« ForrigeFortsæt »
present state of our knowledge are best explained.
v. The hypotheses, which have been framed to account for the origin of the electricity, excited by galvanic arrangements. And,
vi. A general view of the phenomena of electrico-magnetic motion, which, with the principles deducible from them, promise to throw light on some of the most interesting, but obscure operations of nature.
It will be for us to abridge the account of Dr. Henry.
286. It has been stated above, that electricity is excited by friction; but in Voltaic electricity friction is not necessary. All that is required is the simple contact of different conducting bodies with each other; and it has even been found by Dessaignes that two discs of the same metal, heated to different temperatures, give sufficient electricity to excite contractions in the legs of a frog prepared for the purpose. Conductors of electricity have been divided into perfect and imperfect, the former comprehending the metals plumbago and charcoal, the mineral acids, and saline solutions; the latter, or imperfect, including water, alcohol and ether, sulphur, oils, resins, metallic oxides and compounds of chlorine.
have the strongest attraction for oxygen, are those which form the positive pole. But when the fluid menstrua afford sulphur to the metals, the metal, which under the existing circumstances has the strongest attraction for sulphur, determines the positive pole. Thus in a series of copper and iron plates, introduced into a porcelain trough, the cells of which are filled with water, or acid solutions, the iron is positive and the copper negative; but when the cells are filled with solutions of sulphuret of potassa, the copper is positive and the iron negative. When one metal only is concerned, the surface opposite the acid is negative, and that in contact with solution of alkali and sulphur, or of alkali is positive. Elements of Chem. Phil. p. 148.
291. Of Simple Galvanic Circles.—When a piece of zinc is laid upon the tongue, and a piece of silver under it, no sensation is excited while the metals are kept apart; but immediately that you bring them into contact a metallic taste is perceived. This instance affords an example of the arrangement of two perfect conductors, which are the metals, with one imperfect one, the tongue, or rather the fluids which the tongue contains. The metallic taste would seem to be occasioned by the excitement of a small quantity of electricity, from the contact of the metals, and its action on the nerves of the tongue.
292. Compound Galvanic Circles, or Galvanic Batteries.-The principle of these is the multiplication of simple ones. Thus if, between a plate of zinc and of silver, a piece of moistened cloth, of the same size with these plates, be interposed, and brought into contact, a simple galvanic circle is formed, as in the instance above adduced; but if these be piled on each other, in the order of zinc, silver, cloth, for several repetitions, we obtain a galvanic battery, termed from its discoverer the pile of Volta. The power of such a combination is sufficient to give a smart shock, as may be felt by grasping in the hands, which should be previously moistened, two metallic rods, and touching with these the The upper and lower extremity of the pile. shock may be renewed at pleasure, until after a few hours the activity of the pile begins to abate, and finally ceases altogether.
293. The metals composing a galvanic battery may be more conveniently arranged in the form of a trough; a happy invention of Mr. Cruikshank in a long and narrow wooden trough, made of baked wood, grooves are cut opposite to, and at the distance of between one-third and three-quarters of an inch from, each other; and into these are let down, and secured by cement, square plates of zinc and copper, previously united together by soldering. The space, therefore, between each pair of plates forms a cell for the purpose of containing the liquid, by which the combination is to be made active. When constructed in this way the trough affords an example of a galvanic combination of the first kind (see the first table above), formed by two perfect, and one imperfect conductor. But it admits of being modified, by cementing into the grooves plates of one metal only, and filling the cells alternately with two different liquids, as diluted nitric acid, and solution of sulphuret of 2 C
potassa. In this case we have a battery of the second order, formed by the repetition of one perfect and two imperfect conductors. See the second table above.
294. Other modifications of these galvanic apparatuses will, as above intimated, be described in the articles ELECTRICITY and GALVANISM. Here it may be sufficient to add, using the words still of the author whom we are following, that every combination which is capable of forming a simple galvanic circle, may, by sufficient repetition, be made to compose a battery. The combinations also which are most active in simple circles are observed to be more efficient in compound ones.
295. To construct a battery of the first order, it is essential that a fluid be employed which exerts a chemical action upon one of the metals. Pure water, entirely deprived of air, appears to be inefficient. In general, indeed, the galvanic effect is within certain limits proportioned to the rapidity with which the more oxidable metal is acted upon by the intervening fluid. The fluid generally used is nitric acid, with twenty or thirty times its weight of water. A battery which has ceased to be efficient has its activity renewed by emptying the cells of their liquor, and uncovering the plates: when the cells are filled with diluted nitric acid, the apparatus continues active, even under the exhausted receiver of an air-pump, or in an atmosphere of carbonic acid or nitrogen gases. But if the cells be filled with water only, all action is suspended by placing it under any of these circumstances. Hence it appears that the oxidation of one or both of the metals composing the trough is essential to the excitement of galvanic electricity.
the copper end be similarly connected, the jar is charged negatively. The shocks do not differ from those of a jar or battery, charged to the same intensity by a common electrical machine.
v. Galvanism, even when excited by a single galvanic circle only, such as a piece of zinc, a similar one of copper, and a piece of cloth, moistened with a solution of muriate of ammonia, distinctly affects the gold leaf of the condensing electrometer. If the zinc end be uppermost, and be connected directly with the instrument, the electricity indicated is positive; if the pin of the electrometer touch the copper, the electricity is negative. A pile, consisting of sixty combinations, produces the effect still more remarkably.
vi. The chemical changes produced by galvanic and common electricity, so far as they have hitherto been examined, are precisely similar. On this last proposition it is necessary to dwell more particularly, and, in so doing, we shall still follow the author from whom, in the present section, we have already so largely extracted.
297. The most simple chemical effects, produced alike by the agency of electricity and galvanism, is the ignition and infusion of metals; when, indeed, the galvanic power is excited to a considerable extent, metallic wires may be ignited and fused, as is the case with a strong electric battery; but, in the former instance, the particles of the wire are not scattered to a distance, as they are in the latter, since electricity seems to act with greater violence than galvanism. Actual combustion, also, of metallic wires may be effected both by electricity and galvanism.
298. But a much more remarkable action is exerted by the elective and galvanic fluids in disuniting the elements of several combinations. One of the first discoveries of the chemical agency of the pile, was its power of decomposing water. Two piles of any metallic wire are thrust through separate corks, which are fitted into the open ends of a glass tube, in such a way that the extremities of the wires, when the corks are in their places, may not be in contact, but may be at the distance from each other of about a quarter of an inch.
299. If the parts of the wire which project from without the tube, be made to communicate the one with the zinc or positive end, and the other with the copper, or negative end of a galvanic battery, a remarkable appearance takes place. The wire connected with the zinc, or positive end of the pile or trough, where it is in contact with the water, if an oxidable metal is rapidly oxidised, while from the negative wire a stream of small bubbles of gas arises. But if the wires employed be of a metal which is not susceptible of oxidation, such as gold or platina, gas is then extricated from both wires, and may be separately collected.
300. When a stream of galvanic electricity is made to act upon confined water, oxygen gas is given out at the positive end and hydrogen at the negative end, and in the proportions which by their union compose water. At an early
9 FEC 1971
period of the enquiry it was found, however, by Mr. Cruikshank, that the water surrounding the positive wire became impregnated with a little acid, and that round the negative wire with a little alkali.
301. It was afterwards discovered, by Sir H. Davy, that the gases constituting water may be separately produced from two quantities of water not immediately in contact with each other; this very important discovery evinced the transference of the elements of a combination to a considerable distance, through intervening substances, and in a form that escapes the cognisance of our senses. But not only the elements of water but saline compositions and even metallic salts were decomposed in the same way by Sir H. Davy, the acid element of the salt being always collected at the positive, and the earthy or alkaline one at the negative side of the arrangement. Sir H. Davy even found that acids by galvanic excitation may be made to traverse opposite principles without combination, or be transferred through solutions of alkali, from the negative to the positive side, while on the other hand alkalis and metallic oxides were found transmissable from the positive to the negative side, through intervening solutions of acids.
302. These very singular and very momentous discoveries rendered clear what before seemed difficult of explanation, viz. why, by the agency of galvanism on water, alkali appears at the negative and acid at the positive wire. Sir H. Davy ascertained that all water, however carefully distilled, contains neutral salts in a state of solution. From these impurities the alkaline and acid elements are separated, agreeably to a law which has already been explained. In the same way, also, the muriatic acid and alkali are accounted for, which some chemists have obtained by galvanising what was before considered as pure water; a fact which has been urged in proof of the synthetic production of both these bodies. Absolutely pure water, it has been demonstrated by Sir H. Davy, yields nothing but hydrogen and oxygen gases. See HYDROGEN in the present
303. Now it has been shown that ordinary electricity, properly managed, is equal to the production of these curious decompositions; and it is fair to conclude, that galvanism and electricity are modifications of the same power.
304. A most important inference has been deduced from the discovery of these facts, viz. that hydrogen, alkalis, metals, and oxides, exist in a positively electrified state, and therefore will be repelled by surfaces which are in the same condition with themselves; that they will, on the contrary, be attracted by surfaces that are negatively electrified; and oxygen, as also the acids, in consequence of the oxygen they contain, being in a negative state, will be attracted by positive surfaces, and repelled by negative
305. To apply this theory to the simplest possible case, the decomposition of water, the hydrogen of this compound being itself positively electrified, is repelled by the positive wire, and attracted by the negative one, while, on the contrary, oxygen being negative, is repelled by the
negative wire, and attracted by the positive one. The flame of a candle, which consists chiefly of ignited charcoal, when placed between a positive and negative surface, bends towards the latter, but the flame of phosphorus, consisting chiefly of acid matter, when similarly placed, takes a direction towards the positive surface. In the case of neutral salts, the negative acid is attracted by the positive wire, and the positively electrified alkali by the negative wire.
306. Thus then, continues our author, a power has been discovered, superior in its energy to chemical affinity, and capable either of counteracting it, or of modifying it according to circumstances. The chemical attraction between two bodies may be destroyed by giving one of them an electric state, opposite to its natural one; or the tendency to union may be increased by exalting the natural electrical energies.
Further remarks on the theory of the galvanic arrangement, and on the points on which there is a seeming difference between Voltaic and common electricity, will best be discussed under the articles ELECTRICITY and GALVANISM, to which we refer the reader.
307. Having thus investigated, to the extent of our limits, the general laws and principles of chemical action, we are now to proceed in our enquiries respecting the individual substances, and their diversified compounds, the consideration of which comes under the cognisance of chemical philosophy; indeed, the whole world of matter, as far as composition is concerned, lies before us; there is nothing with which, in a certain way, the chemist has not to do; and, as far as arrangement is concerned, we should now, had we been writing but some few years since, have adopted an arrangement of this vast mass of materials, something similar, if not quite the same, as that pursued by Dr. Murray, in his excellent work. We should have proceeded to treat of atmospheric air, or at least have here referred the reader to that portion of the work in which it is treated of; we should then have gone on to the consideration of water, and its base; to acids, their bases and composition; to alkalis, with their bases; to earths, and their bases; metals, and their combinations; and thence into the three great divisions of matter, mineral, vegetable, and animal.
308. The very curious and extensively operating circumstances to which we have just referred at the end of the preceding section, have, however, given rise to a modification of these arrangements, founded on the principle that bodies are divisible into two great classes, viz. electro-negative, and electro-positive. Upon such assumption is founded the division and arrangement which Dr. Henry adopts; and it appears, to say the least, to have this in its favor, that the student finds all along as he goes, more clear and decided illustrations of the magnificent discoveries of modern times, and has a better opportunity furnished him for appreciating these discoveries, and of applying them to their respective purposes.
309. This arrangement, therefore, we shall
likewise, to a certain extent, adopt, although it may be open, as what artificial classification is not? to some objections; it of course leaves untouched the animal and vegetable kingdoms, or the materials of organic existence, which therefore, as in other treatises, will fall to be considered separately, and after inorganic existence shall have been disposed of. The objections which apply to the subdivisions, till recently very generally observed, of combustible and non-combustible, will be best stated, because most easily understood, as we proceed in our investigations.
310. ELECTRO-NEGATIVE BODIES.-Oxygen. This is only known as a separate principle in a gaseous state of existence, and even in this state it is combined with caloric; in the article AIR several substances are mentioned as those from which oxygen gas may be obtained, and it is there stated, that the chlorate of potass yields it in the greatest purity. We have likewise given in that paper the general character and habits of oxygen, which need not be here repeated.
311. Oxygen was long supposed to be the only supporter of combustion, and in the Lavoisierian theory it was treated of as essential to that process. It is now found, however, that other bodies are equally entitled to rank as supporters of combustion, among which are chlorine and iodine. The hypothesis of combustion proposed by the French philosophers, has indeed been found altogether unstable, both as it respects the supposed necessity of oxygen for the process, and its condensation, and as it endeavoured to explain the heat and light at times evolved. Numerous are the instances in which oxygen, in the process of combustion, instead of being solidified, actually becomes gaseous during the operation; the light, moreover, depends upon the combustible, and not upon the measure of oxygen consumed, and there are several cases of combustion, as just intimated, in which no oxygen is present. Combustion is much more probably dependent upon the electrical conditions of bodies, and ought at any rate to be considered rather as an intense chemical action generally, than dependent, as Lavoisier conceived, upon a particular principle or form of matter. It will be inferred from what has been advanced above, that all bodies acting powerfully upon each other are in the opposite electrical states, and heat and light may be evolved as a consequence of the annihilation of these opposite conditions, occasioned by their combination.
312. Substances capable of combining with oxygen, afford one or other of the following products: 1. An acid. 2. An alkali, or earth,
or 3. An oxide.
313. We have already observed (see ACID) that the theory of Lavoisier, which regarded oxygen as the universal principle of acidity, is not consistent with more recent observations and discoveries; but that acids are often the product of oxygenation will be seen as we proceed. It is not easy, as we have before remarked, to give very precise definitions of acids, since some bodies have all the other characteristics of acids at the same time that they do not impart sourness to the taste of the alkalis and earths too
there is some want of precision in respect of their distinctive designations, but they are generally known by their tendency to combine with the acids, and by this union losing their indiv.dual characters. See ALKALI and EARTH.
314. Oxide is a term applied to bodies that have a less quantity of oxygen united to them than that which is sufficient to produce acidity; these bodies may often be brought to the condition of positive acidity by causing them to combine with more oxygen, and the loss of the acidifying portion of oxygen may be again so managed, and effected only in such quantity, as that the acid shall be reduced to a state of oxide.
315. Chlorine. This substance was discovered by Scheele in 1774. It was named by the discoverer dephlogisticated muriatic acid. In the French nomenclature it was denominated oxygenated muriatic acid. It may be obtained in a gaseous form, by mixing black oxide of manganese with muriatic acid, and heating the mixture over a lamp in a glass retort. The gas is soon evolved, and may be collected over warm water very conveniently; cold water soon absorbs it.
316. A mixture of eight parts of muriate of soda, three of black oxide of manganese, four of sulphuric acid, and four of water will, if properly heated, evolve chlorine.
317. This gas has a pungent and disagreeable smell of a suffocating kind, and it is of a yellowish green color, hence its name from xλwpos, green.
318. It is heavier than common air; when dry it suffers no change by being subjected to the most intense cold; but in its common state it may be condensed into a liquid form, and, when exposed to a freezing temperature, the aqueous part of the gas is deposited in the form of crystals; this, however, is again taken up by the gas upon the re-application of heat.
319. Chlorine is not altered by exposure to very high temperatures. When it is suddenly and greatly condensed, by mechanical pressure, heat and light are evolved. Electricity does not alter it. When a burning taper is introduced into a jar of chlorine, the flame becomes immediately red, a dense smoke is emitted from it, and it is soon extinguished. But many bodies, such as phosphorus, and even several of the metals, when finely powdered, are spontaneously ignited upon being immersed in chlorine, and burn in it very brilliantly. The combustion indeed of phosphorus in this gas is vehement.
320. Chlorine is heavier than common air, 100 cubic inches weigh 75.375 grains.
321. It was once imagined, as may be inferred from its former names, to be composed of oxygen and muriatic acid. It is now treated of as a simple body; and the fact of its not being changed by electricity is in favor of this supposition.
322. Chlorine and oxygen unite so as to form oxides and acids.
323. The euchlorine or protoxide of chlorine was discovered by Sir H. Davy; it may be obtained by mixing muriatic acid with chlorate of potass, and stirring the mixture with a platinum
326. What has been called deutoxide, or tritoxide, or with more propriety the Peroxide of Chlorine, is procured by triturating fifty or sixty grains of the powdered chlorate of potass with a little sulphuric acid, so as to form a thick paste, which is to be put into a retort and heated, but not to the boiling point. The gas may be received over mercury. It has a lively yellow color, more brilliant than the euchlorine, and it is more absorbable by water. Its saturated solution in water is of a deep yellow color, imparts an astringent taste, and it may be kept unchanged in the dark; the rays of light, however, decompose it and form from it chlorine and chloric acid.
327. Chloric acid.-Gay-Lussac, was the discoverer of this compound of chlorine and oxygen; it is obtained by adding dilute sulphuric acid to the chlorate of barytes; but this is a compound that exists only in the liquid state; and Sir H. Davy has even disputed the simple combination of chlorine and oxygen; he considers the liquid acid of Gay Lussac to be constituted of two proportions, in the atomic composition of hydrogen, one of chlorine, and six of oxygen. Under the word ACID the reader will find it stated, that Dr. Murray has argued for the existence of hydrogen as an acidifying principle generally, and not as a mere constituent of the water with which substances are combined; and this statement of Sir H. Davy, in reference to the composition of the chloric acid, in some measure harmonises with that assumption.
328. Perchloric acid-In the process of obtaining peroxide of chlorine a peculiar salt is formed, which was first noticed by count Stadion; its taste is somewhat like the common muriate of potass. At the heat of 412° it is resolved into oxygen and muriate of potass, in the proportion of 46 of the former to 56 of the latter. From this salt sulphuric acid at 28° disengages the
perchloric acid, which consists of chlorine and oxygen; but it does not exist independent of water, or a base. See CHLORINE, in the body of the work.
329. IODINE. This newly discovered substance may be obtained from a solution of kelp or barilla, or from the ley of ashes of marine plants, which furnish the mineral alkali. The following process is given. Lixiviate powdered kelp with cold water, evaporate the lixivium till a pellicle forms and set aside to crystallize; evaporate the mother liquor to dryness, and pour upon the mass half the weight of sulphuric acid. Apply a gentle heat to this mixture in the flask of an alembic, and fumes of a white color will arise and become condensed in the form of opaque crystals. The iodine first passes into the receiver in the form of beautiful violet vapors. The crystals are to be quickly dried upon blotting paper.
330. Iodine was first discovered in 1812, by M. Courtois, a manufacturer of saltpetre at Paris. Vauquelin, Gay Lussac, and Davy, have ably and fully investigated its properties. See Annales de Chemie, 90th, 91st, and 93rd vols. and the Philosophical Transactions for 1814.
331. Iodine, like chlorine, is electro-negative, and therefore introduced here. It is solid at the ordinary temperature of the atmosphere, but extremely volatile, and at a temperature somewhat under 80° emits a violet vapor. It produces a yellow stain upon the skin. It is sparingly soluble in water, much more so in alcohol and æther. The color of the solution is yellow. The color of iodine is of a bluish black, its lustre is metallic, and its taste acrid. Its name is from wong, violaceous, on account of its vapor being of a beautiful violet color. Its specific gravity
332. Iodine combines with oxygen and with chlorine, and by this combination produces two acids which have been named Iodic and Chlori
333. Iodic or Oriodic acid. This compound of oxygen and iodine cannot be obtained immediately, for iodine does not undergo change by being merely heated with oxygen, or even with chlorate of potass. It is, therefore procured by the intervention of protoxide of chlorine. We may introduce iodine into a small flask, and disengage the chlorine oxide from it by a due admixture of chlorate of potass; or 100 grains of chlorate of potass may be introduced into a small retort with 400 grains of liquid muriatic acid of the specific gravity 1105; annex to the retort a small globular receiver having a bent tube issuing from it, and passing to the bottom of a small flask containing about fifty grains of iodine; carefully apply the heat of a lamp to the retort, by which oxide of chlorine will be disengaged, and which will be decomposed and absorbed by the iodine. A compound is then formed, which consists of chloriodic and oxiodic acids. The former is separable by a gentle heat, the latter remains as a white, semitransparent, sour, and inodorous body, very soluble in water. It consists of 117-7 iodine, 37.5 oxygen. (Brande).
334. Iodous acid.-Sig. Sementini procured a yellow fluid by distilling iodine and chlorate of