits apparently no change at the electro-positive metal, or zinc, if the local action be destroyed; although in fact it is the zinc which is being dissolved. On the contrary, the electronegative metal, which is in reality undergoing no change, exhibits a copious disengagement of gas, which arises from the transferrence of the hydrogen to that plate, while the oxygen is all absorbed by the zinc.
(14.) This leads us at once to the proximate cause of the voltaic current; for it is found that the amount of action on the zinc is exactly proportionate to the quantity of electricity produced; hence, zinc appears to be the fuel of the battery, holding the same place as coals in a fire. From these and various other facts, Dr. Wollaston, Dr. Faraday, and with them most of the present experimenters in this country, believe that the chemical action of the acid solution on the zinc, or rather of the zinc on the water of the acid solution, is the source of the electric current in the voltaic battery; and this is termed the chemical theory of the pile. The Germans again, and others, following Volta, believe that the chemical action is the effect of the electric current, and that the power is produced by the contact of two dissimilar metals; and this latter has received the name of the contact theory.
(15.) In opposition to the Contact Theory, Dr. Faraday has described in the Philosophical Transactions, curious instances, where the connection of a simple battery, excited by dilute sulphuric acid, was not made through any metal whatever; but through a liquid capable of being decomposed by the stronger energies of the dilute sulphuric acid. He found that a solution of iodide of potassium was best adapted to show this interesting fact.
(16.) Which ever theory be adopted, the use of the negative metal is by no means apparent; for the quantity of electricity developed, cæteris paribus, is exactly as the surface of negative metal exposed; thus, provided there be no obstacle to overcome, if the surface of this be doubled, the quantity of