B 3 103 THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID A COURSE OF TWELVE ELEMENTARY LECTURES GALVANISM; ILLUSTRATED WITH UPWARDS OF 100 ENGRAVINGS Ixperfmettts ana apparatus. BY WILLIAM STUKGEON, Lecturer to the Manchester Institute of Natural and Experimental Science ; formerly Lecturer on Experimental Philosophy, at the Hon. East India Company's Military Academy, Addiscombe ; Author of " Lectures on Electricity ;" Editor of " The Annals of Electricity ;" and of " The Annals of Philosophical Discovery," &c. &c. LONDON: PUBLISHED BY SHERWOOD, GILBERT, AND PIPER: SOLD ALSO BY CHARLES GILPIN, 5, BISHOPSGATE-STREET-WITHOUT. MANCHESTER: w. IRWIN, 39, OLDHAM-STREET. MDCCCXLIII. MANCHESTER : PRINTED BY W. IRWIN, 10, MERCHANTS' SQUARE, MARKET-ST. AND 39, OLDHAM-STREET. 5s PREFACE. THE Lectures which now, for the first time, are brought before the public through the medium of the press, are arranged in nearly the same order as that in which I am in the habit of delivering them. I have, however, introduced some few experiments whose resulting phenomena are not sufficiently striking to be introduced with advantage at the lecture table ; and there are some that I have not had an oppor- tunity of repeating : such, for instance, as those on the heads and trunks of oxen, and on the human body. I have, however, had an opportunity of Gal- vanizing the bodies of four young men, who were drowned by falling through the ice, at Woolwich ; but two hours having been occupied, by the usual routine of medical treatment, before the batteries were employed, there was not the remotest chance of obtaining beneficial results. Notwithstanding this loss of time, however, the bodies, with the exception of one, were still alive to the Galvanic influence. Some of the four opened their eyes and moved the lips ; and one of them bent the elbow when the cur* rent traversed the arm from the shoulder to the hand. But no motions of the chest could be produced by any applications of the battery that were tried. Throughout the whole of the course, I have selected such facts only, as best suited my purpose, and arranged them in that order which appears to me best calculated for teaching the rudiments of the science ; and in order to keep in view the analogy which subsists in the phenomena emanating from different sources of electric action, I have introduced several original experiments never before published. K7J371893 VI In the electro-chemical department, I have endea- voured to simplify the theoretical explanations by retaining familiar terms and long established prin- ciples. I am aware that there are some few philo- sophers of the present day, whose theoretical views would appear to differ widely from those which I have attempted to maintain ; but as I have not yet seen any thing like a clear and unequivocal code of laws, in any of the writings of those with whom I differ, nor even an attempt worthy of the name, to substantiate their hypothesis, I can see no cause for abandoning the good opinion which I entertain of a theory which is based on well established laws, and embracing principles applicable to every known phenomenon in electricity, and well calculated for explanation. Various means have been resorted to for measur- ing the relative degrees of Galvanic force ; but as every attempt has failed, I have introduced no instrument of that class, excepting the galvanometer ; and even that instrument, for no other purpose than showing its use in ascertaining the electrical relations of metals, and of Galvanic combinations, without any intention whatever of measuring their difference offeree As this work is neither a history nor a treatise, the names of many distinguished philosophers who have contributed to the advancement of Galvanism are left unnoticed, the most eminent of whom are the late Dr. Henry, of Manchester ; Donovan, of Dublin ; and Humboldt, OErsted, Berzelius, De Luc, Arago, Biot, Ampere, Dessaignes, Gay Lussac, Thenard, Dela Rive, and others, on the Continent. That there are errors in the work, will necessarily be expected ; but they are mostly of a typographical character, few in number, and easily rectified by the reader, the only important one being "Basium" instead of " Barium," page 153. The experimental department, I believe, will be found free from error ; and I have endeavoured to simplify the whole process in each experiment, in order that the amateur may find no difficulty in his attempts at repetition. VII In fact, 1 have endeavoured throughout the whole work, to simplify the principles of Galvanism so as to be comprehensible to every capacity ; and to place before the student, a series of facts, which, both for number and order of arrangement, has no precedent in the English language. For the information which I possess respecting the application of Galvanism to Engineering, I am especially indebted to General Pasley ; by whose permission I have had an opportunity of being present at some of his experiments in the river Medway ; and by whose instructions I have been favoured with every facility for becoming acquainted with the ar- rangement of the apparatus, and every part of the process ; and I with pleasure avail myself of this opportunity of acknowledging these, and many other marks of kindness with which I have been honoured by that highly distinguished scientific officer. Institute of Natural and Experimental Science, Manchester, July 20th, 1843. CONTENTS. LECTURE I. PAGE INTRODUCTION. Historical sketch of the progress of Galvanism, during the first period of its cultivation, viz., from 1790 till 1800. Discovery of Galvanism by Madame Galvani. Professor Galvani's experiments and theory of Animal Electricity. Professor Volta opposes Galvani's theory. Volta's letters to Mr. Tiberius Cavallo. Mr. AValsh's experiments on the Tor- pedo. Sultzerian experiment. M. M. Creve and Fabroni's discoveries. The invention of the Voltaic Pile 1 LECTURE II. Various branches of Electricity. Dissected frogs convulsed by electric sparks. Electro-polarization of frogs. Electro-polari- zation of the human body. Dissected frogs convulsed by secondary electric sparks. Galvanism illustrated by copper and zinc, and a dissected frog. Frogs Galvanized without immediate contact of a Galvanic pair of metals. The dancing dead frog 19 LECTURE III. Frogs Galvanized by one metal. Frogs (Galvanized by their own electric powers. Frogs Galvanized by the combined action of their own electricity and that of other animal bodies. Aldini's experiments on the heads of oxen, on the (bodies of oxen, and on dissected frogs. Creve and Fabroni's experiments, which first showed electro-chemistry. Leichtenberg's experiments, with variations. Predominent influence of metals, in Galvanic experiments 36 LECTURE IV. Professor Volta's experiments with copper and zinc plates. Electro- scope with condenser. Structure of the Volaic pile. Structure of the Couronne , des Tasses. Method of extending the Voltaic pile. Structure of Cruickshank's battery. Sir Humphry Davy's experiments with frogs and one metal. Structure of Wilkinson's battery. Structure of Wollaston's battery 52 LECTURE V. PAGE First Principles of Galvanism illustrated. Experiments with copper and zinc plates. Electro-polarity. Dry electric column; its theory. Experiments with dry electric columns. Oxydation detrimental to its action. Zamboni's piles. Permanency of their action. Butterfly experiment. Dry electric battery of one metal only. Single gold-leaf electroscope, with experi- ments 67 LECTURE VI. Variation in the power of a dry electric column. Various combina- tions of Voltaic batteries. Quantity and intensity. Glass charged by the battery. Discharging rod. The battery's power of polarizing exterior bodies. Electrical tension of the battery. Leyden jar charged by the battery, A series of jars charged. Method of producing a rapid series of discharges from glass surfaces. The law by which Voltaic batteries charges coated glass 82 LECTURE VII. Galvanic batteries charged with brine and acid solutions. Professor Aldini's experiments on the detached heads of oxen. On several heads at the same time. Experiments on dead rabbits, cats, &c. Physiological experiments on the human subject. Why resus- citation failed in some of the experiments. Mr. Halse's suc- cessful experiments on drowned dogs. Dr. Wilson Philip's interesting physiological experiments. Medical Galvanism. Directions for performing medical Galvanism 95 LECTURE VIII. Variations of Fabroni's experiment. Methods of ascertaining the direction of Galvanic currents. Electro-decompositions. Theory of the battery. Cylindrical batteries of copper and amalgamated zinc. Cast-iron battery. Modern batteries, by Messrs. Daniell, Mullins, Grove, and Smee. Dr. Hare's de- flagrator 1 19 LECTURE IX. Galvanometer and its uses. Various experiments. On the chemical and electrical theories of Galvanism. Tables of Galvanic pairs. Decompositions of water. Experiments showing the prin- ciples of electro-chemistry. Couronne des Tosses of wires employed in the theory 141 XI LECTURE X. PAGE Dr. Cruickshank's discoveries. Decomposition of acid compounds. Transfer of acid and alkalis. Decomposition of iodide of potassium and other compounds. Theory of the action, Decomposition of the common metallic salts, and the theory of electro-typing, electro-gilding, and electro-silvering 162 LECTURE XI. Leichtenberg's experiment compared with electro- decompositions. Illustrative experiment with three jars and a pendulous ball. Theoretical principles explained. Electro- polarization of bodies immersed in water. Decompositions, at several places in the same Galvanic circuit. Secondary Batteries of Ritter. Parallel experiments in common, and Galvanic Elecricity. Polarization of a series of wires within the battery. Processes of Electro- typing, Electro-gilding, and Electro-silvering 173 LECTURE XII. Electro-polarization of living fishes and frogs. Method of procuring food by the Torpedo and Electrical Eel. Snails and leeches Galvanized. Galvanic Fence. Why porter acquires a peculiar flavour when drank out of metallic vessels. Unequal distribu- tion of the electric fluid on the surface of individual metallic masses. Theory of chemical action of metals on their solvents. Philosophical tree. Protection of copper on ships' bottoms. Decomposition of potash and soda. Metallo-Chromy. Bec- querel's method of imitating the chemical processes of nature. Mr. Fox's experiments. Decompositions by various batteries. New decomposing chamber. Electro-calorific phenomena. Distinction in eflpcts produced by quantity and those produced by intensity of the electric fluid. Exploding gunpowder by Galvanic action. Deflagrations of charcoal and metals. Fusion of Metals. Experiments illustrative of the distinction between the electric matter and the calorific matter 191 APPENDIX. Description of the Torpedo and the Gymnotus Electricus 212 ELEMENTARY LECTURES ON GALVANISM. LECTURE I. THE field of science that opens to our view, under the combined terms Galvanism and Voltaism, otherwise called Galvanic and Voltaic Electricity, is so extensive in its domain ; so fertile in the number, diversity, and importance of its phenomena, that, to the philosophical world, it has become a treasure of inestimable value, by expanding its boundaries to an immense extent, and shedding a refulgent light over mysteries which the operations of the physical world had presented from the earliest times of philosophical reasoning, till the period which gave birth to researches in this almost boundless region of Electricity. The brilliant series of discoveries which had been made by the employment of the electrical machine and extensive batteries of Leydenjars, had just ceased to develope their most resplendent phenomena ; and but a short series of years had elapsed to give philo- sophers an opportunity of becoming familiar with what had been done in that department of Electricity, when Madame Galvani, the wife of an eminent physician and philosopher of Bologna, accidentally observed a novel phenomenon, which gave rise to new trains of B 2 reasoning, and to experimental results which, even- tually, have been the means of directing the minds of philosophers to entirely new and distinct branches of study. The discovery which Madame Galvani had the good fortune to make, happened about the year 1790 ; and is, perhaps, as singular, with respect to the circumstances under which it occurred, as any recorded in the history of scientific events. It appears that this lady, who, for some time previously, had been in a declining state of health, was requested by her medical advisers to use stewed frogs as an article of diet, being a customary restorative in that country. One day a few of Galvani's friends being assembled in his laboratory, began to amuse themselves by taking sparks from the prime conductor of an electrical machine, which stood on the same table on which was placed a dish, containing a number of frogs already dressed for cooking. Madame Galvani being present, and intently looking at these delicate morsels, was struck with the occasional and sudden convulsive twitchings whichf some of them exhibited whilst the machine was in action. Observ- ing these novel appearances still closer, she detected a correspondence, in point of time, between the con- vulsive movements and the emission of sparks from the prime conductor : and observing also, that the blade of a scalpel, which accidentally lay on the edge of the dish, touched some of these little animals, the idea arose in her mind that they were convulsed by electrical agency, which further inquiries proved to be the fact. Galvani was not present at the time, but was informed of the phenomena by his lady, immediately after his return. A short time previous to this discovery, he had instituted a series of experiments, with a view of proving that muscular action is effected by electrical agency ; and being still en- gaged in the inquiry, he was delighted to receive the intelligence, considering it as corroborative of the 3 physiological views he had taken. Elated by this supposed confirmation of his hypothesis, Galvani became inspired with new ardour in the prosecution of his researches : and by pursuing the new line of investigation which Ms wife's discovery had pointed out, his future experimental undertakings were attended with the most happy results. Galvani first entered on a series of experiments upon dissected frogs, which he submitted to the action of electric sparks ; and by this means he soon verified his lady's conjecture respecting the cause of the convulsive motions which she first observed amongst those which were accidentally electrized in the laboratory. These experiments were pursued with different degrees of electric action, until it was discovered that the minutest quantity would convulse the delicate parts of frogs, when properly prepared for the experiments. Such, for instance, are the hind legs and thighs, with the crural nerves laid bare, and attached to a small morsel of the spine. These were found to be the parts most sensitive to electric action ; and so exquisitely susceptible are they of electric excitement, that they appear to constitute a more delicate test for feeble electric action than a Bennet's gold-leaf electroscope.* The success which attended these experimental inquiries redoubled Galvani's diligence, and induced him to prosecute them still further. In his next series of experiments the electric action of the atmo- sphere was employed in place of that excited by artificial means ; and the muscular contractions and nervous irritability were displayed in prepared frogs, and other recently killed animals, as decidedly from the one source of electric action as from the other. . Whilst contemplating on the great variety of means by which he was enabled to produce the con- vulsive movements in recently killed animals, his * For a description of this instrument see vol. I., " Elec- tricity," p. 40. previous views concerning electro-muscular action began to expand, so as to embrace the whole animal frame as a natural electrical machine, in a continual state of excitement ; and which, like the rubber and glass parts of the ordinary electric machine, has its muscular and nervous systems in different electric conditions. These grand views of animal electricity, "which were developed to the mind of so excellent a physiologist, and dexterous manipulator as Galvani, by the experimental results he had already obtained, could hardly fail to disclose other means of experi- menting than those he had hitherto pursued. Accord- ingly we find that, in order to test the accuracy of his new hypothesis, he laid aside all extraneous elec- tric action, and operated with that only which he thought naturally belonged to the animals themselves. Keeping in view the supposed distinct electric conditions of the nervous and muscular systems, he laid bare the crural nerves of a frog, and having removed all muscular matter from between the thighs and the dorsal spine, he macle a communication between these two parts by means of a thin metallic wire, which operated as a discharging rod, and the animal was convulsed as decidedly as by the former modes of experimenting. In this case, Galvani sup- posed that the animal was agitated by a discharge of its own electric fluid, which proceeded through the metallic arc from the nerves to the muscles, as in the discharge of coated glass from the positive to the negative side.* Galvani varied the character of his conducting arc, by employing gold, silver, copper, zinc, &c. ; but in all cases the convulsive motions were produced ; and to a . greater extent, as the animal was in a more vigorous condition. Fortunately for this branch of science, Galvani not only employed different metals for the simple conducting arc, but combined the dif- ferent metallic wires in such a manner as to form * See "Lectures on Electricity," p. 100. compound arcs ; by which means one metal could be applied to a muscle and the other to a nerve at the same time, the metals themselves being also in con- tact with one another. By the employment of these compound arcs it was soon discovered that the convulsive motions were displayed in a much higher degree than by the application of a single metal alone. Although these novel modes of experimenting yielded results in perfect accordance with their author's views of animal electricity, it was not enough for this profound and sagacious inquirer to rest his hypothesis solely upon them ; for as he had employed metals in conjunction with the animal fibres, it was necessary to inquire how far these were concerned in the process, since there was a possibility, at least, that each individual metal was endued with an elec- tric action peculiar to itself, which, if true, would be a means of modifying the results. Galvani now partially insulated the metallic arcs from the parts of the animal on which he operated, by means of inferior conducting substances, such as water and other aqueous liquids, and found that, although the phenomena were not of so striking a character as when the metal actually touched the subjects of experiment, the convulsive movements were still as decisively produced. He even proceeded so far in these minute inquiries as to dispense with the metal entirely ; and convinced himself that water alone performs the function of a metallic arc, though in a less degree. Notwithstanding, therefore, the inter- ference of any electric action that might be due to the metals, when those bodies were employed, the phenomena exhibited during their absence could not be attributed to that source of excitement. Galvani very properly considered them as corroborative of his hypothesis, and believed that he was justified in publicly announcing them as such. The announcement of Galvani's discoveries and extensive experimental inquiries in this novel branch B 2 6 of research, excited the attention of every physiologist of eminence throughout Europe ; several of whom immediately entered on a repetition of the novel facts, and examined them in the most scrupulous manner. Professors Valli, Fowler, Pfaff, Volta, and many other naturalists, entered ardently into the new path of inquiry, and their labours soon enriched the science with novel modes of experi- menting, with an abundance of facts, and with dif- ferent modes of reasoning on them. But the most valuable results at this period of the inquiry emanated from the labours of M. Volta, then professor of natural philosophy at Como, and one of the most eminent electricians of his day. Having repeated Galvani's experiments, and satisfying himself with the correctness of the results, Volta turned his attention to other modes of experi- menting, from which he developed facts which he fancied would overturn the whole system of reasoning established by Galvani, and soon became the most formidable opponent to the hypothesis of animal electricity. Volta showed, though anticipated by Cavallo, that the simple contact of the dissimilar metals is sufficient to develope an electric action appreciable by the electroscope : and upon this simple fact alone he insisted that the whole of the phenomena attributed to animal electricity by Galvani, might easily be traced to the electricity of the metals which were employed in the experiments. The professor of Bologna replied to this opposi- tion of his hypothesis by new experiments, and extended reasoning upon them : but notwithstanding the clearness of his statements, and the unequivocal character of his experimental results, even when no metal was present, Volta strenuously combated the hypothesis of animal electricity : and although he became convinced, even by his own inquiries, that other heterogeneous bodies, besides those of a metallic character, would develope electric action by simple contact alone, and was forced to acknowledge that the muscular and nervous fibre came under the denomination of heterogeneous bodies, and that by their simple contact electricity would be developed ; and thus absolutely proved the correctness of those theoretical views which he was attempting to obli- terate, still he persisted in denying Galvani any share of merit for his hypothesis, even though he found the principles on which it was based con- venient for his own purpose, and even indispensable for the completion of an hypothesis which he was then building for himself f from the frame-work of which they could not possibly be excluded, Volta, however, who, in his letters to Cavallo-, gave the first intelligence of Galvani's discovery to the philosophers of this country, gives every degree of credit to the Professor of Bologna fot the acute- ness of his inquiries, the dexterity of his manipu- lations, and the novelty of the phenomena he had developed. He even acknowledges, in his first letter, that Galvani had discovered " an animal electricity, properly so called, appertaining not only to frogs, and other cold-blooded animals, but also to all warm-blooded ones, as quadrupeds, birds, &c. ; a discovery," says Volta, " which makes. the subject of the third part of his (Galvani's) work a subject quite new and interesting."* Volta had hitherto alluded to the earliest of Galvani's experimental inquiries, in which the elec- tricity of the machine and that of the atmosphere had been employed ; and closes his account of them with the following uncourteous remarks : " It was chance that presented to M. Galvani the phe- nomenon just described, but at which he was more astonished than he needed to have been, had he given due attention to the effects of electric atmo- spheres." * Volta's Letters to Tiberius Cavallo, F. R. S. Phil. Trans, for 1793. These letters were written at PAVIA, to the University of which Volta had removed from that of COMO. 8 The doctrine of " Electric Atmospheres" has been explained and experimentally illustrated in my twelfth Lecture on Electricity. It is a doctrine that cannot be too well studied by those who are desirous of becoming acquainted with the general principles of electric action : and as it enters essen- tially into that branch of Electricity of which we are now treating, it ought to be borne in mind in almost every step that we take, as we proceed in the theo- retical illustrations. The action of electric atmo- spheres, here spoken of by Volta, as explanatory of Galvani's experiments, was no doubt the true cause, when primarily considered, though not the immediate cause of the spasmodic movements exhibited : for the electrosphere* of the prime conductor, whether that of the machine or that of the kite string, produced electro-polarity in the scalpel, or other metal, in connexion with the frog ; and as these metals could not sustain the electric pressure which occasioned their polarity, without suffering loss, a portion of electric fluid escaped from them to the frog, and returned as a secondary discharge, simultaneously with each prima- tive discharge of a spark from the prime conductor. The cause of these secondary discharges is precisely the same as that of lateral discharges of the third kind, as illustrated in my eleventh Lecture on Elec- tricity, p. 209. In referring to Galvani's second class of experi- ments, in which no extrinsic electric action was employed, Volta says, " Thus he happily evinced the existence of a true animal electricity in almost all animals. It appears proved indeed by these experi- ments, that the electric fluid has a continual tendency to pass from one part to another of a living organized body, and even of its lopped members, while they * I have taken the liberty of employing the term " Electro- sphere," as being more appropriate than " Electric Atmo- sphere," to express the electric state of the surfaces and vicinal space of electrized bodies. retain any remains of vitality : that it has a tendency to pass from the nerves to the muscles, or vice versa, and that muscular motion is due to a like transfusion, more or less rapid. Indeed it seems that there is nothing to be objected, either to the thing itself, or to the manner in which M. Galvani explains it by a kind of discharge similar to that of the Leyden phial." The opinions here stated are in perfect accordance with those of Galvani. The succeeding paragraph of Volta's letter, however, evinces a very different feeling to the doctrine which, just before, he had seemed to admire ; although to an electric logician it would be difficult to ascertain the cause. There is certainly nothing in the following passages of Volta's letter, which could in the least tend to vindicate such a sudden and total change in his views of Galvani's hypothesis. " M. Galvani," says Volta. " following up the idea he had formed after his experiments, and to follow, in every point, the analogy of the Leyden phial and the conducting arc, pretends that there is naturally an excess of the electric fluid in the nerve, or in the interior of the muscle, and a corresponding defect in the exterior, or vice versa ; and he supposes consequently that one end of that arc ought to com- municate with a nerve which he considers as the conducting thread, or knob of the phial, and the other end with the exterior of the muscle. But had he but a little more varied the experiments, as I have done, he would have seen that this double contact of the nerve and muscle, this imaginary circuit, is not always necessary. He would have found, as I have done, that we can excite the same convulsions and motions in the legs, and the other members of animals, by metallic touchings, either of two parts of a nerve only, or of two muscles, and even of different points of one simple muscle alone." " It is true," continues Volta, " that we succeed not quite so well in this way as in the other ; and that in this case we must have recourse to an artifice. 10 which consists in employing two different metals, which is not necessary in experimenting after Gal- vani's method, at least while the vitality in the animal, or in its amputated members, is in full vigour : but in short, since, with the armings of different metals applied, either to the nerves only, or to the muscles alone, we succeed in exciting con- tractions in these, and the motions of the members, we ought to conclude that if there are cases (which, however, appears very doubtful) in which the pre- tended discharge between the nerve and the muscle is the cause of muscular motion, there are also cir- cumstances, and more frequently, in which we obtain the same motions, by a quite different way, a quite different circulation of the electric fluid. Yes, it is a quite different sort of method of the electric fluid, of which we ought rather to say we -disturb the equilibrium, than restore it, in that which flows from one part to another of a nerve, or muscle, &c., as well interiorly by their conducting fibres, as exteriorly by means of applied metallic conductors, not in con- sequence of a respective excess or defect, but by an action proper to these metals when they are of dif- ferent kinds. It is thus that I have discovered a new law, which is not so much a law of animal electricity, as a law of common electricity ; to which ought to be attributed most of the phenomena which would appear, from both Galvani's experiments and mine, to belong to a true spontaneous animal electricity, but which are not so ; but are really the effects of a very weak artificial electricity." There does not appear in any part of this reason- ing of Volta, nor in the experiments which he brings forward in support of it, any cause whatever to detract from the hypothesis of Galvani. Volta him- self, acknowledges that when a muscle only, or a nerve alone, is operated on, that in order to succeed in producing the convulsive motions, "we must have recourse to an artifice, which consists in employing two different metals, which is not necessary in 11 experimenting after Gal vani's method." The reason is obvious. By Galvani's method the muscles and the nerves were the source of electric action ; whilst by Volta's ' ' artifice, " (a method of experimenting first employed by Galvani,) the dissimilar metals consti- tuted the electric source. The principal part of Galvani's hypothesis which Volta seemed so desirous to overturn, is that of the animal electric charge being similar to the charge of a Leyden phial. At the time this controversy was carrying on, but very little, if any thing, was known, respecting the different electric states of conducting bodies when in close contact with one another ; the general opinion being that insulation, to some certain extent, was absolutely necessary to maintain their respective con- ditions : and as the Leyden phial showed this effect in the most prominent manner, there can be no wonder at Galvani adopting the coatings of that instrument, as a comparison to the nervous and muscular electric conditions : which he thought to exist ; although, at the same time, he intended his words to imply nothing more than that these systems were relatively in different electric states ; a fact well attested both by his own experiments, and by those of other philosophers, and ratified by analogy, even from the experimental labours and the reasonings of Volta himself. The doctrine of Franklin holds it as a principle, that the same bulk of each kind of matter has, naturally, a certain quantity of the electric fluid belonging to it, which may be called its specific quantity : and the experiments of Lichtenberg and Cavallo, several years prior to the discovery of Galvanism, showed that bodies, both conductors and non-conductors, were in different electric conditions during contact. I have already shown a specimen of this fact, by a very beautiful experiment, towards the close of my Lectures on Electricity, and shall have to allude to several others of the same kind in the present course. 12 That there is the faculty of keeping in different electric conditions, the organism of living beings, is amply manifested in the Torpedo, and the Electrical Eel of Surinam, whose shocks, especially of the latter fish, are too violent to be received without experiencing considerable pain. The benumbing sensations produced by approaching the hands towards different parts of the Torpedo, have been known as a fact from time immemorial : and if this faculty was not attributed to an electrical influence previously to 1772, Mr. Walsh's experiments at Rochelle, about that time, were sufficiently decisive on that point. In Mr. Walsh's letter to Dr. Franklin, dated Rochelle, 12th July, 1772, he observes, " It is with particular satisfaction I make to you my first communication, that the effect of the Torpedo appears to be absolutely electrical." He then proceeds to say, " I will not at present trouble you with the detail of our experiments, especially as we are daily advancing in them ; but only observe, that we have discovered the back and breast of the animal to be in different states of electricity." " By the know- ledge of this circumstance we have been able to direct his shocks, though they were small, through a circuit of four persons, all feeling them ; likewise through a considerable length of wire, held by two insulated persons, one touching its lower surface, the other its upper. When the wire was exchanged for glass, or sealing wax, no effect could be obtained ; but as soon as it was resumed, the two persons became liable to the shock. These experiments have been varied in many ways, and repeated times without number, and they all determined the choice of conductors to be the same in the Torpedo as in the Ley den phial. The sensations, likewise, occasioned by the one and the other in the human frame are precisely similar. Not only the shock, but the numbing sensation which the animal sometimes dispenses, expressed in the French by the words enyourdissement and sour mill em cut, mav be exactly imitated with the 13 phial, by means of Lane's Electrometer,* the regu- lating rod of which, to produce the latter effect, must be brought almost into contact with the prime conductor which joins the phial." I have made this copious extract for the purpose of showing that Galvani's hypothesis of animal elec- tricity was well supported by the facts established in Mr. Walsh's experiments, at least eighteen years previous to Madame Galvani observing the commo- tions in the frogs ; and the electric powers of the Torpedo were well known to both Galvani and Volta, at the time of their controversy respecting the elec- tricity of other animals. Besides the well-established proofs of the ex- istence of dissimilar electric organisms in these inhabitants of the waters, there are no less direct proofs of the electric agency residing in other animal bodies. The experiments of Professors Valli and Aldini, in which no metals were employed, led both of these eminent physiologists to the conclusion, that the living animal is endowed with electric powers : and they showed by the most satisfactory experi- ments, that these powers are brought into a state of activity, and capable of convulsing the animal in which they reside, by the mere contact of a muscle with a nerve. But it would be needless to multiply facts in this place, since we shall have to advert to them again as we proceed in our next lecture, in which we shall show the most efficient methods of making the experiments already alluded to, and several others, which not only favour the hypothesis of animal electricity, but which carry conviction to the unprejudiced mind, that, as in metals and other inorganic matter, so in organised inanimate, and all animated, beings, there resides a specific electric charge ; that this charge is not uniformly distributed even in one and the same individual body ; and that, * For a description of this instrument, with its application to the Leyden phial, see " Lectures on Electricity," p. 172. c 14 in the animal system, the extent of the charge in the nerves is very different to that in the muscles ; so that each individual organism in the animal hody has its specific charge of the electric matter. Notwithstanding the irrefutable facts demonstra- tive of animal electricity, that were brought to Volta's notice, he still persisted in maintaining the contrary opinion, and with the most assiduous perseverance he multiplied and diversified his experiments in order to give it support. Volta's theoretical views were now so steadfastly centred in the electricity of the metals, that for a while he made no enquiries in which they were not employed ; and his experiments, eventually, led to the grandest results. Volta having excited convulsive motions in the muscles and the members of both small and large animals, without laying bare any nerve, by the application of dissimilar metals to the muscles strip- ped of their integuments, began to think of doing the same in the human subject ; and after pretending to devise a variety of plans to operate on the living body, he claims for himself the credit of discovering a mode of experimenting, which happened to be well known to the philosophers of Europe many years previously. " Having covered the tip of the tongue and a part of its upper surface, to the extent of some lines, with tin leaf, he applied the convex part of a silver spoon more advanced on the flat of the tongue, and in- clined the spoon till its handle came in contact with the tin foil.'' By this mode of experimenting, Volta says that he " expected to see the trembling of the tongue, and for that purpose placed himself before a looking-glass. But the expected motions did not take place : however, he felt instead of it, a very unexpected sensation, a pretty sharp sensation on the end of the tongue."* SULTZER, a German metaphysician, published * Volta's letter to Cavallo; Phil. Transactions for 1793. 15 the very same experiment in his " Theory of Plea- sures," in the year 1767. This author states, that "if two pieces of metal, one of lead the other of silver, be joined together at their edges so as to form but one plane, and thus placed on the tongue, a taste similar to that of vitriol of iron will be experienced. But when either piece of metal is applied separately to the tongue, no such taste is produced." Hence this experiment, which by some writers is ascribed to Volta, is obviously due to Sultzer, who published it twenty-five years previous to the date of any of Volta's enquiries on this subject. When Volta perceived the " sharp taste on the end of his tongue," he appears to have been " much surprised at the event. ' ' When speaking of Galvani's earliest experiments, Volta had remarked, that "he was more astonished than he needed to have been, had he given due attention to the effects of electric atmospheres." The same remark is applicable to his own astonishment at the effect produced on his tongue by a repetition of Sultzer's experiment. In- deed, the remark retorts on Volta in a still more forcible manner, when it is considered that Galvani's experiment was perfectly original, and no similar effect to that produced was then known ; whilst the Sultzerian experiment with its attendant phenome- non, were well understood when Volta enrolled them amongst his own list of discoveries. Indeed, the Sultzerian experiment had been re- peated and modified in different ways by CREVE and FABRONI, prior to its repetition by Volta. The former physiologist considered that in the Galvanic experiments their effects were due to chemical irrita- tion, and not to any electrical influence. He says, that, " when a communication is established between two metals, or between a metal and charcoal, the water by which the muscles or the nerve is surrounded is partly decomposed. The oxygen, one of its ele- ments, having a greater affinity with the charcoal, or with the metal, than with the hydrogen, quits the 16 latter. The decomposition merely takes place in tHat portion of water immediately in contact with the metals ; but the sphere of influence which this decomposition possesses is not enclosed by such limitation, as the following experiment amply proves. "Place a metallic apparatus in a glass filled with water, and afterwards introduce the tongue to the liquid, at the distance of an inch from the metals. An acrid and astringent impression is produced, characteristic of metallic irritation. The tongue is affected because it is placed within the sphere of action of the decomposed water ; and the nearer it is brought to the place where the metals are in contact with each other, the greater degree of sensation is experienced." It is somewhat singular that in this early part of the enquiry, CREVE should have fallen upon one of the most important facts in Voltaic Electricity. The decomposition of the water in connexion with the metals, is a necessary condition of continued electric currents from every form of Voltaic apparatus hitherto known. The superior action on the tongue, when near to the point of metallic contact, is also a fact perfectly analogous to other phenomena, now well known to proceed from a similar arrangement of metals and liquid matter. And to this physiologist is justly due, not only the discovery of this fact, but also the chemical hypothesis of Galvanic action, which are generally supposed to have emanated from the researches of other philosophers, of a much later period of the history of the science. The theoretical views of FABRONI were analogous to those of Creve, attributing the whole phenomena, both GALVANIC and SULTZERIAN, to chemical opera- tion. In repeating the Sultzerian experiment with his tongue partially dried by wiping it with a towel, FABRONI discovered that the sensation was reduced to such a degree as scarcely to be distinguishable. Hence he concluded that the saliva, the lymph, or some kind of humidity, had an influence in producing 17 the full effect ; and that there is a high degree of probability that it is this humidity that either wholly or partially forms a sapid combination .with the metal. FABRONI also made experiments with metals and water, from which he was convinced that chemical action took place. From the duration of this species of action, FABRONI supposed that it could not be electrical, because at that time electrical action was considered to be momentary only. Moreover, from the oxides and saline crystals which he obtained by a connected pair of dissimilar metals in liquids, he was led to the final conclusion, that to a chemical operation the whole phenomena are attributable. Fabroni, however, did not think that by the contact of two metals the development of electricity was impossible ; but he was decidedly of opinion that electricity was not the primary agent in pro- ducing the phenomena in the Sultzerian experiment. He was convinced, from his own experiments how- ever, that metals have a reciprocal action on each other, and that it is this action which is productive of the phenomena at the time of their contact. This philosopher went even so far as to satisfy himself that this action gives a new power to the metals, enabling them to decompose chemical compounds, and to transfer their elements to new combinations ; and he showed by a very simple experiment, that light is also developed by this species of chemical action. Volta's views of metallic electricity induced him to prosecute his enquiries on the metals alone, inde- pendently of any connexion with the animal body ; and by the assistance of his condenser, an instrument he had previously invented,* he was enabled to show that the mere contact of two dissimilar metals is suffi- cient to develope electric action appreciable by the * For a description of the principles of the condenser, and the method of employing it, the reader is referred to my " Lectures on Electricity," page 46 48. c2 18 gold leaf electroscope. This discovery was the first grand result that Volta arrived at in the enquiry ; and although he perverted its application by adducing it as a proof of the erroneousness of Galvani's hypo- thesis of animal electricity, he was eventually led to turn it to account in the most efficient manner in the construction of the pile which so justly bears his name. By this happy discovery, Volta has put into the hands of philosophers the most valuable electrical apparatus they ever yet possessed. Volta's pile was first made known in this country in the year 1800. Its discovery consummated a long and successful career of enquiries which its author had closely pursued, and marked a new era in elec- trical science. Under the various modifications that the pile has since received, it has been the means of revealing to philosophers new and highly interesting classes of facts, and of establishing novel branches of study of the highest importance in experimental science. The activity of the pile, Volta, for a long time contended, was attributable to the electricity of the metals. Finding, however, that non-metallic bodies could be rendered available in the formation of the pile, he eventually relaxed the rigidity of his theory : but although, whilst thus modifying his theoretical views, he indirectly sanctioned Galvani's hypothesis, he never directly and openly acknow- ledged its correctness ; but, on the contrary, when Doctor Valli had shown that by merely bringing a muscle and a nerve into contact with each other, the convulsive motions were still produced, Volta turned this fact to his own account, and vauntingly replied that, in order to render Valli' s experiment successful, two things were essentially necessary. The organs of the animal brought into contact should be of dis- similar kinds, and that a third substance should intervene between them. This attempt to explain away the hypothesis of Galvani, although lauded by many philosophers from that time to the present, was as strong a proof as the 19 Pavian Professor could have given of its correctness, for by this mode of explanation, the muscles and nerves are acknowledged to be in different electric conditions, which was the grand principle in the doctrine of animal electricity. As to the intervening medium between these organs, which Volta con- sidered essential to the effect, the idea arose from CREVE'S and FABRONI'S associations of metals and water, and from which he was so happily led to the formation of the pile, which is neither more nor less than a series of similar associations of metals and liquids, properly arranged for accomplishing concert of action. LECTURE II. HAVING pourtrayed an historical sketch of the progress of Galvanism for the first ten years of its cultivation, I will now endeavour to illustrate the various facts that were developed, by a series of experiments, which will lead you gradually from the simplest case of Galvanic action to those which appear more complicated ; and eventually to some of the most magnificent displays of electricity that have hitherto been produced. In speaking of electricity in this place, it may be necessary to state before proceeding further, that electricity is divided into several distinct branches, and that Galvanism is one of them ; hence it is, that instead of the word Galvanism, we frequently say, Galvanic Electricity, which implies the same thing. The term Galvanism, was first applied to this branch of science in honour of Professor Galvani, who first cultivated it through a long series of experimental enquiries, before he made it known beyond the circle of his own friends. We also employ the term Voltaism, and Voltaic Electricity, in honour of Pro- fessor Volta, who was the inventor of the Voltaic 20 pile, an instrument which, in its various forms, is extensively employed in many interesting researches, and has been the means of developing more important facts than any other piece of electrical apparatus. It has even created new branches of science, for electro-chemistry and electro-magnetism owe their existence to its wonderful powers. In proceeding with our experimental illustrations it will be as well, in the first place, to operate with apparatus as similar to those originally used as we can get them prepared, and employ them in the same manner as they were employed by their respective authors. By these means an opportunity will be afforded for obtaining a more perfect idea of the progressive steps by which Galvanism made its first advances, than by any other mode of illustration. I shall, however, in many parts of these lectures, introduce various pieces of apparatus of a very different construction to those which were first employed, and, occasionally, novel modes of experimenting will be resorted to. When I have passed through the necessary series of experiments for illustrating the discoveries during the first period of Galvanism, or up to the year 1800, when the Voltaic pile was first made known in this country, I shall no longer observe the chronological order by which the subse- quent facts were discovered, but proceed by that route which appears to me most likely to develope the principles by which Galvanic phenomena are displayed, and those upon which the science is based, in the most simple manner. When frogs are used as an article of diet, it is only the thighs which are thought to be sufficiently delicate for this purpose ; and, consequently, it was on these parts of the animal alone that Madame Galvani's discovery was made. This fact having begun a new era in electricity, it is usually, and indeed very properly, resorted to as the first step in experimental illustrations in this beautiful and interesting field of science. In fig. 1, you will see that I have placed the hind legs of some skinned frogs on a plate, and have reared against the edge of the plate the blade of a knife, at a considerable distance from the prime con- ductor of an electrical machine. When the ma- chine is in motion I will take sparks from the prime conductor to the brass ball which I hold in my hand ; Fig. 1. and you will observe these limbs of the frogs con- vulsed at the precise time that a spark strikes -the ball ; although the latter has no connexion whatever with the knife or the frog's limbs. At first sight this fact appears to be involved in mystery, especially to those who have not had an opportunity of becoming acquainted with the general principles of electric action, as laid down in my lec- tures on Electricity ; but when I refer you to the doctrine of electro-polarization, as explained in the fourth, fifth, and sixth lectures of that course, you will easily understand that the blade of the knife becomes polar by the electric action of the prime conductor, prior to its delivering a spark to the ball which I hold in my hand. Much of the fluid natu- rally belonging to the blade of the knife is also driven out at the farther extremity, by the superior electric pressure from the prime conductor, exerted on the nearest extremity of the blade : hence, it is not only electro-polar, but also electro-negative, so long as the prime conductor continues to be charged. But when I suddenly discharge the prime conductor by taking a spark to the ball which I hold in my hand, the electric pressure on the vicinal end of the knife is suddenly lessened, and the latter immediately recovers its lost quantity of fluid from that body which, by vicinity and conducting character, is most suitably disposed to supply it. In the present case, this supply is from the limbs of the frog, nearest to the knife, which, though long detached from the rest of the animal, retains sufficient irritability, or sensitiveness, to become convulsed by each of these momentary discharges. By directing your attention to figs. 6 and 26, pages 36 and 81 (lectures on Electricity), you will easily understand that in this experiment, the limbs of the frog became as decidedly electro-polar as the blade of the knife. And in order to convince you of this fact, I will now introduce a perfectly novel experiment amongst our illustrations. I have here arranged the hind limbs of three frogs in nearly the same right line with the axis of the prime conductor of the machine. They are Fig. 2. insulated from one another, and also from the table, by resting on small slips, or tables, of varnished glass, which are supported on glass stems. I also hang from each pair of limbs, two pair of pith balls, as represented by fig. 2. When the machine is put into action, a divergency takes place in every pair of the electrosopic balls, which, by testing in the usual way, are found to be negative and positive alternately, in precisely the same manner as if suspended from the extremity of a series of metallic wires when ex- posed to a similar electric influence (see Electricity, fig. 6, page 36) ; and, consequently, each pair of limbs is polar, being negative in those extremities which are directed towards the prime conductor, and positive at the remote extremities. Whilst the frogs are thus arranged I will take a few sparks from the prime conductor, and you will observe that every frog in the series is convulsed simultaneously with the passing sparks. Our next business will be to show that this series of animals' limbs, whilst under the influence of the charged prime conductor, will display the electric star and pencil as distinctly as those phenomena are exhibited by polarized wires. For this purpose, however, the room must be rendered as dark as possible, in order that these faint electrical illumina- tions may become clearly distinguished. Whilst on this theoretical topic, I will bring forward another experiment, which will show that, whilst under the influence of the electro-positive action of the prime conductor, the frog's limbs are rendered electro-negative. For this Fig. 3. purpose I suspend the hind legs of a skinned frog by a silken thread, and hold it at some distance above the prime conductor of the machine, as represented by fig. 3. Whilst the machine is still in action, I remove the limbs of the frog to a distance, and place it on the cap of a Bennet's gold-leaf electrometer,* and I find that the leaves diverge with negative electric action. Hence we learn from this series of experimental illustration, that animal matter, in common with metals, is susceptible of electro-polarization and elecro-negation, when subjected to different degrees of electric pressure on the two opposite parts of its surface : and upon the same principle animal matter, whether a whole animal body, or only a detached part, would become electro-positive by being placed in the vicinity of the negative conductor ; for in that * This instrument is described at page 42, &c., in the volume on " Electricity." 24 situation the animal would] become electro-polar, by lessening the electric pressure on that particular side next to the conductor, and the remote side being thus rendered electro-negative, would present an opportunity for the introduction of fluid from the surrounding air. Another method of showing the electro-polariza- tion of the animal body, is by placing a person on the electrical stool, with both arms stretched out as far from one another as possible, and holding in each hand a pith ball electroscope. In this position one hand is to be directed towards the prime conductor of a machine, and the other in the opposite direction. The moment that the machine is put into action both pairs of balls diverge ; and by testing them by either excited glass or sealing-wax, that pair of balls nearest to the prime conductor is found to be electro- negative, and the more remote pair electro-positive. The two hands are therefore in different electric states, and, consequently, the arms are electro-polar. If, whilst this person is in the electro-polar state, I present a large brass ball to any part of his body, the remote electroscopic balls will lessen their divergency ; they sometimes collapse and fall to- gether, whilst the negative balls, or those nearest the machine, will diverge further than before. These changes are occasioned by lessening the electric pressure on that part of the body towards which the ball is presented, which causes an accumulation of the electric fluid at that place to a greater extent than at any other part of the body, and at the same time increases the facility for the fluid to retire from that hand and arm which are directed towards the prime conductor. If the ball approaches the body sufficiently near to receive a spark, the whole body will be rendered electro-negative. When an assistant is at hand, the effects of secondary electric action on the animal system are shown in a very elegant manner, by the arrangement represented by fig. 4. The principal operator, w, holds in one hand a flat porcelain dish, on which is stretched the hind limbs, E, of a skinned frog, and Fig. 4. in the other hand a metallic wire. The thumb of that hand which holds the dish is placed upon the crural nerves of the frog, and the point of the wire held in the other hand rests upon the thick part of the thighs, whilst its upper end is slightly inclined towards the prime conductor, c, of an electric machine. Whilst thus arranged, the assistant takes a spark from the conductor to the ball, D, and the limbs of the frog are instantly convulsed. A series of sparks received by the ball, D, are productive of contemporaneous secondary electric movements in the wire, the operator's hands, and the limbs of the frog, which cause corresponding convulsive motions in the latter, in a very remarkable manner. If, instead of the electrical machine, we were to employ an electrical kite, at a time when the air D would yield a good supply of fluid, the whole of the experiments already offered to your notice might be easily repeated, by arranging the apparatus in a similar manner, with respect to the reservoir (fig. 79, page 185, " Lectures on Electricity"), as hitherto has been done, with respect to the prime conductor of the machine. Hitherto we have passed through a series of experiments, which, with some trifling modifications for the purpose of explaining the principles of action, are similar to those first instituted by Pro- fessor Galvani ; and as we have employed electricity in its most indisputable and universally acknowledged forms, we cannot hesitate for a moment to attribute to electricity the CHIEF, if not the sole, agency in the production of the phenomena. We must now, however, lay aside the electrical machine, the electrical kite, and every source of palpable electric action, and resort to a very different mode of experimenting to that hitherto pursued. We must now prepare our frogs in a particular manner, according to the directions of Galvani, and operate on them as first shown by that distinguished philosopher. The preparation of the frogs is thus to be carried on. I take hold of the hind feet with my left hand, whilst an assistant takes hold of the fore feet ; and, whilst the animal is thus stretched between us, I cut it in two with a pair of scissors, directly across its body, a little below the shoulders. The fore part of the body is thrown aside as useless, and the entrails cleared away from the other part, which is intended to be the subject of experiment. The skin being partially cleared from the upper part of the remaining spine, my assistant takes hold of the latter between his thumb and fore finger, holding it fast till I have stripped the whole of the skin from the thighs and legs. The crural nerves, which are two bundles of white silken-like cords, parallel to one another, are now to be cleared of all fleshy matter, and from the 27 bones which are behind them, so as to be free from every other part but the thighs, and a morsel of the spine, which I leave attached at their superior extremities. Thus prepared, I place the morsel of spine upon a piece of zinc, z, and the feet upon a piece of silver, s, as represented by fig. 5. With this arrangement the Fig. 5. limbs remain perfectly motionless. I now, by means of a copper or silver wire, c, make a con- nexion between the silver and zinc, at which moment the thighs, F, and the whole of the lower extremities, are thrown into violent agitation, quivering and stretching themselves lengthwise, in a manner too singular for language to describe. If, however, I keep the wire in close contact with the silver and the zinc, the phenomena exhibited by the frog's limbs are but of momentary duration ; but they are again renewed when the wire, c, is withdrawn. Thus it is, that by keeping one end of the wire in close contact with either the silver or the zinc, and moving the other end rapidly so as to make a series of momentary contacts with the other metal, the limbs may be kept in a state of continuous agitation. The wire, c, when thus employed, is called the conducting arc; and the whole arrangement, when the wire unites the other two metals, as represented in fig. 5, is called a Galvanic circle. When either end of the conducting arc, c, is lifted up so as to quit the metal on which it rested, the circuit, or 28 circle, is said to be opened, and a renewal of the contact closes the circle ; hence, in Galvanic language, the frog, in the last described experiment, is agitated by a series of alternate openings and closings of the Galvanic circuit. For some time after Galvani had discovered these extraordinary facts, it was the practice to fold the crural nerves and morsel of spine in a piece of tin foil, for no other purpose, it was said, than that of securing better contact ; but I should wish it to be distinctly understood that, when the nerves were so armed with the tin foil, it was that metal, and not the zinc, on which it was placed, that tended, in conjunction with the silver, on which the legs were laid, to bring about the convulsive movements. To prove this fact I will enclose the crural nerves in one piece of tin foil, and the feet in another piece ; both parts are thus said to be armed. I now place one of these armed parts upon a piece of sheet zinc, and the other upon a half-crown, taking care that no part of the animal touch either of these metals. I now close the circuit by a conducting arc, but no motions of the animal are perceptible. I open and shut the circuit rapidly for many times, but still the usual phenomena do not appear. If, however, the limbs had been those of a healthy vigorous frog, and the experiment carried on immediately after their removal from the body, some slight agitation would have taken place by this mode of proceeding ; but the subject now operated on is too feeble, and too much exhausted of its irritability, to be affected by it. Having now satisfied ourselves that the animal's powers are too dormant to be awakened by this mode of experimenting, I will remove the tin foil from the feet, and place them upon the bare silver coin. Under these circumstances, we soon discover that the dormant animal powers are aroused into a state of great activity whenever the circuit is completed by the conducting arc. To vary the experiment 29 once more, I remove the tin foil from the crural nerves, and place them naked upon the zinc plate. On closing the circuit now, the convulsive struggles become still more violent than when the crural nerves were encased in the tin foil ; proving that zinc is a more suitable metal than tin foil for the display of this class of phenomena : and if we were to replace the silver by tin foil, we should soon discover, by the same criterion, the inferiority of the latter metal. It is thus by experimenting with different kinds of metal, that we are enabled to understand which two, when acting in concert, are productive of the greatest effect. Zinc has hitherto held a supremacy as one of the metals in experiments of this kind ; and silver, gold, or platinum, the other. Copper and zinc answer very well in the capacity of a Galvanic pair, as the combination is termed, and copper is more frequently employed than any of the noble metals. In the experiments on frogs by a Galvanic pair of metals, it is not necessary to place the feet upon the silver, and the crural nerves upon the zinc ; for the convulsive motions will be produced when the animal is placed on these metals in the reverse order, that is, with the feet upon the zinc and the nerves upon the silver. Nor is it of much consequence which metal is employed for the conducting arc. Indeed, a metallic wire is not absolutely necessary for this purpose ; for charcoal, or even water, as I shall presently show you, will answer very well for the conducting arc: and I will now show you, that a morsel of charcoal may substitute even the silver in the formation of a Galvanic pair. The experiment I am now about to introduce, will explain several particulars in performances of this kind ; it is, therefore, highly instructive to an attentive observer. I tie, by means of thread, a small piece of charcoal to the spine of a prepared frog, and upon a slip of zinc, to which one end of a metallic wire is soldered, I place the thighs of the D2 30 animal, as represented by fig. G. I Fig- <> now take hold of the wire, and complete the circuit by bringing its loose end into contact with the char- coal, and immediately the animal is convulsed ; and, as in previously shown experiments, it is again con- vulsed when the circuit is opened'. In this case, it is proved that charcoal answers all the purposes of silver in experiments of this kind ; and also, that it is not necessary that the feet should rest on one of the elements of the Galvanic pair ; which is still further proved, if necessary, by cutting them off, and operating on the remaining parts only. And another variation of the experiment shows, that the convulsive motions can be produced by operating on the exterior muscles only, and thus dispensing with the crural nerves. If, for instance, I remove the crural nerves by scissors, and place the thick part of the thighs upon a piece of zinc, and the feet upon a half-crown, neither metal will be in direct contact with the nerves. Still, however, it will be found, on completing the circuit with the conducting arc, that the usual phenomena are produced, though in a minor degree. If, whilst using the zinc and charcoal, as repre- sented by fig. 6, we permit the two to be firmly united by the conducting wire, we may open and shut the circuit by lifting the frog's thighs from the zinc, and again bringing them into contact with that metal. The limbs are agitated as decidedly by this operation, as when the circuit is opened and closed by the conducting wire. Indeed, it is a matter of no consequence in what part of the circuit the opening be made, the effect is still the same ; but as the apparent struggles of the animal are best shown when the limbs are quite free to move, the most commodious mode of operating is, by employing the wire for opening and closing the circuit. There is 31 also another particular to be observed when the best effect is required. This is the bending of the knees of the frog so as to draw the feet up to the thick part of the thighs, when placed on one of the metals ; for as there is always a tendency to stretch or throw the legs outwards in the direction of their length, when the circuit is closing, these motions are best shown when the limbs are previously bent in the manner described. I will now bring forward a few experiments which will inform you that to Galvanize the limbs of a frog, a metallic contact with it is not absolutely necessary. Fig. 7 will give an idea of the apparatus I shall first employ in this illustration. The two Fig. 7. small jars, which may be either of glass or of porcelain, are nearly filled with water. The two portions of water are connected with each other by a metallic arc, having a piece of zinc soldered to one end and a small silver coin to the other, as represented by the figure. I take hold of the feet of a prepared frog with the finger and thumb of one hand, and in the other hand I hold a metallic wire. Thus prepared, I dip the morsel of spine, which hangs by the crural nerves, into the water of one of the vessels, and close the circuit by touching the other piece of water with the wire. The frog is strongly convulsed at every contact that is made between the water and the wire. If I permit the wire to remain immersed, and occasionally open and close the circuit by alternately lifting and dipping the frog from, and into, the water, the phenomena produced are precisely the same by this process as by the other. The experiment is pleasingly and instructively varied, by having a circuit of several persons between the limbs of the frog and the water in the vessel. Suppose, for instance, eight or ten persons join hands, so as to form a chain from one end to the other. The disengaged hand of one of the extremi- ties of the chain, or range of persons, is to take hold of the legs of the frog, and the disengaged hand at the other extremity of the range, is to be immersed in one portion of the water. This done, the pendent spine of the frog is to be occasionally dipped into the other portion of water, which, in every case, will occasion the usual convulsive commotions. In this case the conducting arc is the chain of persons forming the greatest portion of the circuit. This experiment may be varied in different ways. For instance, the spine of the frog may be permitted to remain immersed in one portion of the water, and the circuit may be closed and opened by the hand at the other extremity of the arrangement. It may also be varied by keeping immersed both the frog at one end and the hand at the other end, the openings and closings being accomplished by any two adjoin- ing hands in the arrangement. But the most singular effect is produced by holding the spine in the hand, and permitting the pendent toes of the frog to just touch the surface of the water, the remaining part of the circuit being previously completed. By this simple immersion of the limbs, the most singular antick motions are exhibited by them. The toes are thrown out of the water at the first contact, which opens the circuit ; they immediately fall down again and close it, at which moment they are again con- vulsed and thrown out, and again the circuit is 33 broken. They again fall down, and are again thrown out ; and thus the action may be kept up for half an hour, or longer, if required, a dance by the detached limbs of the frog being performed all the while, to the no small amusement of the spectators. If, instead of having so long a circuit, which always weakens electric action, there were but one person engaged in the experiment, immersing one of his hands in one portion of the water, and with his other hand taking hold of the crural nerves, the frog steps are displayed, on the aqueous stage, in a first rate style, and especially when the experiment is made on the limbs of a vigourous frog, and immediately after their being detached from the body. In all cases where the hands form a part of the circuit, they ought to be well moistened with water, to increase the conduction of the skin. By paying attention to this particular, the electric action meets with less obstruction, and the animal commotions are more eminently displayed. The next experiment which I shall offer to your notice requires nice electrical discrimination to un- derstand it : and, of course, it will require all the attention you can afford to give it that degree of interest which it really deserves. Having an already prepared frog, I hold it by the crural nerves in one hand, and permit the limbs to hang pendent over a small plate of zinc which is placed on the table ; and by permitting the toes to touch the zinc plate you will behold the most singular frog's hornpipe you ever witnessed. But what can be the cause ? You see only one metal, and no Galvanic circuit ! In all our former experiments a circuit could easily be seen ; and permit me to tell you, that no animal commotions by Galvanism can possibly be produced independently of a complete circle of conductors. Therefore, although the circuit in this case is not so palpable as in our previous experiments, the pheno- mena that you have witnessed are sure indications of its existence ; and as these experiments are intended 34 to teach, and not to mystify, I will now let you into the secret of the arrangement, by which means you will as easily trace a Galvanic circuit in this case as in any other. If you will have the goodness to remove the zinc plate, and examine the spot where it laid, you will perceive the head of one of the nails which holds the top of this old table to its frame. Now this nail being in contact with the zinc whilst the experiment was carrying on, you can have no difficulty in dis- cerning a Galvanic pair of metals ; and by consider- ing that the table is not an insulator, though a very bad conductor, it serves to form a part of the con- ducting arc. The floor also which lies between the foot of the table and my boots, forms another part of the circuit. Then, since you are well aware that the human body is a good conductor, and that the soles of my boots conduct partially, you are easily led to admit, that the circuit was complete whenever the frog's toes touched the zinc plate, and that it was opened every time they sprang from it. To convince you that the circuit was in the direction I have pointed out, we will repeat the ex- periment, and afterwards slightly vary it. Being satisfied that the frog dances as before by the same arrangement, I will now step on a resinous cake, and the dancing immediately ceases ; because the resinous cake insulates me from the floor, and pre- vents the circuit from being closed. But if, whilst standing with one foot on the resin, I touch the floor with the toe of my other boot, the circuit again becomes closed, and the dancing frog resumes its hornpipe. To a perfect stranger, these performances, when the process is concealed, have a truly magical appearance. The illusion, however, is much more perfect, even sufficiently so as to puzzle a moderately skilled Electrician, when the zinc plate has no nail in contact with it. Old kitchen tables are sometimes burnt in various places on the upper surface, and when a piece of bright zinc is laid upon one of these burnt spots, a Galvanic pair is formed between the metal and the charcoal, which produces more violent commotions in the animal's limbs, than by the iron nail and the zinc. When the zinc is touched by a piece of silver or copper, held in the other hand, the circuit is shor- tened, and the Galvanic pair of metals more active than the zinc and iron ; hence, on both these accounts, the dance is much more animated than by the other mode of experimenting. If the muscles of the thighs are separated so as to hang together by no other tie than the nerves and spine, the commotions are still as active as before. Indeed, a single limb, entirely removed from the other, performs its steps in a very animated manner. When the legs are cut from a very lively and vigorous frog, the commotions are produced inde- pendently of any direct contact of the metallic elements of the Galvanic pair. If, for example, a piece of zinc be let fall to the bottom of a tumbler of water, and the feet of a prepared frog held in one hand, and a copper or a silver wire in the other, as represented by fig. 8, and the crural nerves of the animal be immersed in the water, then, as soon as the metallic wire touches another part of the water, a circuit is formed, which has the power of agitating the pendent limbs. This effect, however, is com- paratively slight ; but another action is pro- duced, more violent than the former, by pressing down the wire until it touches the zinc. The frog is now strongly con- vulsed. To give the best effect in this experiment, the hands of the operator ought to be well moistened 36 either with simple water, or with a solution of common salt, which is still better ; and if the vessel contain a solution of salt (muriate of soda) instead of water, the action is much enhanced. There is something very beautiful and instructive in this experiment. The first part of it shows that a Galvanic circuit can be formed independently of any direct contact of the metals forming the pair ; and the latter part teaches us to understand that, although the metals are surrounded by the water, which is a good conductor, and which consequently forms a- distinct circle with the metals when brought into contact, yet another circle is formed through the operator and the frog. LECTURE III. HITHERTO our experiments have been performed by means of two dissimilar metals, or of one metal and charcoal, which formed what is called the Galvanic pair ; but it was shown by Galvani, that the presence of two metals is not absolutely necessary to accom- plish commotions in the limbs of a frog, when the animal is strong and in a lively condition, prior to its being dismembered for experiment. It is only necessary, to produce the effect, that a silver wire form a conducting arc between the crural nerves and the muscles. It is true that the twitchings of the thighs, legs, and toes, are but feeble by this process ; but that they are absolutely produced is a fact which is easily proved by a repetition of the experiment. It is one of those facts on which Galvani laid great stress in support of his hypothesis of animal elec- tricity, although he still suspected that the con- ducting metal had some influence in producing the effect. Galvani, hawever, proposed several methods of experimenting, independently of the aid of metals ; 37 and the experiment now about to be described, shows in the most satisfactory manner, that neither metals, charcoal, nor any intervening body whatever, is absolutely requisite to convulse the limbs of a pre- pared frog. It is an experiment first instituted by Professor Aldini, the nephew of Galvani, and shown at the Institute of Bologna, in the year 1794.* Fig. 9 will afford a good idea of the method of making this experiment. The frog is held up by a Fig. 9. glass rod, H, which passes between the crural nerves, R, attached to a morsel of the spine, s. By means of another glass rod, M, one of the legs is so managed as to bring the joint, L, into contact with the crural nerves ; and, by this simple process, the limbs become convulsed. To insure success in this deli- cate experiment, it is well to immerse the whole of the limbs and nerves in a solution of common salt (muriate of soda), previously to making the experi- ment. The animal's motions are very slight, but sufficiently decisive to show the fact. There can be no doubt entertained, but that an electric action from the metals, when employed, operates extensively in the production of those violent struggles which the animal displays ; but in the * Aldini's work, p. 17. F, 38 experiment last described, no metal being employed, and the animal completely insulated by means of the glass rods, the convulsive movements were obviously due to an electric action emanating from the animal alone. The effects on animals, due to a commotion or discharge of their own electric fluid, from one part of the system to another, is still more satisfactorily shown by a series of prepared frogs, as represented by fig. 10. In this figure, M N is. a bent glass rod, Fig. 10. and A B two prepared frogs. The feet of the lower frog are tied, by thread, to the spine, c, of the upper one ; and the feet of the upper frog, A, are tied to the projecting arm of the glass rod. A thin glass rod is now to be introduced between the crural nerves, E, of the lower frog, and lifted up until the spine, D, is brought into contact with the feet or legs of the upper frog, A. At every con- tact, both frogs are convulsed. I have produced a similar effect by a chain of six frogs, the whole of which were convulsed whenever the contact between the spine, D, and the legs, A, of the upper frog, was completed. Aldini, who was ever anxious to support the views of his uncle, prosecuted his enquiries on animal electricity with much vigilance and care, and produced many striking effects by operating on recently killed animals only, independently of the employment of any metal in his experiments. The two following experiments were successfully performed by this indefatigable physiologist. They are experiments I have never had an opportunity of repeating ; but the arrangements are so judiciously chosen, that there can be no doubt of satisfactory results being obtained. 39 Having provided the head of an ox, recently killed, Aldini pushed a finger of one of his hands, well moistened in a solution of common salt, into one of its ears. The spine of a prepared frog, which he held by the feet in the other hand, was then made to touch the upper part of the tongue ; convulsive motions in the frog were immediately produced, and were repeated whenever the contact was made between the frog and the tongue of the ox. Fig. 1 1 Fig. 11. STEEL is a representation of the arrangement in this famous experiment, which was performed in London in the year 1803. 40 The next described experiment, also Aldini's, is similar to the former, the only difference being in the length of the circuit. Having placed the trunk of a decapitated calf on an insulated table, a longitudinal incision was made in the breast, for the purpose of exposing a long series of muscles. Aldini then arranged two insulated persons in such a manner, that the one with a finger, well moistened in a solution of common salt, touched the spinal marrow of the calf, whilst the other applied the morsel of spine, hanging by the crural nerves of a prepared frog, to the muscles of the trunk. The disengaged hands of the two persons being previously moistened in the salt solution to improve the conduction of the skin, the circuit was closed by uniting them in the manner represented by fig. 12, which is also a Fig. 12. 41 representation of the whole arrangement. Every time this circuit was closed, the frog was strongly convulsed ; but no commotion occurred by simply touching the muscles of the calf with the spine of the frog, when the outer hands of the operators were not united. These highly interesting experiments can be repeated under those circumstances only in which the larger animals employed can be had as soon as killed. Aldini had many opportunities of experimenting on the bodies of decapitated criminals, and made several successful inquiries of this kind, by employing the human body in the same manner as the trunk of the calf in the last described experiment. The same sagacious physiologist made several other experiments in animal electricity, which can be easily repeated by any one who can command a frog of strong vital power. The frog is to be prepared in the usual way, and, after moistening it in salt water, the operator takes hold of the legs, and brings the crural nerves to his own tongue. The Galvanic circuit, in this case, is through the frog and the operator, from the hand to the tongue. The usual commotions are produced at every contact between the nerves and the tongue. If several persons form a chain amongst them- selves by joining hands, well moistened in salt water, and that the person at one of the extremities of the chain hold the legs of a prepared frog, contractions are produced whenever the circuit is closed by the disengaged hand, at the other end of the chain, and the crural nerves of the frog. Many persons have failed in repeating these experiments, from their not operating on large frogs of great activity ; and others by not attending strictly to the necessary preparations. The experimental illustrations hitherto brought forward in these lectures, would, of themselves, be sufficient evidence to convince most physiological enquirers, who have no preconceived notions to sup- port, that in the animal frame the different organisms E 2 42 of which it is constituted, are naturally charged with the electric fluid in different degrees of density, or intensity, as we frequently express ourselves ; as, for instance, the nerves and muscles have each a natural charge peculiar to itself, and in accordance with its susceptibility ; for the susceptibilities of becoming electrically charged, differ considerably in bodies of different kinds, indeed in those of the same kind, when differing in structure. Hence, since the organi- zation of muscle and nerve differ from each other, so must their natural electric charges differ also. But the most palpable cases of animal electric action are those displayed by the Torpedo, and the electrical Eels, which are endowed with such formidable electric powers as to destroy other fishes by their discharges, which they seem capable of delivering at their own will. It is thus that they procure their prey amongst the smaller tribes of fishes, and probably parry off their enemies amongst the larger ones, which otherwise, from their size and voracity, might devour them in abundance. I will not, however, interrupt the regular train of our elementary experimental illustrations, by a description of the electric organs of these remarkable fishes, in this place ;* nor will I dwell longer on animal electric action, since the experiments we have passed through afford a sufficient type of all that has been done in this department of our subject. There can be no doubt that, besides the electric action due to those animals which have been operated on, the sensitiveness of those parts of the frog employed in the experiments, render them exceedingly useful in the capacity of electroscopes, the convulsive motions of which were the only indications of electric action. The tongue, however, is an organ whose suscep- tibility of delicate electric impressions fits it admira- bly as an electroscopic apparatus in certain enquiries * These particulars will be found in the Appendix. 43 in this branch of physics. The experiment of Sultzer, already named in our first lecture, is pro- bably the first on record in which the tongue was intentionally employed as an indicator of a peculiar development now well known to be electrical. This experiment, and those variations of it by Creve and Fabroni, as pursued by Volta, Cruickshanks, Davy, and others, have been the means of throwing much light en chemical action, and affording more rational views of many processes of nature than those pre- viously taken. To proceed with this experiment in the most efficient manner, a plate of bright sheet zinc, and a similar plate of silver, gold, or platinum, ought to be employed : the one covering a portion of the upper part of the tongue, and the other the lower part, having their outer edges projecting so as to be conveniently held by the hands. Whilst the outer edges of the plates remain separate, no pecu- liar sensation is experienced ; but the moment that these edges are brought into contact with each other, the tongue is made to quiver, and a disagree- able acid taste is produced. The quivering of the tongue is but of momentary duration, if the metals be kept close together ; but the acidity in the mouth increases as the period of contact is prolonged. On making the metallic contact, a sudden flash of light sometimes darts across the eyes ; and as this phenomenon is best observed when the eyes are closed, the flash obviously passes through some part of the organ of vision. But the flash of light is most successfully produced, when one of the metals is placed on the tongue, and the other pressed close upon the gums beneath the upper lip. With this arrangement, the flash is constantly produced at the moment of contact of the outer edges of the metals. In both these experiments, the effects produced are greater as the surfaces of the metuls exposed to the tongue are increased ; and if that organ were to be bathed in salt water the moment before the metals 44 are applied, the effects would be still greater. A draught of porter considerably enhances the effects. The experiments of Messrs. Creve and Fabroni are those which next require our attention. The inquiries of these two philosophers were chiefly directed to the cause of Galvanic phenomena, which the few experiments that they instituted, led both of them to imagine was of a chemical nature. If a slip of zinc, and another of copper, silver, gold, or pla- tinum, be placed in a sloping position in a glass vessel nearly filled with water, in the manner repre- sented by fig. 13, and if now the tongue -pig. 13. be applied to any part of the surface of the water between the metals, nothing particular is observed ; but if whilst the tongue is still immersed, the lower edges of the plates be made to touch one another, an acrid taste is immedi- ately produced. This is an interesting variation of the Sultzerian experiment; for it teaches us that metallic contact with the tongue is not absolutely necessary to pro- duce the peculiar sensation in that organ, which, in this case, as in the original experiment, is the electro- scope. The effect on the tongue, however, is far more feeble than that produced by the original experiment of Sultzer. This experiment is susceptible of other interesting variations. When the vessel holding the water is somewhat large a glass basin, for instance the tongue may be immersed at different distances from either of the metals, whilst they remain in contact with each other. When the tongue is about half-way between the metals, the effect is scarcely perceptible, but becomes stronger as it approaches either of them. Moreover, the acrid taste is experienced, although the tongue be immersed in any other part of the water than that which lies directly between the metals, which shows that the influence occupies every part of it. This latter fact was not noticed by 45 the above named philosophers, nor did they give any satisfactory reason for the effect being greatest near the metals. From some chemical effects which Fabroni observed, he supposed every Galvanic phenomenon to have a chemical origin. This saga- cious philosopher discovered that the zinc was always oxidized in these experiments, and consequently, that the water was decomposed ; and by employing solutions of various kinds of salts, he was enabled to form new compounds in crystallized forms. To Fabroni, therefore, the honour is due of making the first electro-chemical discovery, and of inventing the chemical hypothesis of Galvanism, which, even at this day, is the favourite phantom of certain chemists, both in this country and on the continent. More- over, as Fabroni' s experiment is the first in which a Galvanic pair of metals were associated by means of inorganic matter an association which constitutes one complete member of the Galvanic battery, which now universally bears that name the invention of that noble instrument necessarily dawned with Fabroni's experiments, and its history is to be traced to the admirable researches of that philosopher. To repeat this memorable experiment in the most satisfactory manner, we place a piece of very clean zinc at the bottom of a glass of water, and upon the zinc is to rest a slip of very clean copper, or silver. In the course of a few minutes, or a quarter of an hour, the zinc around the point of contact with the other metal will become blackened, and by per- mitting the action to continue uninterruptedly for several hours, the whole of the upper surface of the zinc will become covered with the black matter, which is a thin layer of the oxide of zinc. On in- specting the copper before the apparatus is disturbed, its surface will be found covered with exceedingly minute bubbles of gas, which is hydrogen. If the slip of copper has an edge resting on the zinc, a line of black matter (oxide of zinc), directly under it, will be formed in the course of a few minutes ; and 16 as the copper can be curved in any way the experi- menter thinks proper, devices of any required form may be obtained on the upper surface of the zinc. I have the word FAB RON i formed on the lower edge of a slip of copper, by means of which I occasion- ally in my lectures, imprint the name of the author of this first of electro-decompositions of water by a direct Galvanic association of metals. As it is not my intention to enter very minutely into the theory of Galvanism, until the next and sub- sequent lectures, I shall, in this place, offer to your notice only a few experiments from which Galvanic phenomena have been supposed to have an electric origin. I have already stated that, in the Franklinean theory of electricity, it is supposed that each indi- vidual kind of matter has a specific quantity of the electric fluid naturally belonging to it, so that no two bodies of the same size, if of different kinds, contain equal quantities ; which view of the natural distribu- tion of the electric fluid amongst terrestrial bodies, appears to be perfectly correct, at least under certain circumstances. I shall, however, have to show you, as we proceed, that the distribution of electric matter, even in one individual body, may be, and more fre- quently than otherwise is, very different in different parts of it ; depending on its state of aggregation, crystallization, compactness, state of its surface with regard to polish, temperature, &c. ; and if it can be shown that different parts of the surface of one and the same piece of metal, which are attached to each other by the best conducting union, are nevertheless in different electric conditions at one and the same time, it is easy to imagine that two distinct kinds of matter may be in different electric conditions, even when in close contact with each other. The experiment of Lichtenberg, already explained in my " Lectures on Electricity," shows this fact in a more striking and interesting manner than any with which I am acquainted ; and as it is one of high 47 importance in preparing the mind for reasoning on the modus operandi of Galvanic action generally, I shall repeat it in this place. The apparatus required for the performance of this experiment are an elec- trical machine, a small Ley den jar, a flat cake of pitch, a small tin can with a glass stem for insulation, and a spring puff or bag, containing a mixture of sulphur and red lead. The resinous cake being quite dry and free from dust, I place the little can upon it at some short distance from the middle of the surface. I then electrize the inside of the jar negatively, either by placing its knob in connexion with the negative conductor of the machine, or by holding its outer coating against the prime conductor : (the former way is preferable, because the hand has already hold of the body of the jar) which done, I present its ball to the can, from which it recovers a small quantity of its lost fluid. The can, consequently, becomes electro- negative, and its rim in connexion with the resinous cake renders a ring of the latter also negative. I now lift up the can by its glass handle, and for a moment touch it with my finger to restore its electric equilibrium. The can is then placed on another part of the resinous surface, the jar dis- charged, and again recharged with its ball at the prime conductor. A spark is now transmitted from the ball of the jar to the can, which renders it electro- positive ; and a considerable portion of this charge is transmitted to the resinous cake, spreading outwards by its repulsive attribute, on every side of the can ; and when the latter is removed by its glass handle, the electric fluid advances inwards also, until the forces on opposite sides of the centre, balance one another, and completely arrest the fluid's further advance. The cake being thus differently electrized on two parts of its surface, is placed in a vertical position ; and then standing at a considerable distance, I project towards it the mixed powers from the spring bag, 48 which, whilst traversing the air between the bag and the electrized surfaces, became influenced by the latter, and attracted by them, according to the strict prin- ciples of electric action : the sulphur attaching itself to the positive surface, and the red lead to the nega- tive one. The beauty of this experiment is beyond all description, and, in a theoretical point of view, it is one of the highest importance. Fig. 14 will serve to Fig. 14. give an idea of the arrangement which the powders assume on their respective places on the resinous cake, but nothing short of a personal view of the phenomena can convey to the mind their real beauty and importance. A momentary glance at these electrical pictures delights the eye, by the beauty and singularity of their configurations ; and when it is known that they are of electrical production, the curiosity becomes quickly aroused, and the attention immediately fixed upon them. To the mind of the philosopher these phenomena convey a rich fund of information. Volumes have been written to show the similarity of positive and negative electric action, as indicated by the spark, the shock, the attractions, and the repul- sions, &c, ; and much has been said and written with a view of supporting the contrary opinion. But the experiment now under contemplation, carries convic- tion to the mind more forcibly than all the rest that have been brought forward on either side of the 49 question. The first view of these pictures is of itself sufficient to distinguish a striking difference in their configurations, and the mind is readily impressed with the idea that their formation arises from different causes. On close inspection, we perceive a seeming animation in the positive or stellar configuration, which expands "before our eyes, gradually and uni- formly on every side, until the repulsive electric forces become too attenuated to advance the particles of sulphur any further from the centre of action. Inward also from the ring on which the metallic can stood, the particles of sulphur are seen creeping in converging paths towards a centre which they can only approach, but can never reach, being arrested in their advance by a balance of those very forces which urged them on towards it. The negative picture, on the contrary, is dull, without life, and a perfect contrast to the other, yet not without interest. It shows, in the first place, that it is not endued with any force similar to that in the other picture. No repulsive forces are indicated by the motions of the minutest particle of the powder ; nor are any ramifications produced, even though the apparatus be permitted to rest unmolested for many hours ; the picture remains unchanged from first to last, a mere heavy looking ring of red lead. Around this ring, however, at a considerable distance, and also within it, when its dimensions are considerable, there adheres a portion of the sulphur, whose subse- quent motions, with regard to the centre, are in the reverse order to those in the positive picture ; for the particles of sulphur within the ring radiate towards it from the centre, and those exterior to the ring travel inwards ; hence both portions advance upon the ring of red lead, according to the laws of elec- trical attraction between two dissimilar electrized bodies. In short, the phenomena exhibited by this experiment afford the most satisfactory demonstration of the distinctive character and capabilities of these two specks on the resinous surface, and of the just I 50 appropriation of the terms positive and negative, as expressive of their real electric states. On the positive surface, we have occular demonstration of the existence of a repulsive force ; on the negative surface there is no indication whatever of any such force being present. Moreover, we learn from this experiment that, although intimately connected with each other, the sulphur and red lead were in different electric conditions. This fine experiment is susceptible of many variations, each of which is full of beauty and interest. Metallic forms of the outlines of animals, trees, houses, and other objects, may be employed in place of the can ; and these may be rendered positive or negative at pleasure, according to the fancy of the operator. Different kinds of powders may also be used for the formation of the pictures. When the different coloured lakes, pigments, and other im- palpable powders are used, the pictures, may be made exceedingly elegant ; and they may be rendered permanent, and framed, when formed on glass, wax, &c., which substances answer quite as well as pitch. When formed on glass, another piece of glass is placed upon the picture, which is thus held fast between the two. The wax, or other substance that will melt by a gentle heat, will easily adhere to the electrical pictures, by holding over them a hot iron, or by presenting them to a gentle fire. Many other methods might be pointed out which will readily occur to the ingenious electrician. The Rev. Abraham Bennet was the first philosopher in this country who repeated Lichtenberg's experi- ment ;* and as it led him to others in which he dis- covered a difference in the natural electric conditions of * The Rev. Abraham Bennet, when curate of Wirksworth, Derbyshire, diversified this experiment probably more than any other person. This is the same ingenious electrician who invented the gold-leaf electroscope. Many years previously, however, a similar electroscope, made of two narrow strips of tin foil, was used by Baccaria. 51 dissimilar metals, a knowledge of which is important in this place, I will point out the method of experi- menting pursued by this philosopher. Into a small copper shovel, furnished with a glass handle, is put some coarse zinc filings, or other morsels of that metal ; the shovel is then held over .the cap of a gold- leaf electrometer, and the zinc filings permitted to roll out of it on to the cap. The gold leaves diverge with positive electric action. If now the shovel he tested by an electroscope, it will be found to be electro-negative.- These metals still exhibit the same relative electric action, when the shovel is of zinc and the filings of copper, which proves that the mode by which the experiment is made does not interfere with the electric character which the metals acquire by their simple contact with each other ; a fact which is still further supported by another mode of experi- menting presently to be brought forward. Some experimenters use a metallic sieve in place of the shovel, and shake the filings of the other metal through the sieve, so that they may fall on the cap of the electroscope. By a similar process, the electric character of any metal, acquired by contact with any other body, metallic or otherwise, may easily be ascertained. The idea of electric action being developed by the metals employed in his experiments, seems to have originated with Galvani himself; and the experiments last described, have shown the correct- ness of this theoretical view. This principle now enters essentially into the theory of Galvanism ; so far at least as the metals are concerned in the development of Galvanic action. But the original experiment of Lichtenburg proves also that other bodies than metals develope electric action by mere contact with one another ; and as Galvanic arrange- ments are never entirely of metallic matter, but con- sist partly of metallic and partly of non-metallic bodies, the electric actions of all the bodies which enter into a Galvanic circle are essentially concerned, 52 either directly or indirectly, in producing the resultant or principal effect. In all the Galvanic experiments hitherto described, in which metals were employed, those bodies had obviously a predominant influence over the other matter which entered the circle ; and what is to be particularly remarked is, that through the instrumentality of the animal electroscope (pre- pared frog), we are enabled to detect the action thus displayed, independently of the ordinary means of insulation. LECTURE IV. VOLTA, who had studied the electricity of the metals with great assiduity and success, brought forward, in the year 1800, his interesting experiment with the metallic discs, which still bear his name. The Voltaic discs are of copper and zinc, one of each, and usually of about six inches diameter. Each disc is furnished with a glass handle, as represented by fig. 15, for the purpose of insulation from the hands of the operator. The other pig. 15. face of each disc is perfectly flat and polished, to insure as exten- sive contact as possible, when those faces are brought together. As the electric action developed by these discs is exceedingly feeble, when compared to that shown with the metallic filings in Bennet's experiments, it is usual to employ a condenser in connexion with the electroscope, in order to obtain satisfactory results. The electroscope, with its condenser, is represented by fig. 16.* Before the experiment is performed, * To those who have read my "Lectures on Electricity," this instrument will be familiar ; but as this volume is likely to fall into the hands of many who have not seen the other work, a description of its use in this place may be useful, and even convenient to all. For the theory of the condenser, I must refer the reader 136 I showed that a Galvanic pair consisting of copper and rolled zinc, has a much superior action to that exhibited by a pair consisting of copper and cast zinc. Indeed, I made several small batteries in which the metals were cast and rolled zinc. In the same pam- phlet I have detailed several curious results which I obtained by experimenting with iron, which led me to the formation of Galvanic batteries with iron and amalgamated zinc, which I found were more powerful than when the latter metal was associated with cop- per. I also discovered that, in this capacity, cast iron has a superior action to hammered or sheet iron ; hence, it was easy to perceive, that a battery consist- ing of cast iron and amalgamated rolled zinc, would be the most powerful of the whole. Such, indeed, is the excellency of this battery that, notwithstanding the several forms that have subsequently appeared, it yet stands pre-eminent, both as regards economy and action, in all those employments where quantity, without high intensity, is required. The cast iron battery, in its present form, consists of a series of hollow cast-iron cylinders, about eight inches high and four in- ches diameter, to each of which is united, by means of a stout copper wire and solder, a cylinder of amalgamated zinc, as re- presented by fig. 58, in which i is the iron, and ^1 z the amalgamated zinc. The iron cylinders are placed in porcelain jars containing diluted sul- Published by Sherwood, Gilbert and Piper. Previous to the pub- lication, Mr. Kemp, of Edinburgh, published a series of very interesting experiments, in which he employed a soft paste of the amalgam of zinc. See "Annals of Electricity," vol.i. page 81. In most of the batteries that have been contrived since the publication of my pamphlet, amalgamated zinc forms one of the metals. 137 , phuric acid, and each zinc cylinder is placed within the iron of the next jar in the series, as is represented by fig. 59. The jars holding the acid liquor are not Fig. 59. shown in the figure. R and n are portions of the conducting wires, and the arrows indicate the direction of the current when the circuit is closed. We shall have to employ this battery as we proceed in our electro-chemical illustrations. Some time previous to the appearance of my pamphlet, M. Becquerel, of Paris, had taken advan- tage of the action of two dissimilar liquids and one metal, (as first shown by Sir Humphry Davy), by means of which he was enabled to form some chemical compounds similar to those formed by nature, and of which we shall have to say much more in another place. In the year 1836, Professor Daniell brought for- ward a battery of large size upon nearly the same principle, employing two liquids and two metals. This formidable looking battery consists of a series of copper cylindrical vessels, usually two feet hi^h, and about four inches diameter. In each copper ves- sel is placed another cylindrical vessel made of porous pottery ware. This latter vessel is of the same height as the copper one, but very narrow. It is charged with diluted sulphuric acid, in which is placed a rod of amalgamated zinc ; and the annular space between N 2 138 the porous vessel and the copper one, is filled with a strong solution of sulphate of copper. The rod of zinc in the first copper is connected with the second copper, and the zinc of the second copper with the third copper, and so on. The connexions are made by means of copper wires and binding screws, in the manner represented by fig. 60, in which the porous Fig. 60. cylinders are represented by the dotted lines, and the zinc rods by the full straight lines between them. Shortly after Professor Daniell's battery made its appearance, Mr. Mullins brought forward one, con- structed of the same kind of materials, but with the zinc exterior to the copper. It consists of a series of porcelain jars, in each of which a cylindric scroll of zinc is placed, which is nearly the size of the in- terior of the pot. Within this zinc cylinder is placed a cylinder of sheet copper, surrounded by a piece of bladder. The space unoccupied by the zinc, between the bladder and the pot, is filled by a solution of sul- phuric acid ; and the interior space is filled with a solution of sulphate of copper. The pairs are con- nected in series by means of copper wires. The most powerful battery that has hitherto made its appearance, is one contrived by Professor Grove. Its metals are platinum, and amalgamated rolled zinc ; and the liquids employed are pure nitric acid, and diluted sulphuric acid. The platinum is immersed in the former, and the amalgamated zinc in the latter liquid. The metals and liquids are placed in rectan- 139 gular parallelopipedon pots, two of the vertical sides of which are broad squares, and the other two and Fig. 61. the bottom are comparatively narrow. Within the exterior pot, which is acid proof, is placed another which is porous. The latter pot holds the nitric acid and the platinum plate ; and the space which is un- occupied by this pot, holds the zinc and diluted sul- phuric acid. The zinc sheet is amalgamated, and bends round the bottom of the porous pot, and pre- sents itself to both of the square sides. An edge view of the zinc is pretty well represented by the shaded part c, of fig. 29, page 66. The connexions of the pairs with each other are the same as in all other batteries, with the exception of the mode of holding the plates together, which, in this battery, is by means of small brass vices with binding screws. Fig. 61 is a representation of a vertical section of three of the pots and their metals, as seen edgewise. Mr. Smee has fitted up a battery, each com- pound metallic element of which consists of rectan- gular plates of amalgamated zinc, between which is a platinized silver plate of the same size, and parallel to them. The liquid employed is a solution of sul- phuric acid. Several batteries have lately appeared whose elements are charcoal and amalgamated zinc ; plum- bago and amalgamated zinc, &c. But, with the 140 Fig. 62. exception of the Deflagrator, invented several years ago, by Dr. Hare, of Philadelphia, it will not be necessary to dwell with descriptions of any more of them in this place. The battery which Dr. Hare calls Deflagrator, consists of copper and zinc. The copper is formed into cases, about seven inches high, three inches broad, and half an inch in thickness ; and each case contains a plate of zinc, which is held at equal dis- tances from its sides, and prevented from touching it by grooved strips of wood. Each plate of zinc is soldered to the next copper in the series, in the man- ner represented by fig. 62, which is an edge view of a series of six pairs in sec- tion. The zinc plates z z z z z, reach downwards to the bottoms of the copper cases which are open at top and bottom, but not at any other place. One of the vertical edges of each zinc plate reaches just to the top of the copper case, but the other reaches much higher in order to meet the bent copper strap which is soldered to the upper edge of the next copper case. This form of the zinc leaves a triangular portion above the effective portions of the metals, as will be seen by looking at fig. 63, which is a re- presentation of a copper case c, attached, by a strap, to the upper angle of the zinc plate z, the trian- gular part of which only is seen, the other part being hid from view by another copper case in which it is placed. A series of fifty pairs, with thin veneers between the cases, is fixed in a wooden frame, open at bottom and top : and three of these series are placed, endwise, in a ma- Fig. 63. 141 liogany trough, which holds the acid liquor. In order to throw the whole series of 150 pairs into, or out of, action at pleasure, a second acid trough is fixed to the other, in such a manner that their open tops may be at right angles to each other, forming a right angle all along the connected edges from one end to the other. The two troughs revolve on an axis which is coincident with their angle of connexion ; the whole being supported by two upright posts, which are portions of a strong wooden frame, and through which the pivots pass. When one of these troughs has its open top horizontal, that of the other is vertical ; so that by a quarter of a revolution the acid is transferred from one to the other ; and as the Galvanic series is in one only, they may be brought into action, or thrown out of it, at pleasure, by these simple motions of the apparatus. LECTURE IX. ALTHOUGH our business in this course of lectures is not a display of Electro-Magnetism, there is a certain piece of electro-magnetic apparatus, called a Galva- nometer, which is almost indispensible in Galvanic illustrations in the present state of the science. Its principal value consists in the facilities which it affords in ascertaining the relative electric states of two bodies ; two metals, for instance, when united by wire and immersed in an acid or saline liquid. There are several shapes of the galvanometer, but that consist- ing of a hollow coil of copper wire, with a moveable magnetic needle Fig. 64. within, is most in common use. Fig. 64 represents a vertical section of the coil with its contained needle, IN- Fig. 65. which, when out of action, is to stand with its axis in the plane of the coil. The cups at the extremities of the coil-wire are for the purpose of holding mer- cury. If they be brass cups they are soldered to the wire, and their insides are amalgamated by means of a solution of nitrate of mercury ;* but if they be wood, as is frequently the case, the wire passes through the bottom of each, and projects a little upwards within. The portion within being amalgamated forms an union with the mercury, which is afterwards placed in the cup. Fig. 65, represents the horizontal face of the in- strument, which consists of a compass card gra- duated into degrees ; the upper part of the coil, and the magnetic needle, which is represented in the figure as if deflected by the action of an elec- tric current traversing the coil- wire. Although we cannot enter into an ex- planation of the theoreti- cal principles upon which this instrument operates, until we arrive at our lectures on Electro-magnetism, it will be necessary, in this place, to point out one essential consideration in its practical indications. In the first place, then, since the needle when at rest assumes a magnetic north and south position, the instrument must be so adjusted that the needle may * To prepare the nitrate of mercury, dissolve a portion of mercury in nitric acid, and evaporate the liquid till that which is left appears to be crystallizing. Dissolve this again in pure water and again crystallize. These crystals dissolved in water is the article used for these amalgamations. Any clean piece of copper or brass dipped into this liquor, becomes immediately covered with metallic mercury ; and in this condition is said to be amalgamated. 143 rest parallel to the wires in the upper part of the coil. This done, any electric current traversing the coil in a certain direction, will deflect the north end of the needle towards the east : but if the current traverses the coil in the opposite direction, the north end of the needle will turn towards the west. Hence, when we once know which way the north end of the needle turns by any current, the direction of which is known, the direction of any other current may be ascertained by the indications afforded by the deflected needle. It has already been shown, that the current brought into existence by a pair of copper and zinc plates when united by a wire, and immersed in an acid li- quor, traverses the connecting wire from the copper to the zinc. If, then, we employ this Galvanic pair as a standard of reference, and also mark the direc- tion which the north end of the needle takes when these metals are connected with certain cups of the Galvanometer, we can, from these standard data, at any time ascertain the direction of an electric current which traverses the coil- wire, by the action of any other pair of metals. Let us, for example, begin with the standard pair of copper and zinc. These plates may be of any size that we please, but when they are about two inches high, and one inch broad, they are as convenient as any. Having selected this size for the standard pair, we shall find it necessary, in some enquiries, to have all the other metals whose Galvanic actions are to be compared with the action of this standard pair, of precisely the same size and shape. But for the mere purpose of ascertaining the direction of the current produced by them, this nicety will not be required. Having adjusted the galvanometer, we place the copper and zinc plates, c and z, in a small porcelain jar, holding an acid liquor, and connect their wires with the mercury in the cups c and z respectively, as represented by fig. 66. The needle is immediately deflected, and we find that its north end has taken a 144 westerly direction. Hav- Fi S- 66 - ing ascertained this fact, we put the wires belong- ing to the copper and zinc into the other cups, so that they may change places. This done, the needle changes its posi- tion, having its north end now turned easterly. Being previously a- ware that the current flows from the copper to the zinc through the con- necting wire, we learn, from these last-performed experiments, that the needle's deflections are sure indicators of the direction of any current which flows through the coil -wire : and in order to facilitate our progress, whilst examining the Galvanic action of other metals, we must note down the position that the needle takes when the copper and zinc have certain connexions with the galvanometer, and fix upon these as standard connexions for this standard pair of metals. If, for instance, we fix upon the cup c, for connexion with the copper plate, and the cup z, for the zinc plate; we know that by these standard connexions the north end of the needle turns westward, which is its standard position for that direction of current. If, now, we remove the copper plate, and intro- duce to its place a plate of brass of the same dimen-. sions, the needle again takes a westerly deflection ; hence, we conclude, that the current through the con- necting wire is from the brass to the zinc, or in the same direction, as by a pair of copper and zinc. But it will be observed, when the needle is at rest in its deflected positions, by the two different pairs of metal, that it stood farther from the north by the copper and zinc, than by the brass and zinc ; we therefore conclude that the copper and zinc produced the more powerful action of the two. 145 We next remove the zinc, and connect the brass plate with its cup z, and the copper plate with the cup c. The needle is again deflected westerly, but nothing like so far as with the brass and zinc. This shows that copper and brass form a Galvanic pair, but of the least power of any pair that can be formed of these three metals. We will now replace the zinc, and with the cup c we will connect another plate of zinc. The result in this case cannot be predicted if both pieces be new, of the same size, and from the same sheet of metal. But the needle will immediately be deflected, which shows that a current traverses the coil wire ; and, consequently, that the two pieces of zinc are in dif- ferent electric states. If we do not disturb the metals in the acid liquor, the needle will slowly retire to the meridian line of the card ; at which position it indi- cates the total absence of an electric current, and a consequent electric-equilibrium of the two pieces of zinc at that moment. But, by watching the needle a little longer, it will be observed to commence a move- ment in the opposite direction to that in which it was first deflected, This indicates another current in the wire, which flows in the opposite direction to the former ; consequently the relative electric conditions of the two pieces of zinc have changed. This is frequently the case with two pieces of metal from the same mass, some of which will change their electrical characters several times in the course of a few minutes. Two pieces of iron are very subject to these electrical vicissitudes. By experimenting with two pieces of well polished wrought-iron, both being of the same size and shape, some very interesting phenomena are produced, which are well worth the notice of every Galvanist. If one of these pieces of iron, already connected with the cup z of the galvanometer, be immersed in a solution of muriatic acid, in which the water is be- tween twice and thrice the quantity of acid; and the other piece immersed shortly afterwards, and 146 connected with the cup c, the north end of the needle starts towards the west, showing a current to be running through the coil-wire, from the piece of iron last immersed. If, now, the first immersed piece be taken out of the acid liquor, and after a few seconds again immersed, the needle starts the contrary way, indicating a current from the last immersed piece. By removing from the acid liquor, and re-immersing first one piece of iron and then the other, several times, it is found that the current invariably flows from the last immersed piece. If one of the pieces be agitated in the liquor, the same effect is produced as by taking it out ; so that by shaking either piece whilst the other remains undisturbed, a constant current may be kept up for a considerable length of time ; and the piece which is shaken, and that which is at rest, will have the same electrical relation to each other as copper has to zinc, but the current produced is much more feeble than by the latter, or standard Galvanic pair. We will now take two other pieces of metal, one of which is cast zinc, and the other rolled zinc, and connect them with the galvanometer ; the cast- zinc with the cup c, and the rolled-zinc with the cup z. The acid liquor in the jar may either be the nitric, the muriatic, or the sulphuric. The needle's north end takes a westerly direction, indicating a current from the cast to the rolled piece ; and as this current is steady for a long time, and an opposite current never produced by this pair, we infer that there is a constant electric relation with these two pieces of zinc, similar to that existing in the standard pair. We now remove the cast zinc and replace it by the plate of copper. The needle is now deflected to a much greater angle than before. When it has come to rest we note its position on the card, or the degree to which the north end points. We now remove the rolled-zinc, and put the cast piece in its place. The needle becomes deflected as before, but when it has come to rest we find that the- deflection from the 147 meridian line of the card, is not so great as when the copper and the rolled-zinc formed the Galvanic pair. From these results it is natural to infer that a Gal- vanic battery formed of copper and rolled-zinc, would be more powerful than a battery formed of copper and cast-zinc, which is absolutely the case. This fact was first announced in the pamphlet already noticed ; and from that date, 1830, rolled-zinc has been more generally used than cast-zinc, which was the only shape in which zinc was previously employed in the construction of Galvanic batteries. We may now try the effect of two equal pieces of rolled zinc, having previously amalgamated one of them.* If we connect the amalgamated piece with the cup c, and the pure zinc with the cup z, an elec- tric current will traverse the coil- wire from the pure zinc to the amalgamated piece, as will be indicated by the needle's north end taking a westerly direction. This deflection is something considerable, and by try- ing each piece separately with a piece of copper, in the manner we proceeded with the two pieces of cast and rolled zinc, it is found that a pair of copper and amalgamated zinc, are much more powerful than a pair of copper and pure zinc. Hence it is, that amalgamated zinc is now employed in preference to pure zinc, in most of the modern forms of Galvanic batteries. These examples will be sufficient for giving you an idea of the usefulness of the galvanometer ; but we shall again have recourse to this instrument in some of our subsequent illustrations, by means of which its value, as an implement of research, will become still more obvious. Previously to the invention of the galvanometer, about the year 1822, philosophers had not the facili- ties they now possess for ascertaining the Galvanic relations which metals have to each other. Nor, * If zinc be placed, for a few seconds, in a weak solution of sulphuric acid, and when removed, suddenly plunged into quicksilver, it immediately becomes coated with the latter metal. It is thus amalgamated. 148 indeed, had they any method whatever of making successful enquiries on the action of a single Galvanic pair, excepting in a few combinations, of which the action is considerably powerful. The only available indications for ascertaining the direction of the cur- rent, were, the positions which, after the decomposi- tion of some chemical compound, certain elements took up in the circuit, with respect to the two terminal metals (platinum or gold), which conducted the current to and from the liquid operated on, in the manner already shown by the decomposition of sul- phate of copper, by the action of a single pair of copper and zinc. But as many Galvanic pairs may be formed, whose action alone is too feeble to accom- plish a separation of the elements of any known chemical compound, recourse was had to a series of these pairs, in the form of a pile, or of a couronne des tasses. The relative powers of various Galvanic combinations were ascertained by the quantity of oxygen and hydrogen gases that were liberated in a given time by the decomposition of water. Although these processes were attended with a great deal of labour, the ardour and perseverance of philosophers enabled them to overcome every obstacle that was met with ' in these researches ; and Sir Humphry Davy, who took the most active part in them, ascer- tained, by his own experiments, the electrical rela- tions of a considerable variety of bodies, both liquid and solid, when combined with each other in Voltaic pairs. To those combinations, which consist of two solids and one liquid, Sir Humphry gave the title of the first order ; and those composed of two liquids and one solid, he called of the second order. The following tables, which express the Voltaic relations of both kinds of combinations, were constructed by that eminent chemical philosopher. The solids are called perfect conductors, and the liquids imperfect conductors. 149 FIRST ORDER. Most oxidable Substances. Less oxidable Sub- stances. Oxydating Fluids. Zinc . . . With gold, charcoal, silver, copper, tin, ! Solutions of nitric acid in water, of iron, mercury. muriatic acid, of Iron . With gold, charcoal, ! sulphuric acid, silver, copper, tin. f &c. Water hold- Tin ... With gold, silver, ing in solution charcoal. oxygen, atmos- Lead . . . With gold, silver. . pheric air, &c. /'Solution of nitrate Copper . With gold, silver. . I of silver and mer- | cury. Nitric acid, 1^ acetous acid. Silver . . . With gold. . . . Nitric acid. SECOND ORDER. Perfect Conductors. Imperfect Conductors. Imperfect Conductors. Charcoal . Solutions of alkaline Solutions of nitrous Copper . hydro - sulphurets, acid, chlorine, mu- Silver . . . capable of acting riatic acid, &c. ca- Lead . . . on the first three pable of acting on Tin . metals, but not on all the metals. Iron . the last. Zinc . . . These tables were formed at a time when the general opinion of philosophers was, that it was essential in all Galvanic pairs, that the metal which occupied the place of zinc in the standard hattery, be the more oxidable of the two ; hence, in the table of the first order of combinations, the metals named in o 2 150 the left hand column are more easily oxidized than those in the second column, which stand against them, and with which, respectively, they form a Galvanic pair. The liquids in which the pairs are immersed, are placed in the right hand column. In both tables the electric currents are supposed to proceed from those bodies which occupy the second column, to those which occupy the left hand column, with which they are respectively associated, in the same manner as from the copper to the zinc in the standard battery, which will be found convenient as a type of reference. The doctrine above alluded to, and upon which these tables were founded, although it obtains in a considerable variety of cases, is by no means general, for there are many Galvanic combinations, now well known, in which the current flows, through the connecting wire, from that metal which suffers the greater degree of destruction by oxidation, to the other which suffers least of the two. And as these exceptions to the rule which the above doctrine sets forth, are essential data for the consideration of the theoretical Galvanist, and necessary to be known by every experimenter, I will bring forward a few specimens of them in this place. A specimen of the action in question is afforded in a beautiful and .striking manner by the Galvanic pair which is formed of two equal pieces of zinc, the one pure, and the other amalgamated. In the expe- riment already brought forward, it is seen that the pure zinc operates as copper in the standard battery. Now, by continuing that experiment for some time, and paying attention to the chemical action that goes on in the acid solution, we observe an immense quantity of gas rising from the surface of the pure zinc ; but scarcely a bubble ascends from that which is amalgamated. The former piece undergoes rapid destruction, whilst that of the latter is comparatively slow. So slow, indeed, that it will outlast five or six pieces of pure zinc, combined with it in succession. 151 The deflection of the needle in these experiments, is exceedingly steady, even when the pure zinc is reduced to a mere film of trifling dimensions. " I have observed a deflection of more than 10, for two successive hours, with two pieces, each exposing about one square inch of surface to the action of the acid solution, at the end of which time, the needle was perfectly steady at that angle, although the piece which operated as copper (in the standard battery) was nearly destroyed."* By combining iron with amalgamated zinc, and immersing the pair in a solution of sulphuric acid, a powerful chemical action immediately commences on the surface of the iron, but scarcely any change can be observed on the surface of the amalgamated zinc ; nevertheless, the electric current which the needle indicates, traverses the coil wire from the former to the latter metal. These facts are sufficient to show the incorrectness of the views that were taken respecting Voltaic pairs of the first order; and I will now offer to your notice a few experiments by combinations of the second order, the results of which are directly opposed to those displayed by the combinations in Sir Humphry Davy's second table. These experiments are made with the greatest facility by employing a galvanometer, and a vessel, divided by a bladder partition into two distinct watertight compartments, having the same kind of metal, but different liquids, in each compartment. -j~ A vessel of this kind is easily made by sawing down the middle from top to bottom, a small cylindrical wooden box, the two halves of which, after stretching a piece of bladder over the open face of one of them, are to be held fast together, with the bladder between, by means of twine. Should any leakage be ob- served, it may easily be stopped by wax or cement. * See my Pamphlet, previously alluded to, page 42. t Ibid. 152 If we operate with two equal pieces of iron, and place diluted nitric, acid in one compartment of the box, and water in the other, the metal which is placed in the acid solution suffers rapid oxidation, but that placed in the water scarcely any ; yet the current through the coil wire is from the former to the latter piece. " When a few drops of acid are mixed with the water, the electric energies become very much exalted, and the needle will frequently mark an angle of 75, particularly if the stronger portion of the acid solu- tion be not very feeble ; and these energies seem to improve with an increase of acid in that portion of the fluid."* In the cell containing the stronger acid solution, the chemical action becomes quite violent, and the liquid is continually changing its character, from the rapid disengagement of fumes of nitrous acid and oxide, and the impregnation of the re- maining liquid with dissolved oxide of iron, whilst that in the other chamber suffers but very little change. When nitrous acid is employed in place of the nitric 9 the phenomena are of the same character, but the current is more powerful. The piece of iron which is placed in a solution of muriatic acid, or of sulphuric acid, and which necessarily suffers more rapid oxidation than another piece placed in water, invariably operates in combi- nation with the latter, as copper with zinc in the standard battery. When similar plates of copper are employed, and dissimilar solutions of either the nitric or the nitrous acid, we have first a momentary current from that plate in the feeble solution, to that in the stronger ; but this current soon gives place to an opposite one, which indicates a change in the elec- trical relations of the two pieces to have taken place. This latter current is more powerful than its predecessor, and in consequence of its steadiness and * "Recent Experimental Researches," the pamphlet before mentioned. 153 permanency, being the conclusive one, may very justly be regarded as the proper and essential current due to this combination. When two similar plates of zinc are combined, and immersed in nitric or nitrous acid solutions, of very different degrees of strength, the currents indicated by the needle, are similar to those produced by the two pieces of copper. In strong and weak solutions of sulphuric acid, also, the piece immersed in the stronger solution, and, consequently, that which is suffering the more rapid oxidation of the two, invariably operates as copper in the standard battery. Hence we learn from all these experimental facts, that the rule of Galvanic action set forth in Sir Humphry's tables, although perfectly correct, with respect to the combinations which it exhibits, cannot be entertained as emanating from, or typical of, any universal law that exists in nature. In an additional PART, which Professor Cumming, of Cambridge, has given to his translation of "A Manual of Electro-Dynamics," by M. J. F. Demon- ferrand, another table of the Galvanic relations of metals, in acid solutions, is given, differing, in some respects, from that of the first order, by Sir H. Davy. The following is Professor Cumming's table, by which is to be understood, that any metal in the series, when combined with any of those above it, has an electric relation to the latter, similar to that which copper has to zinc in the standard battery ; but with respect to those metals which are below it in the list, it answers as zinc to copper in the standard battery. Potassium. Bismoth. Tellurium. Basium. Antimony. Gold. Zinc. Lead. Charcoal. Cadmium. Copper. Platinum. Tin. Silver. Iridium. Iron. Palladium. Rhodium. I know of no philosopher on whose experiments, in this branch of science, more reliance can be placed than on those of Professor Cumming ; and as far as 154 the pure metals are concerned, the above table is of essential value. But when the surfaces of the metals are amalgamated with mercury, their Voltaic- relations do not, in all cases, follow in the same order. From the results which we have obtained, with a combination of pure zinc and amalgamated zinc, the latter would necessarily stand higher than Reformer in the scale, according to the order of arrangement in the above table ; and from this it might probably be inferred, that because copper stands much lower in the scale than iron, a combination of copper with amalgamated zinc, would be more powerful than a combination of iron and amalgamated zinc. This, however, is not the case ; for whether in a single pair, or in a compound battery, the latter combina- tion is the more powerful of the two. There have been from the earliest period of Gal- vanism to the present day, two diametrical opposite opinions entertained, respecting the source or cause of Galvanic action, and the facts advanced in favour of each opinion, have assumed the character of a distinct theory. Hence, there are now existing, what are called, two theories of Galvanism ; one called the electrical theory, the other the chemical theory. The former originated with Volta, and supposes that every Galvanic phenomena emanates from the electricity which is developed by the simple contact of the bodies forming the series. The chemical theory, as has already been shown, origin- ated with Creve and Fabroni, and is, at this time, advocated by several eminent men. It supposes that in every Galvanic pair, the action essentially depends on a more rapid destruction of that metal which corresponds with the zinc in the standard battery, than of that which corresponds with the copper ; and it is on this principle that Sir Humphry Davy's tables were constructed. The facts already shown are, however, sufficient of themselves, to refute every hypothesis that can be framed upon such a basis : and the electrical theory. 155 ;ts Volta left it, although substantially correct in its general principles, does not apply them to those minute ramifications, or subdivisions of the action, essential to the propagation and maintenance of continuous electric currents. With respect to the theory of the dry electric column, and the water-charged battery, I know of none that will account for the phenomena upon principles differing from those which have already been explained in these lectures. But to apply the innate electric forces to the production of continuous currents, we must first trace their influence in accom- plishing those molecular changes which take place within the battery ; and also those transportations of matter, independently of which, an electrical equili- brium must of necessity prevail, and a statical repose would ensue in every part of the Voltaic system. These considerations, however, we must defer for the present, for the purpose of making you acquainted with some other phenomena, which may be made available in illustrating the theory of the electro- chemical action both within and without the battery. The decomposition of water by Galvanic action, in the experiments of Creve and Fabroni, was discovered some years previously to the formation of the pile by Volta. It occurred within the liquid which formed a part of the combination. But the discovery of the decomposition of water, exterior to the operating liquid in the combination, fell to the lot of Messrs. Nicholson and Carlisle ; who, by means of Volta's pile, detected the hydrogen that was liberated at the negative polar wire, and observed the change which took place on the positive wire (which was brass), by oxygen combining with it. When platinum wires were employed for the terminals, neither of them suffered tarnish ; and it was observed by these two philosophers, that oxygen was liberated at the posi- tive, or delivering wire, and hydrogen at the negative, or receiving wire. Since these discoveries were first made, a great 156 variety of apparatus have been invented for the purpose of exhibiting this interesting experiment to advantage. Fig. 67 is a representation pig. of one of these pieces of apparatus. It consists of a narrow but very stout glass tube, closed at one end, and embracing a platinum wire, which is hermetically sealed in it. The wire reaches within the tube about two inches, and terminates in a small knob. The tube is filled with pure water, and its open end slightly closed by a cork, through the centre of which passes another platinum wire, with a terminal knob similar to the former. The knobs of these two wires being both within the tube are adjusted to less than a quarter of an inch from each other, and the tube is then inverted, and placed in a glass of pure water, and supported in a vertical position, as repre- sented by the figure. That part of the lower wire which is exterior to the tube, ought to be well covered with varnish, to prevent loss of gas, which otherwise might rise from it. When the apparatus is thus adjusted, the poles of the battery are to be connected with these platinum wires, and the water in the tube suffers decomposition. The liberated gases, hydrogen and oxygen, rise to the upper part of the tube, and displace the water, which descends in proportion to their accumulation. When the surface of the water has descended to below the knob of the upper wire, the circuit becomes opened, and the Galvanic action is consequently arrested. If, now, a spark from an electrophorus,* be transmitted from the upper to the lower knob, within the tube, the gases will explode, and entirely dis- appear, being re-united and again resumed the form of water. As the quantity of water thus formed is but very small, a vacuous space is left, for a moment, in the upper part of the tube, which is suddenly * The Electrophorus is explained in my fourth lecture on Electricity. 157 re-filled with water, by the atmospheric pressure on the surface exposed in the glass vessel. By employing two glass tubes, with a platinum wire in each, the oxygen and hydrogen gases, which are the results of the decomposition, can be collected separately. The two tubes are first filled with water, and then inverted and placed in a large wine glass, which also contains water, in the manner represented by fig. 68. The pla- Fig. 68. tinum wires in this arrangement ought to reach nearly to the open ends of the tubes, for by that means, the decomposition goes on much more rapidly than when they are shorter, and consequently not so near together. One of the wires B, is to be con- nected with the negative pole of the battery, and the other wire A, with the positive pole. It will soon be observed, that the gas collected in the tube B, occupies more room than that in the tube A, and in the course of a few minutes, you may pretty well ascertain their relative quantities, which, by measure, will be as two to one. These are, in fact, the relative proportions of hydrogen to oxygen, in the formation of water. To ascertain this fact by these experiments, however, it is requisite that the water operated on be the purest that can be obtained, and without any admixture whatever, otherwise the experiments would be vitiated. The glass tubes in which the gases are collected ought to be precisely of the same bore, and both graduated into inches and tenths, in order that their relative quantities may be accurately ascertained by mere inspection. When these things are strictly attended to, and the operation carried on as in the last experiment, the proportions of hydrogen and oxygen obtained, in their respective tubes, will be as above stated. And by collecting both gases in one and the same tube, and afterwards exploding them, as in the preceding experiment, it is found 158 that both disappear ; and as no other inference can be drawn than that the gases have again combined, we learn, both from the decomposition and the recom- position, or, in other terms, from analysis and synthesis, the real constitution of water. A more convenient, and at the same time, a more elegant form of apparatus for collecting the gases separately, is that respresented by fig. 69. It consists of a glass vessel, which Fig. 69. resembles the frustrum of a cone. Prior to its being fitted up, it is open at both top and bottom. The opening at the bottom is in the shape of a neck which pro- jects inwards, rising considerably above the level of the exterior of the base. This neck is filled up with a small bung, through which pass two strips of platinum foil, which rise about two inches above the cork, and terminate downwards in two concentric channels, which are formed round the centre of a wooden tray, on which the glass vessel stands. These channels are filled with mercury, each portion of which communicates with a brass cup, by means of a wire which passes under the tray. These cups are fixed to the edge of the tray, as seen in the figure, and are filled with mercury, for the purpose of communication with the connecting wires of the battery. The upper end of the glass vessel is closed by a brass cap, having two poles for the reception of two tall glass tubes, both of which are closed at their upper extremities, but open at bottom. When in their places, each covers one of the platinum ter- minals which rise from the base. The tubes arc furnished with screw ferrules, by means of which 159 they are screwed into the holes in the cap, and kept steady in their places. When this instrument is employed at the lecture table, the water operated on is usually mixed with a little sulphuric acid. The glass vessel is first nearly filled with the liquid, and when the tubes are firmly screwed in their places, the apparatus is inverted for the purpose of filling them ; which done, the glass is placed on its tray, and the battery connexions made with the cups. The upper part of the large glass has a small perforation for the purpose of allowing the air to escape, whilst it fills with the water that is forced out of the tubes, by the gas which collects in them. When the full extent of the power of a battery, in the capacity of decomposing water, is to be ascertained, the platinum terminals must be much larger than those we have hitherto employed, and placed much nearer together. Apparatus suitable for this purpose will be brought forward in our con- cluding lecture of this course. Some very interesting experiments on the electro- decomposition of water, are made by modifying the apparatus of Fabroni. Fig, 70 is a pig. 70. neat little apparatus, contrived by Mr. Silvester, for this purpose. It consists of a small glass jar, nearly filled with water, to which a very small proportion of sulphuric or muriatic acid is added. Through a cork which fits the mouth of the jar, passes a short zinc wire, and also a longer wire of silver, or copper. The lower part of the latter c s is turned upwards, and the upper part is bent at right angles, as seen in the figure. When the upper part of the copper wire is turned away from the zinc, the latter only, gives off gas, but when the two wires are brought into contact, by pressing the horizontal part of the copper against the zinc, a Galvanic pair is formed, and hydrogen in abundance rises from every part of the copper wire. 160 A very instructive experiment is made by em- ploying amalgamated zinc instead of that which is not so treated. Fig. 71 represents F j g 7] a glass jar nearly filled with a limpid solution of sulphuric acid. In the jar is placed, in a sloping position, a strip of amalgamated rolled zinc, which, not having the power to accomplish decomposition, every thing in the vessel remains perfectly tranquil. I now introduce a copper wire to the liquid, which is also inert whilst separate from the zinc. But on pressing it down, so as to bring the two metals into contact, the Galvanic pair is formed, and hydrogen ascends from every part of the copper wire below the surface of the liquid, but not a single bubble of gas escapes from the surface of the amalgamated zinc.* If, instead of a straight copper wire, we employ one formed into a long spiral, like a large cork-screw, and press its lower end against a piece of amalgamated zinc, laid in the bottom of the jar, the effect is much more striking. The gas ascends in the fluid in the form of a cylindrical pillar, increasing in density from the bottom upwards to the surface. By raising the wire from the zinc, the Galvanic chain is broken, and every trace of chemical action immediately lost. And by thus uniting, and disuniting the metals, alternately, we create, and annihilate, chemical action at pleasure. This is one of those instructive experiments from which we learn that chemical action is due to electric agency, and strikes at the root of the chemical hypothesis of Galvanism, for every feature of it disclaims a chemical origin. Another very imposing variation of this interesting * This experiment, and the two following variations ef it, were first published in my Recent Experimental Researches, &c. 161 experiment is accomplished by employing a flat spiral of copper wire, having the two end portions straight, and proceeding in opposite directions from the centre of the spiral, and at right angles to its plane. Fig. 72 will give an idea of its form. The lower portion c, from the point to the spiral, is to be well covered with varnish, cement, or bees' wax, and the point quite bright, or amalgamated. The upper part of the wire answers the purpose of a handle. When the point c is pressed upon the amalgamated zinc at the bottom of the jar, (fig. 71,) gas is immediately libe- rated from the water at every part of the spiral portion of the wire, and ascends in a dense cylindric cloud thence to the surface of the liquid, whilst the lower portion remains as tranquil and limpid as if no such process were in operation in the vessel. Fig. 73. The action of a series of several pairs of copper and amalgamated zinc, arranged in regular order of p 2 162 sequence, may be very beautifully exhibited by a couronne des tasses, of which fig. 73 is a representa- tion. It consists of twelve small glass tumblers or jars, arranged in a shallow circular channel, which margins the upper surface of a round mahogany table, twelve inches in diameter. The boundaries of the channel prevent the glasses from slipping oif the edge of the table. These glasses are furnished with twelve Voltaic pairs of copper and amalgamated zinc wires. The liquid employed is a limpid solution of sulphuric acid, which has become cool after mixing. When eleven only of the metallic pairs are in their proper places, the extreme glasses will be the poles of the arrangement ; and as the circuit is not closed, no chemical action takes place in any of the glasses. But the moment the circuit is completed, by intro- ducing the twelfth pair of wires, gas is liberated at every copper wire in the series, but none rises from the amalgamated zinc. LECTURE X. PERHAPS there is no philosopher to whom we are more indebted for leading us to the true path of Electro- chemistry, than Dr. Cruickshank, the inventor of the battery which still bears his name. Several of his discoveries are coetaneous with the earliest that were made with the Voltaic pile, and were published at the same time with Nicholson and Carlisle's discovery of the decomposition of water.* Some of those facts which I have already offered to your notice, and many others which are now to be brought forward, originated with that excellent chemical philosopher. We place in a glass tube, bent into the form of the letter V, an infusion of red cabbage, in which a * Nicholson's Quarto Journal for July, 1800. 163 few crystals of Glauber's salts (sulphate of soda) have been dissolved, and support it in a wine glass, as represented by fig. 74. To the liquid in each branch of the tube, is introduced a platinum wire, reach- ing nearly to the bend. When the upper extremities of these wires are connected with a Cruick shank's battery of fifty pairs of small plates, a decomposition of the water, and also of the salt immediately com- mences, and in a short time the liquid assumes two distinct colours. That portion in which the delivering terminal is placed, appears red, denoting the presence of acid, and that in the other branch of the tube becomes green, and indicates the presence of an alkali. Hence the sulphate has suffered decompo- sition and its constituents have taken up certain determinate positions in the circuit. The sulphuric acid has assembled at the positive or delivering ter- minal, and the soda at the negative or receiving terminal metal. If, instead of the sulphate of soda, the sulphate of potash had been employed, the results of the experiment would have been similar. The sulphuric acid would still be found at the delivering terminal, and the alkali at the receiving terminal. Or if, to the infusion of red cabbage, a little sulphuric acid were first added, which would produce a red liquor, and afterwards some liquid ammonia, till a neutral blue liquor was obtained. On submitting this neutral liquor to the Galvanic action in the bent tube, the results would be as in the two preceding cases. Now in all these cases there is an obvious law by which the electro-chemical phenomena are displayed, for the alkalis are invariably found at the negative terminal wire, and the acids at the positive one. The immutability of this law is beautifully illustrated by a slight variation of these experiments ; which is 164 done by merely reversing the battery connexions with the platinum terminals after the first results are obtained. When the first results are very perfect, there is at the bend of the tube a well defined plane of demarcation between the red and the green portions of the liquid ; which, as soon as the battery con- nexions are reversed, begins to fade, and soon vanishes altogether ; being obliterated by the mixture of the two colours, which is occasioned by the acid and alkali travelling in opposite directions, and exchanging places in the bent tube. This transfer of the acid and alkali is completed in the course of a few minutes ; and, as was before observed, is a striking illustration of the law of electro- chemical action. We will, however, proceed with a few other experi- ments in which this beautiful law is still further manifested. We may now operate on those compounds which are insoluble in water, and still we shall meet with similar results. If, for instance, we place finely pounded sulphate of baryta in a portion of the infusion of red cabbage, and operate on the mixture in the bent tube, fig. 74, the baryta, which possesses alkaline properties, will be collected in that branch which contains the negative or receiving wire, as will be understood by the liquid becoming green ; and the sulphuric acid, which was the other con- stituent, assembles around the positive or delivering wire, and there converts the liquid into a red colour. A very convenient apparatus for exhibiting the results of electro-chemical decompositions, is repre- sented by fig. 75. It is Fig> 75 a rectangular box made of plate glass, and sepa- rated into two compart- ments by means of a gauze or buckram par- tition. We place in this box a solution of idoide of potassium, with a little 165 starch mixed with it, and a strip of platinum in each compartment. To each plate of platinum, (which in this case are the terminal metals,) is soldered a copper wire, for convenience of connexion with the battery, and if the wire p be connected with the positive pole, (zinc end) and the wire N with the negative pole, (copper end) the platinum belonging to the former will be the delivering terminal, and that attached to the wire N the receiving terminal, to and from the liquid, respectively. The decomposition takes place immediately, as may be understood by the liquid about the delivering terminal turning slightly of a brown colour. This is occasioned by the presence of iodine, which is liberated from the dissolved salt, and which becomes so abundant in a short time, as to cause a dense brown cloud around the delivering terminal, and, eventually, the whole of the liquid in that compartment of the glass box becomes deeply coloured by it. In the other com- partment the liquid continues unchanged, and nothing remarkable appears but a liberation of hydrogen, which rises from the receiving terminal metal. In this experiment, as well as in many others, a secondary action takes place ; and as these secondary actions cannot be more simply exemplified than in the present case, we will avail ourselves of it as a type by means of which others may be understood. The iodide of potassium is a compound, consist- ing simply of iodine and potassium. The first, or primary Galvanic action accomplishes a separation of these two elementary bodies ; and the iodine is drawn to the positive or delivering terminal, and none of it is to be found in the other compartment of the apparatus. The potassium, however, is drawn to this latter compartment ; and the reason that it is not found there in its natural character as a metal, is simply this : as fast as it arrives at the receiving terminal, it is assailed by the oxygen of the water, which, being in a different electrical state, unites with it, and the result of this union, or secondary 166 action, is potassa. The hydrogen, which the oxygen has deserted, finding no other body with which it can unite, quits the liquid in the shape of gas. We will now operate on another anhydrous salt, the chloride of sodium, or the common salt of the table. This salt consists of a metal called sodium, and chlorine, which is a gaseous body. In order to show a secondary action, whilst operating on this salt, we employ a solution of indigo in sulphuric acid. The sulphate solution of indigo being well diluted with water, we put into it a large spoonful of common salt, and place it in the partitioned glass box, fig. 75. The sodium in this case, as in the preceding one, proceeds to the cell in which is the receiving terminal ; and a secondary action taking place between it and oxygen from the water, the formation of soda is the consequence, and the hy- drogen thus deserted flies off in its gaseous state. Whilst these things are going on at the negative terminal, the chlorine which has been separated from the sodium assembles around the positive or delivering terminal, and immediately sets to work upon the indigo, the beautiful blue colour of which it gradually and effectively destroys ; and, if time be permitted, every trace of colour in that particular chamber of the box, will entirely disappear, but the other portion of liquid suffers no apparent change. It would appear from this last series of experi- ments, that the alkalis are not formed until their metallic bases have arrived at the negative terminal, and, consequently, the solutions of those alkalis, which must be subsequent to their formation, is a tertiary event. Taking this view of the process, we can easily understand, that in all cases where the solution of an alkali is one of the ultimate results of electro-chemical decomposition, the liberation of the alkaline base is the primary result, the formation of the alkali is the second result, and the solution of this salt is the third. The same law holds good in the decomposition 167 of those salts whose metallic bases do not enter into new associations during the Galvanic process, for in all cases, the metals which are liberated, assemble at the negative terminal, and the acids with which they were combined, as uniformly assemble at the positive terminal. In illustration of this fact, a few experiments may here be necessary. Into a solution of the sulphate of copper I introduce two platinum wires, which are the polar or terminal wires of a battery. In a few seconds the receiving terminal will be covered with copper, but none will appear on the delivering terminal. We now reverse the battery connexions, making that wire which before received the current from the solution, now deliver the current to it, and that which before was the delivering wire, now be the receiving one. Permitting the action to proceed a few seconds, and then examining the two wires, we find, that the copper has entirely disappeared from that on which it was first deposited, and that the other is now cased with copper. We proceed in a similar manner with a solution of the nitrate of silver, employing gold wires instead of platinum ones. The receiving gold wire is soon covered with metallic silver, which in time shoots into beautiful needle-like crystals, but none appears on the delivering wire. By reversing the wires with the poles of the battery, the silver soon leaves that on which it was deposited by the former part of the experiment, and the other receives a silver coating, with the shining needles as before. These needles seem to be produced to the greatest perfection, when the terminals are silver wires. Gold wires are employed in the reduction of silver and other white metals, on account of the colour being a better contrast to them than platinum ; but in some instances, copper terminals answer very well. If, for instance, we operate upon a solution of the muriate of tin, or the acetate of lead, the result of the experiments is very conspicuous in a very short time, by that 168 portion of the negative or receiving copper wire which is below the surface of the liquid becoming covered with the tin in the one case, and brilliant spangles of lead being formed around it in the other. By adding a little vinegar to the acetate solution, the reduction of the lead is much facili- tated. We now employ a solution of the muriate of gold, and platinum terminals. The auriferous salt imme- diately suffers decomposition, and the receiving ter- minal wire becomes gilt with the liberated gold ; but no change is observed on the positive or delivering wire. If, now, we change the battery connexions, the gold soon quits the wire on which it was depo- sited, and the other receives a similar portion from the liquid. Besides the observations which have been made during the progress of these electro-chemical decom- positions, there are still some others to be offered which are well worthy of attention. You will have observed in some of these experiments, that a consi- derable quantity of gas escaped from the positive or delivering terminal, though never so copiously as from the other metal. This is one of the consequences of employing too much battery-power, by means of which a portion of water became decomposed. When the battery has not too much power, the salt in solution is the only article that suffers decomposition ; under which circumstances, no gas is seen at the delivering metal. This is one of the most important considerations in the arts of electro-typing, electro- silvering, and electro-gilding ; for if the battery employed in any of these processes be sufficiently powerful to decompose the water, the hydrogen which is liberated at the negative terminal interferes with the deposition of the metal, and prevents it from lying in a compact form and with a smooth surface. Another phenomenon which takes place, is the corrosion of the delivering terminal, under some circumstances. When a metallic salt is decomposed, 169 and no oxygen gas escapes from the positive or delivering terminal, that gas which is liberated from the salt enters into an union with the terminal, and with it forms an oxide, which in many cases is soluble in the liquid of the solution. This is a fortunate circumstance in the arts above alluded to, because the liquid becomes charged with new supplies of metal on one hand, at precisely the same rate as it parts with it on the other. Therefore, the delivering terminal in those processes, should always be of the same kind as that of the salt in solution. In the electro-type process, it must be a copper terminal, and in the silveiing and gilding processes it must be silver and gold respectively. The secondary actions which frequently take place during the decomposition of water, which is not quite free from foreign matter, were first noticed by Cruick- shank, and were amongst his earliest discoveries. This philosopher obtained nitric acid at the positive terminal, and ammonia at the negative one, and very justly supposed that the former originated from an union of the oxygen with the nitrogen of atmospheric air which the water held in solution ; and that the ammonia resulted from an union of another portion of the nitrogen with the liberated hydrogen. In the experiments of some other philosophers, muriatic acid and soda were formed under circum- stances where it was supposed the constituents could not have existed ; for instance, in the decomposition of pure water. And it was not until the masterly investigations of Sir Humphry Davy were announced, in the year 1806, that the true source of these com- pounds became properly understood. In a paper that was read towards the close of that year, Sir Humphry showed by several excellent experiments, that not only might these products result from the most trifling impurities in the water operated on, but that the vessels which held the water would suffer decomposition by the Galvanic process, and yield up 170 elements which, by secondary actions, would give the observed results. In some of these experiments, Sir Humphry employed small cylindric vessels made of agate, and the results were various, but the generality of them indicated impurities in the water, which had either existed previous to the Galvanic process, or were ta.ken up from the vessels during the time. Two of these vessels were generally used at the same time, the two portions of liquor in them Fig 76 being united, by a bundle of moistened asbestos A, as represented by fig. 76. The wires with rings at their up- per extremities, were of pla- tinum or gold, and were the terminals for the two poles of the battery. When the two portions of water were tested, after being subjected to the Galvanic process for many hours, an acid in one vessel, and an alkali in the other, was an usual, though not a constant result. When two portions of the purest water that could be procured were placed in two vessels of gold, in Fig. 77. the form of cones, as repre- sented by fig. 77, the results were still unsatis- factory whilst the water operated on was exposed to the atmosphere, and it was not till this perseve- ring chemical philosopher thought of surrounding the gold vessels, with their contents by an atmosphere of hydrogen gas, that he was enabled to prove that the formation of these compounds was not due to new 171 forms of the constituents of the water. By this mode of experimenting, however, Sir Humphry succeeded in decomposing the water without the production of any new compound whatever, and thus set at rest a question, which had for some years pre- viously, puzzled almost every philosopher in Europe. From the accuracy of the views which Davy generally took respecting electro-chemical action, and the immense range of apparatus he had at command, his experiments were generally attended with the happiest results ; and his inventive genius enabled him to contrive those forms of apparatus which gave to his experimental productions the most imposing effect. By introducing two or more vessels into the circuit, connected with asbestos, as in figs. 76 and 77, acid and alkaline matter, and other com- pounds, are collected in the different vessels, perfectly distinct from each other, and may be kept separate for examination as long as the operator pleases. If we place pure water in Fig. 78. the vessel N, fig. 78, and muriate of lime and water in the vessel p, and connect their contents by moistened asbes- tos, as shown in the figure, and also the platinum wires p and N, with the positive and negative poles, respectively, of a battery of even moderate power, the lime soon begins to quit the muriatic acid, and travels over the asbestos to the vessel N, but the muriatic acid remains in the vessel p. If the muriate of lime be first placed in the vessel N, and pure water in the vessel p, the lime will remain in the former vessel, and the acid will find its way to the latter. Therefore it is of no consequence in which vessel the muriate be placed, the final result is invariably the same. If instead of the muriate, the sulphate of lime, 172 or the carbonate of lime, be powdered, and mixed with water in one of the vessels, the acid and lime still take up their respective positions in the circuit, being found at the positive and negative terminals respectively, which shows that lime is one of those bodies which have a natural tendency to be placed in the same part of the circuit, as hydrogen and the metals invariably proceed to. If we employ three Fig. 79. vessels, and arrange them as in fig. 79, and in one of the exterior vessels we place pure water, and in the other a solution of the sulphate of potash ; and in the centre vessel an infusion of litmus. The battery, when connected with the terminals p and N, will accomplish de- composition of the salt, in which ever of the exterior vessels it be placed ; and the acid and alkali will traverse the liquid in the centre vessel in opposite directions, without interfering with each other. The characteristic colours which indicate the presence of acid and alkaline matter, will be beautifully dis- played in the central vessel i, one half of the liquor in which will appear red, and the other half a heavy green. Litmus paper immersed in the liquid con- tained in the positive exterior vessel will turn red, and turmeric paper will indicate the presence of free potash in the other exterior vessel. From what has already been brought forward it will appear obvious, that in all cases of electro- decompositions there exists an immutable law by which the whole of the phenomena are brought into existence, and it will readily occur to the experi- mentalist, that these facts are mere samples of the immense number that are within his reach, and which he may vary in many ways. 173 LECTURE XL BY contemplating the facts already before us, upon the strict principles of electricity, we are immediately struck with the analogy which is presented when these are compared with the results of Leichtenberg's experiments, in which dissimilar particles of matter are separated from each other by electric forces. In those experiments, the different kinds of matter pro- ceed to differently electrized surfaces, which is pre- cisely the case in all Galvanic decompositions ; and their determined positions with respect to those sur- faces are as uniformly manifested by the one process as by the other. Moreover, when by the Galvanic process the result is simply a decomposition of some compound of two elements only, it is a precise coun- terpart to the separation of the powders in Lichten- berg's experiments ; and a more exact analogy does not exist in any two cases within the boundaries of experimental science. Therefore, since we are per- fectly aware, that the separation of the particles of matter in the one case is accomplished by electric forces, we very naturally infer, that the separation of dissimilar matter by the other process, is also due to electric forces. By viewing the results of simple electro-decom- positions in this light, they may be very easily understood, since it is only necessary to be borne in mind, that each species of matter having a specific degree of electric tension, from its natural peculiar susceptibility of becoming charged, will, whenever a group of other bodies are sufficiently near, invariably tend to move towards that, the electric tension of which differs most from its own ; and whenever an oppor- I tunity presents itself, the two bodies will come into the most intimate union with each other. I am not aware of a better illustration of this fact than that afforded by a pith ball, suspended by a silken fibre, at equal distances from the balls of Q 2 174 three circumjacent Leyden jars of equal size, but charged to different degrees of intensity. The ball, as would be expected, invariably proceeds to the highest charged jar ; but when it has arrived at the same degree of intensity as that of the ball of the jar, it quits the latter, and flys to the jar which has the lowest charge of the three.* In Leichtenberg's experiment, although the powders do not quit the electric surfaces to which they are first drawn, they select and proceed to those surfaces upon the same principles and according to the same law, as the ball selects and proceeds to one particular jar, at each trip, in preference to any of the others. The red lead and the sulphur are in different electric conditions ; and, consequently, each selects that electrized surface of the resinous cake (fig. 14, page 48.), whose tension is the most remote from its own. By keeping in view this simple, but universal law of electric action, whilst contemplating electro- decompositions of simple compounds, there can be but little difficulty in understanding the process. If, for instance, we select the iodide of potassium, and operate upon it with the electric machine, its two elements, iodine and potassium, become separated ; and, like the red lead and sulphur, each body selects, as it were, and proceeds to, that terminal wire whose electric tension is the most remote from its own. The same reasoning is also applicable to the decomposition of water ; the constituents of which, being in different electric conditions, travel to those respective terminals between which, and themselves, the electric force is greatest : and the same law holds good in the electro- decomposition of every compound, whatever may be the number of its constituent elements. But, as is the case with iodide of potassium, the primary effects * This is a beautiful and very instructive experiment, when the intensities of the three jars are considerably different from one another. Their balls ought to be placed at equal distances from one another, and all in the same horizontal plane, and something higher than the suspended pith ball, when hanging quiescent in the mid- space. 175 are frequently concealed by the products of secondary actions. There appears, however, to be an opinion enter- tained, by some persons, that a series of conducting bodies in contact with one another, cannot be in dif- ferent electric conditions at one and the same time ; or in other words, they cannot, individually, be electro- polar. Hence, in a continuous, or closed electrical circuit, whether belonging to the machine, or to the Galvanic battery, every part must be in the same degree of electric tension, under which circumstances, the cases which we have here pointed out would not be analogous, and the law by which the phenomena are displayed in Leichtenberg's experiment, would not be applicable to electro-decompositions. The idea, I believe, originated with Mr. Singer, whilst attempting to controvert some peculiar views of Sir Humphry Davy. It would be difficult to say from what train of reasoning Singer arrived at this view of an electric circuit, more especially as he was one of the first electricians of his day, and has, in his treatise on the subject, described some experiments, the phenomena of which, not only cannot be ex- plained upon the principles he advocated, but are the most palpable evidences opposed to them. In my sixth lecture on Electricity, page 80, I have pointed outran universal law, which will assist us materially in this place. " Whenever a body is delivering the electric fluid to another body, the former is electro-positive to the latter ; and, as the terms positive and negative refer to the relative inten- sities only, without any reference whatever to the relative quantities of fluid that the bodies absolutely contain, it is obvious, that in all cases where a trans- ference of the fluid takes place, the delivering body must be more intensely charged than the receiving one." In illustration of this law, I have advanced the following experiment. " We arrange a series of insulated pointed wires, in the manner represented by fig. 80, having one 176 end of the series directed to the pointed wire in the Fig. 80. end of the prime conductor, and the remote extremity of the series in the opposite direction. The whole series of points will become luminous the moment that the machine is put to work. The point pro- jecting from the prime conductor, and the remote point in every wire will throw out a brush of electric fluid into the vicinal air, and the nearest points will be tipped with electric stars, indicating that those points are occupied in receiving fluid at the same time." Hence every wire in the circuit is both receiving and delivering fluid to and from the vicinal air ; and, according to the above law, not only are those wires electro-polar, but every individual portion of intervening air must, of necessity, be electro-polar, being, relatively, positive and negative, at the delivering and the receiving surfaces, respectively. If now we arrange in a line several small piles of paper, well soaked in a strong solution of iodide of potassium, on a long slip of glass, placing upon every adjacent two, a wire of platinum, so that its extremities may just rest upon the edges of the paper without touching the next wire in the series, we shall have an opportunity of making an experiment somewhat similar to the last one. And if we cover the ends of the wires by continuing the piles a little higher, we shall have an arrangement of alternate metal and liquid, as decidedly as that of metal and air in the former case. And when presented to the prime conductor, the electro-polarizations are similar in both cases. Every wire has both a receiving and a delivering point, to and from the liquid, and the result is a decomposition of the salt, the constituents of which become arranged in the circuit according 177 to the same law that regulates the phenomena in Leichtenberg's experiment. We may now form a similar arrrangement, and place it between the polar wires of a Voltaic battery. The result is precisely the same as by operating with the machine. The liberated iodine is found at those extremities of the wires which are most remote from the positive, or zinc end of the battery, but none is found near to the other ends of the wires. The experiment of Mr. Singer, before alluded to, is similar to the last one. A series of platinum wires are arranged in the axis of a long glass tube, by the assistance of cubical pieces of cork, in the manner represented by fig. 81. The ends of the Fig. 81. tube are each filled with a cork, through which a platinum wire passes, and when the tube is filled with water, and the terminal wires of the series attached to the poles of a battery ^ the whole arrrange- ment becomes polar. Every wire has both a re- ceiving and a delivering extremity, and oxygen is liberated at the latter, and hydrogen at the former. If the tube be filled with an infusion of red cab- bage, rendered blue by proper proportions of sulphuric acid and liquid ammonia, the polar actions of the series of wires become remarkably well shown, by the red and green colours produced at their positive and negative extremities, respectively. When the tube is filled with a solution of any of the metals, each in- dividual wire collects the liberated metal at its receiving termination, but none is found at the de- livering one. With a straight glass tube, the gases which are liberated, accumulating at the upper side, press upon the liquid and force a portion out of the tube, and sometimes force out the cork alsor This incon- venience is avoided by having a series of bent tubes, 178 with platinum wires, joining their liquid contents, and arranged as represented by fig. 82. The wires Fig. 82. in this apparatus, assume their respective electro- polaraties, as precisely as by any other arrange- ment ; and each pole performs its peculiar function accordingly. When six or eight of these platinum wires are arranged in one series, and the bent tubes contain a solution of copper, tin, or zinc, the deposited metal being in close contact with one end of each platinum wire, a series of Voltaic pairs become formed by the two metals, and the arrangement of these metals, in this secondary battery, being in the opposite direction to those in the primative battery, the two operate against each other. This checks the action of both ; and, in some cases, the action of the secondary battery, so far counteracts the force of the primative one, as to reduce it to below the requisite degree for carrying on the decomposition. An ingenious Prussian philosopher, named Ritter, made secondary batteries on this principle, probably without being aware of it, as the theory of them was first made known by Sir Humphry Davy. Ritter formed a pile of alternate plates of one kind of metal, and paper moistened with water, or with a solution of some salt. When this pile had been subjected to the action of a powerful Voltaic battery for some time, and afterwards removed from the circuit, it was found to possess all the properties of the original battery, as though it had been constructed of two metals. This pile is frequently alluded to by writers on Galvanism, and is called the secondary pile of Ritter. 179 Keeping in view the relative electrical states of the delivering and the receiving bodies, we may bring forward two other parallel experiments, one of which is by the machine, and the other by the Voltaic bat- tery. If we immerse two small brass balls, or even blunt-ended wires, in a tumbler of water, and connect one with the prime, and the other with the negative, conductor of the machine when in action, sparks are seen passing from the delivering to the receiving ball, as decidedly as in the open air ; and therefore these balls are, relatively, positive and negative, as defi- nitely in one case as in the other. We next connect these two wires with the two poles of a powerful Voltaic battery, and after having brought the two terminals together beneath the sur- face of the water, we gently withdraw the one from the other, and we have a beautiful play of the electric fluid between the two. If, instead of using metallic terminals in the water, those wires terminated with charcoal, the display of fire would be still more splendid. Now, since no electric discharge can pos- sibly take place independently of SL pre-polarization* , these experiments demonstrate an electro-polarity of the metallic and charcoal terminals, although sur- rounded by water. Having thus shown the means of recognizing the polarizations of those bodies which are placed in a Galvanic circuit exterior to the battery, we will now offer an experimental illustration of those polariza- tions which take place within the body of the apparatus. If we had no other facts to offer than those attendant on the experiments of Fabroni, they alone would be sufficient to prove the existence of an electro-polarity within the battery, when the circuit is closed. The following, however, being of a more striking character, may be introduced with advantage in this place. If we employ the bladder-partitioned vessel, described in page 151, and a Voltaic pair of platinum * Lecture vii, on Electricity, page 99. 180 and zinc wires, and place in the platinum compart- ment, a solution of sulphate of copper, and in the zinc compartment, a feeble solution of sulphuric acid, the platinum immediately becomes cased with copper, which indicates its electric condition, relatively to the liquid in which it is placed. If in the platinum cell, a solution of gold be placed, the wire soon becomes gilt by the gold which is liberated from the solution. By a similar arrangement, with a solution of nitrate of silver, and either a gold or a copper wire, the silver would be liberated, and by attachment to these wires, would encase every part of them which was immersed in the liquid. When a solution of the acetate of lead is operated on, the result of the experiment is very beautiful, by employing an apparatus such as that represented by fig. 83. It consists of a glass Fig. 33. tube, one end of which is closed by a piece of bladder ; a zinc basin, and a copper wire. In the zinc basin is placed a weak solu- tion of either the sulphuric or the muriatic acid, and the glass tube is to be nearly filled with a clear solution of acetate of lead. The copper wire being passed through a cork that fits the bore of the tube, one end of it is placed in the acetate solution, and the bladder- closed end of the tube is immersed in the acid liquor, and rests on the bottom of the basin. When the exterior part of the wire is brought into contact with the handle of the basin, the Galvanic circle is complete ; and the polar character of the copper wire is soon discovered by its receiving the particles of lead which are liberated from the solution. The recovered lead shoots into beautiful spangles, or shining leaves, which grow to a considerable size. . When a straight vertical wire is employed, like that represented in the figure, the lead tree is liable to slip off; but if the wire be 181 twisted into a spiral form, this accident is prevented, and the tree will sometimes grow to the full dimen- sions of the tube. Now, in all these cases the polarized metals were within the battery, of which they formed their respective portions. It will now be interesting for you to know, that portions of metal placed within a battery of which they do not form a part, will also become electro-polar. We will first employ a pair of copper and zinc, and unite them by wire, in the manner shown by fig. 53, page 127. We immerse this pair in a solution of sulphate of copper, and between these metals we place two or three short pieces of platinum wire, arranged in the manner shown by the darts. In one moment every wire becomes tipped with copper at those ends which are presented to the zinc. The copper accumulates in time, but none ever appears at the other extremities. By reversing the wires, the copper first deposited soon quits them, and a similar deposition takes place at the other extremities. If instead of placing the wires above one another, they are laid side by side at the bottom of the vessel, or even placed in one row, their polarization is still as complete, and the copper will invariably attach itself to their negative or receiving poles. When zinc is immersed in cuperous solutions, it liberates a considerable quantity of the copper, which by attachment, soon slackens the power of the principal current ; but for short illustrative experi- ments of this kind, that circumstance does not interfere, nor does it injure the battery for future experiments. Therefore, we will now employ a Cruickshank's, of fifty pairs, the cells ofwhich shall be charged with a solution of sulphate of copper. We now take ten short pieces of platinum wire, each of which is stuck in a strip of card, which holds it by the middle. The upper ends of the cards are fixed to a light wooden beam, at proper distances from one another, to allow of the wires being R 182 immersed in any ten successive cells of the battery ; with their axis in the direction of the battery's length. When the battery is charged, and its poles united by a wire, we dip the ten wires into ten neighbouring portions of the cupreous solution. When we lift them out again, we have ten copper and ten platinum extremities ; each wire having become electro-polar whilst in the battery. We reverse the wires in the battery, and in a few seconds the former copper ends have become platinum ends ; and those which before exhibited a platinum surface, are now covered with copper. From these facts, and many others that might be made, we are led to understand, that within the battery, as decidedly as without its poles, there exist electric currents. That the metals are polar, and that besides their important functions as the primary elements in the battery, they have others imposed upon them, and take an active part in conduction as well as in the excitation of the elec- trical element. Notwithstanding the importance which necessarily attaches to the polarization of the elements of the battery, there is yet another theoretical point to be considered before we can have a proper idea respect- ing the continuance of the current. In the first place, then, there are no continuous electric currents known independently of motion in some of the generating elements. Electric currents from the common machine, depend upon the motion of the glass. Those from a thermo-electric apparatus de- pend upon motion of the calorific matter ; and those generated in magnetic electricity, are continuous only whilst the magnet or magnetized body is in motion. Indeed, it is a law, which might be mathematically demonstrated, that no continuous electric current can possibly exist in any piece of apparatus, indepen- dently of a pre-existing motion in some other element employed. Therefore, the continuous currents which flow through a Galvanic circuit, must, of necessity, depend upon the motion of some of the elements 183 within the battery ; and that such motions do exist amongst the elementary constituents of the liquids em- played, we have ample proof. Previous to closing the circuit by introducing the twelfth pair of metals, in the courrone des tasses, fig. 73, page 161, an electric equilibrium prevails, and no commotion can be detected in any of the glass vessels. But the moment that the polar union is completed, the concentrated electric forces burst from their imprisonment, tear asunder the elements of the liquified compounds, and distribute them, with the rapidity of,lightning, to new positions in the circuit, according to their electric relations with those elements of the battery which are either absolutely fixed in those positions, or otherwise constrained to remain there. Hence it is, that decomposition is the first act performed by the electric forces ; and the distribution of the elements thus separated, gives a new distribution to the electric fluid, from moment to moment, a"s rapidly as it can be polarized at the fixed elements of the battery. The secondary actions which are incessantly going on, and forming new compounds, in a closed circle, also assist in disturbing the electric fluid ; and when one of the metals is unamalgamated zinc, the local electric actions, both primary and secondary, which take place on its surface, tend to keep up a continual change in its electric character. Hence, although the original force is the electrical, a succes- sion of consequential events, amongst the elementary atoms within the battery, are essential to a continuous disturbance of the electric fluid, as decidedly as a pulsatory current, in the dry pile, depends upon the pendulous movements between its poles, which are events consequent upon the pre-existence of electric forces. ELECTRO-TYPING, ELECTRO-SILVERING, AND ELEC- TRO-GILDING. The theory upon which these arts are founded, has already been illustrated by the experi- ments on metallic solutions ; and one of the grand points to be kept in view, is to keep the battery 184 power within such limits as to decompose nothing more than the metallic salts in solution ; because, should any part of that power be exerted on the water, so as to accomplish its decomposition, the liberated hydrogen gas would interfere with the metallic deposition on the receiving terminal metal, which, in all these processes, is that which is intended to be covered. When any article is to be electro-typed, for the purpose of obtaining fac similies of an original production, a medallion for instance, the surest way of arriving at the wished-for result, is to employ the medallion itself. Let us suppose that it is of copper. Then the vessel divided into two compart- ments by a bladder partition, already described, (page 151,) would answer well for holding the liquids to be employed. The medallion is to be furnished with a thin copper wire, which passes round its edge, and there made fast by a twist. The other end of the wire is to be furnished with a piece of zinc ; and this Voltaic pair is then to be placed in the two chambers of the box, in the manner represented by fig. 84, in which M is the medallion p . g ^ and z the zinc, with their faces parallel to each other. The cell in which the medallion is placed is filled with a strong solution of the sulphate of copper, and the other cell is filled with water. A Galvanic action com- mences, and in a very short time that face of the medallion next to the zinc, becomes covered with copper which has been liberated from the solution, and the coating thickens with the time allowed, until the whole is extracted from the liquid. If, however, the liquid be fed by fresh portions of the sulphate, the process will go on as long as you please, and the thickness of the copper deposited on the medallion will be proportional to the quantity of the salt decomposed. The zinc must either be of sufficient thickness to 185 last all the time, or new pieces must succeed each other during the process. The metal thus deposited is in a firm compact mass, and may easily be peeled from the medallion, of which it has taken an exact impression, and with a face of equal polish, and, generally, much more brilliant. As, however, the off surface of the medal- lion, when left bare, receives a thin layer of copper, it is well to cover it with lac varnish prior to the Galvanic operation. This first electro-type is now to be employed in precisely the same manner as the original medallion was employed in the first process ; and the electro-type formed in this matrix will be a fac simile of one side of the medallion. And, with care, a great number may be formed in the same matrix. To obtain fac similies of the other face of the medallion, a similar process must be pursued. A matrix for each side of the medallion may be formed at one and the same time, by employing a vessel with three compartments ; in the centre one of which the medallion is suspended in the cupreous liquor, and in each of the others, a piece of zinc with water. The edge of the medallion, with its surrounding wire, are to be covered well with varnish, which will prevent metallic deposition on them. By similar processes, fac similies of brass, silver, gold, and platinum medallions, or coins, may be obtained. But when the metal of the original is lead, tin, zinc, &c., a battery must be employed, otherwise the acid liquor would deface them. When the article to be electro^typed is large, such as a copper-plate from which prints are taken, it may be laid horizontally, with its face upwards, at the bottom of a water-tight box, and covered with a solution of the sulphate of copper. Over this is to be placed a diaphragm of bladder, parchment, or strong paper, stretched on a wooden frame, and supported an inch, or more, above the copper plate. The diaphragm with its frame, forms a tray, in which water and a sheet of zinc is placed ; and when the 18G two metals are united by wire, the Galvanic circle is complete, and the upper surface of the copper plate receives the liberated copper from the solution. When the action has proceeded about twelve hours, both portions of liquid should be removed and new ones introduced to their places. By paying attention to the renewal of the liquids and the zinc plate, the electro-type will be sufficiently thick, from about two days' action, to be peeled off the original. By a similar process, many fac similies of the original may be obtained, by operating with this first electro- type production. From these secondary electro- types, prints may be obtained as decidedly as from the original plate, and exhibiting all the minutiae of the picture with the same degree of precision. When an original article cannot be obtained for an electro-type operation, a mould may be formed on it, in which an electro- type may be deposited. The best moulds are metallic, and the simplest way of procuring them for coins, or small medals, is to squeeze the original between two pieces of clean sheet lead, in a strong vice, or by some other means. The lead yields to the pressure, and takes a tolerable fair impression from the two faces of the original. Another method of obtaining metallic moulds is to employ the fusible metal, which consists of bismuth, lead, and tin, in the proportions of two of bismuth and one of each of the other metals. This alloy fuses below the temperature of boiling water, and may be kept in a pasty condition whilst worked in the hands. When in this soft state, the article to be electro- typed is to be pressed upon it, with all the precautions necessary to exclude every particle of air. When the alloy has become solid by cooling, it is to be removed from the other metal, and used as a matrix in the electro-type process. These moulds are to be attached to the negative pole of a battery of two, three, or more elements in series, and immersed in a solution of sulphate of copper. To the positive pole of the battery is to be attached a plate of copper, 187 which is also to be immersed in the cupreous solution. The decomposition commences, and copper is depo- sited in the mould, and at the same time the copper plate oxidizes, and dissolves in the liquid, by which means a supply is kept up. When the metal of the electro- type is sufficiently thick, it is to be removed from the mould, and the latter being re-immersed in the solution, another electro-type commences its formation. The copper plate terminal must be attended to, in order to keep up the necessary supply. The battery may be either of Darnell's form, or that of Mr. Mullins : or it may be the cast iron battery, with a very weak solution of sulphuric acid. The size of the Galvanic metals must be regulated according to the work to be performed. Moulds of bees' wax are sometimes employed, but they are never so good as those made of metal. The wax moulds are lined with a fine layer of black- lead, laid on with a soft brush. The process of electro-silvering is somewhat different to that of electro-typing. The principal point to be observed, is to procure the most suitable solution of silver to work upon, or such a solution as may easily be decomposed, and from which a fine and firm coating of silver may be obtained. There are several solutions of this kind in common use. The cyanuret of silver, the ferrocyanuret of silver, the hyposulphite of silver, &c. The articles to be silvered are to be well cleaned from grease, dirt, and all foreign matter ; and it is well to dip them into a very weak solution of nitric acid, the moment before they are to be submitted to the battery action. The simplest battery that can be used, is that of which the article to be silvered forms a part, similar to that described for the electro -typing process, and represented by fig. 84. In one of the cells is placed the silver solution, and in the other a weak solution of sulphu- ric acid ; and when the article and the zinc are united by wire, they are placed in their respective liquids. In a few minutes, the silvering of that side of the 188 article* next to the zinc will be complete ; and pro- bably a slight coating may appear on the other sur- face. But when the article is to be silvered on every side, the apparatus represented by fig. 85, will be Fig. 85. found convenient. It consists of a porcelain jar, within which is placed another jar ; but thislatteris porous,-)- and contains the solution of silver. The space between the two jars holds a hollow cylinder of zinc, which sur- rounds the porous jar, and a weak solution of sulphuric acid. A wire rises from the zinc, and bent at right angles for the purpose of supporting and keeping contact with the article in the silver solution. When a separate battery is used for the silvering process, the solution of silver is placed in a vessel called a decomposing cell, which F - 86 may be a wine glass, a jar, or any other convenient article. In fig. 86, v v represents the decomposing cell ; and the jar B, containing a copper and a zinc plate c, and z, is a sepa- rate battery of a single pair. The zinc is connected with the article M, to be silvered ; and the copper is connected with a silver plate o, which will dis- solve as the process goes on, and keep up a charge of silver in the liquid. The arrows and darts indicate the course of the electric current. Gilding is carried on in precisely the same manner, and the auriferous liquids are similar to * The article may be a copper coin for a first trial, t Apparatus for this purpose may be had of the Philoso- phical Instrument Makers. 189 those in which the silver is dissolved. Some times a battery of four or five elements in series is employed in both processes. This hastens the work, which is sometimes accomplished in a few minutes. It is never necessary to have a Galvanic series of more than four or five pairs ; and the size of the plates must be regulated by the magnitude of the article which is to receive the silver or the golden covering. The mode of connecting the article with the battery, may be any that is found most convenient. It is a remarkable fact, and one of great im- portance in the above arts, that, although some metallic moulds and other articles will dissolve, in the liquids employed, when not connected with the battery, they are completely protected when united with the negative pole, and become the receiving terminal. If, for instance, we place two copper wires in a weak solution of nitric acid, such as would slowly dissolve the metal of both. When the dissolution has proceeded for a few seconds, and we connect the wires with the poles of the battery, the action is immediately arrested on the receiving terminal wire, but increased on the other. From this fact we learn, that the. article to be electro- typed, silvered, or gilt, ought to be the last body that is introduced to the Galvanic circuit, in order that its surface may not be injured by local action. Mr. De la Rue, the ingenious manufacturer of such a variety of fancy and other articles, seems to be the first person who observed electro-type impressions of great accuracy, in some of his Gal- vanic apparatus ; but we are indebted to Professor Jacobi, and Mr. Spencer, for its first appearance as an art. Advantage is taken of the gilding process in the art of etching copper plates. The surface intended to receive the etching, is first covered with a thin coat of gold, through which the tool can make the finest lines imaginable, without danger of breaking 190 the gold covering. The biting liquor having no power on the gold, acts in the lines only. A similar application of copper has been made, by covering steel plates with it, and tracing through the copper. An attempt has lately been made to take advantage of the secondary actions as a source of printing on calico and other fabrics, a specimen of which may be interest- ing in this place. An iron block, with any device, in relief, on one of its surfaces, is procured. The article, calico, for example, is well soaked in a solution of ferrocyanuret of potassium, and afterwards dipped in a solution of nitrate of soda. When the redundant liquid is squeezed out, the calico is spread smoothly on a sheet of tin foil, which is to be connected with the negative pole of a battery, and the iron pattern is placed upon the calico, and connected with the positive pole. By this means, the surface of the pattern becomes oxidized, and the oxide combining with the ferrocyanuret in solution, forms prussian blue, and a very exact blue impression of the design is left on the calico. Other metals produce other colours, and by having various metals in the pattern, a variety of colours may be printed at the same time. The iron battery of about ten jars, fig. 59, page 137, will print remarkably quick. With twelve or fourteen pairs in series, the impressions might be made almost as speedily as the pattern could be adjusted to its proper place on the fabric. The idea of electro-printing, seems to have originated with Mr. Baggs, of London. By a similar process to that above, writing ink may be easily formed. For this purpose we employ a solution of common salt, and mix with it an infusion of galls. A portion of this liquor is placed in each of two wine glasses, and connected by tow, or loose spun cotton, well soaked in salt and water. The terminals are to be iron wire. The positive terminal is soon oxidized, and a black liquor, (ink) is produced, but no such colouring is observed in the other vessel. Fifty pairs of metals in series. 191 answer very well for this experiment. If we want a blue ink, we have only to employ a solution of the ferrocyanuret of potassium, in place of the infusion of galls. LECTURE XII. ALTHOUGH we have given several cases of the electro- polarization of bodies, when surrounded by liquid conductors, there are still some others to be noticed which may probably give a pretty accurate idea of the mode by which the Torpedo, and Electrical Eel, arrest their prey before delivering the fatal shock. When small fish are placed in a basin of water, the two opposite sides of which are connected with the poles of a Galvanic battery, the current, which spreads through every part of the water, soon disturbs them, whatever part of the water they may be swimming in, and very frequently draws them into a line with the terminal wires, gradually exhausting their strength until they become quite lifeless. When several frogs are in the water, they very soon become arranged in the axis of the current, one behind another, and there remain quite motionless, though not dead, until the current is discontinued. Some of the frogs will take hold of one of the wires with its fore feet, and there hang in the water during the whole time. But when the current is powerful, they become stunned, and unable to swim for a long time after the process has ended. A volley of discharges, by running one of the conducting wires over the edges of the plates, has the best effect. It appears to me that these animals, when in the axis of the current, cannot avoid being electro-polar, for they are in the best possible position for being forced into that state ; and I have no means of under- standing why they are drawn into that position, unless by a similar action on them whilst at some 192 distance from the axial-plane.* Admitting this to be the case, it is not difficult to suppose that the battery of the electric Eel would polarize fish at some dis- tance from it, and would thus draw them closer, until they were in a proper position to receive the fatal blow. Leeches and snails are very much disturbed by touching zinc and another metal in contact with it, at the same time. Neither of them will travel across the joining of these two metals. Advantage has been taken of this fact, to keep snails from young garden plants, by placing a Galvanic fence of copper and zinc, round the beds in which they are growing. It is very likely that animals suffer much from a decomposition of their fluids during Galvanization. This chemical action is shown in a very decisive manner, at those points of the skin of a dead animal , which the polar wires touch, and putrefaction soon follows. There seems to be in Galvanism, an agent of immense value to the epicure, by means of which he might obtain venison much earlier than by the usual natural process. The porter drinker, also, derives his favourite flavour from the Galvanic action occasioned by the saliva, the porter, and the metallic pot, which form a Galvanic group. A zinc pot renders the porter too sour to be agreeable. There are many facts which induce us to believe , that there is not an uniformity of electric tension amongst the particles of any one piece of metal, and that it is on this account that they individually act on acid and other liquids Voltaically , and thus accomplish their own dissolution. In illustration of this view, I will offer an experiment. We nearly fill a glass tumbler with an exceedingly weak solution of sulphuric acid, and place at the bottom of the vessel, a piece of clean rolled zinc. * As the wires usually reach to the bottom of the water, the greatest force of the current must be in a vertical plaiu between them. This is what I mean by the term n.ri