« ΠροηγούμενηΣυνέχεια »
Berzelius and Poutin. A portion of pure barita, moistened with water, is placed on a plate of platina, which is rendered positive by a Voltaic battery, while a globule of mercury, placed in the paste, is made negative: an amalgam is thus obtained, from which the mercury is expelled by distillation in a tube of glass free from lead, filled with the vapour of naphtha, and hermetically seated. Sir Humphry Davy had before obtained it only in combination with
Strontium and calcium are procured in the same manner from strontia and lime; strontium much resembles barium, calcium is a little brighter and whiter. When the vapour of potassium is made to pass through ignited barita or lime, or some of their compounds, some potass is formed, and the earths are either partially or completely reduced to a metallic state.
Magnesium may be obtained in either of these ways, though more slowly by the electrochemical process: when the vapour of potassium is employed in a thick tube of platina, a small quantity of mercury may be afterwards introduced, which will amalgamate with the metal, and when expelled, will leave it in the form of a dark grey metallic film, not acting so rapidly on water as any other of the alkaline metals.
Aluminium, glycinium, zirconium, silicium, and itrium, have been obtained less distinctly in separate forms. Aluminium, for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound, withstands all attempts to decompose the earth by electrifying mercury in contact with it: but when a particle of iron is employed, with an electrical power capable of fusing it, the iron is whitened, and effervesces with water, affording a small portion of alumina. By means of the vapour of potassium also, some gray metallic particles may be obtained from ignited alumina: and glycina, similarly treated, affords a dark coloured substance, which regains the earthy appearance when heated in air, or moistened. Similar particles obtained from zirconia are found to be partly metallic, and partly of a chocolate brown colour. Silicium seems to have an appearance somewhat resembling plumbago; its alloy with iron may be obtained like that of aluminium. Itria also, treated with potassium, affords potassa, and acquires a partial appearance of metallization.
Nor have the same powerful means of analysis been less successful in demonstrating the composition of the boracic acid, from which our author has obtained a substance too little resembling a metal to be termed boracium, but which, from its analogy to carbon, he had thought it right to distinguish by the more appropriate than elegant name boron. It is procured either by the electrical decomposition of the boracic acid, or by iguiting that acid with an
equal weight of potassium in a tube of iron. It is of a dark olive colour, neither fusible nor volatile in any heat to which it has been exposed; it sparkles very brilliantly when thrown into oxygen gas, and a portion of it is converted into boracic acid.
When the fluoboric acid gas is decomposed by the combustion of potassium, it affords fluate of potassa, and the boracic acid only seems to be deprived of its oxygen; but when potassium is burned in the silicated fluoric acid, there is reason to think that both the silica and the fluoric acid undergo a partial decomposition, since the gas affords a smaller quantity of fluate of lime than before the operation of the potassium: but the base of the fluoric acid has never been separately exhibited, and from the readiness with which the fluoboric gas enters into combinations, our author thinks it not altogether impossible that the fluoric acid contained in it may be a simple empyreal principle analogous to oxygen and to 'chlorine.' His opinions on the nature of these substances, which constitutes the third great feature of the present work, require to be illustrated in his own words; p. 240.
'Scheele considered chlorine as an element of the muriatic acid, and hence called it dephlogisticated marine acid. By that chemist it was regarded as an undecompounded body. Lavoisier and Berthollet asserted that it was a compound of muriatic acid gas and oxygen. This idea is now universally given up; but some chemists in France and Scotland conceive that it is a compound of oxygen, and an unknown body, which they call dry muriatic acid. The weight of chlorine, its absorbability by water, its colour, and the analogy of some of its combinations to bodies known to contain oxygen, are arguments in favour of its being a compound; and it is possible that oxygen may be one of its elements, or that oxygen and chlorine are similarly constituted. I have made a number of experiments with the hopes of detecting oxygen in it, but without success; none of its compounds with inflammable bodies will afford this principle; charcoal, intensely ignited in it, undergoes no change, nor is it altered by the strongest powers of electricity. Should oxygen ever be procured from it, some other form of matter, possibly a new one, will, at the same time, be discovered as entering into its constitution, and till it is decompounded, it must be regarded, according to the just logic of chemistry, as an elementary substance. P. 237. Chlorine and oxygen are capable of existing in combination, and they form a peculiar gaseous matter. They do not unite, when mixed together, but when existing in certain solids, they may be detached in union. To make the compound of chlorine and oxygen, hyperoxymuriate of potassa is introduced into a small retort of glass, and twice as much muriatic acid as will cover it diluted with an equal volume of water. By the application of a gentle heat, the gas is evolved, and it must be collected over mercury. I discovered this elastic substance in its pure form in January 1811, and gave to it the name of euchlorine, from its bright yellow-green colour. Its smell is not unlike
that of burnt sugar. It must be collected and examined with great care, and only in small quantities at a time; a very gentle heat causes it to explode, sometimes even the heat of the hand; and its elements separate from each other with great violence, producing light. None of the metals that burn in chlorine act upon this gas at common temperatures; but when the oxygen is separated they then inflame in the chlorine. The proportion in which chlorine combines with bodies may be learned from the decomposition of euchlorine; the oxygen in which is to the chlorine as 15 to 67 in weight. If euchlorine be considered 'as consisting of one proportion of oxygen to one of chlorine, then 67 will be the number representing chlorine, which is most convenient as being a whole number. If euchlorine be supposed to contain two proportions of chlorine and one of oxygen, then the number representing chlorine will be 33.5. It will hereafter be shewn that whichever of these data be assumed, the relations of the number will harmonize with those gained from various other combinations.'
The doctrine of the simple proportions of combinations, exemplified in these numbers, which was the fourth point that we mentioned as particularly requiring to be noticed, is thus stated in the introduction.
'Experiments made by Richter and Morveau had shewn that, when there is an interchange of elements between two neutral salts, there is never an excess of acid or basis; and the same law seems to apply generally to double decompositions. When one body combines with another in more than one proportion, the second proportion appears to be some multiple or divisor of the first; and this circumstance, observed and ingeniously illustrated by Mr. Dalton, led him to adopt the atomic hypothesis of chemical changes, which had been ably defended by Mr. Higgins in 1789; namely, that the chemical elements consist of certain indestructible particles, which unite one and one, or one and two, or in some definite numbers.' p. 56.
P. 117. Mr. Berthollet, to whom the first distinct views of the relations of the force of attraction to quantity are owing, has endeavoured to prove that these relations are universal, and that elective affinities cannot strictly be said to exist. He considers the powers of bodies to combine as depending, in all cases, upon their relative attractions, and upon their acting masses, whatever these may be: and he conceives that in all cases of decomposition, in which two bodies act upon a third, that third is divided between them in proportion to their relative affinities and their quantities of matter. Were this proposition strictly correct, it is evident that there could be scarcely any definite proportions. When an alkali precipitates an earth from its solution in an acid, the earth, according to Mr. Berthollet's ideas, ought to fall down in combination with a portion of acid. But if a solution of potassa be poured into a sulphuric solution of magnesia, the precipitate produced, after being well washed, affords no indication of the presence of acid; and M. Pfaff has shewn, by some very decisive experiments, that magnesia has no action upon neutral combinations of alkalis and
sulphuric acid; and, likewise, that the tartarous acid is entirely separated from lime, and the oxalic acid from oxyd of lead, by quantities of sulphuric acid merely sufficient to saturate the two bases; and these are distinct and simple instances of elective attraction. Again, when one metal precipitates another from an acid solution, the body that falls down is usually free from acid and oxygen: thus zinc precipitates lead and tin, and iron copper; and the whole of the oxygen and the acid is transferred from one metal to the other.'
Having exhibited this outline of the general doctrines which Sir Humphry Davy has undertaken to maintain, we must pause to consider how far he seems to have left any thing further to be desired, with regard to the perfect establishment of either of them. His electrochemical discoveries, and his decomposition of the alkalis and earths, must ever remain incontestable memorials of his ingenuity and success; but on the subject of the oxymuriatic acid gas, we cannot help thinking his tone (p. 335) somewhat more decisive than the present state of the investigation altogether authorises. We do not see the absolute necessity of considering every body as simple which has never been decompounded, provided that there are strong analogical reasons for suspecting that it is really a compound. In the present instance, there are considerable difficulties on both sides, and being much disposed to suspend our judgment until further evidence can be obtained, we must confess that a new nomenclature, founded on the adoption of the new opinion, and tending to carry with it à tacit persuasion of its truth, appears to us to be somewhat premature. Either hypothesis may be employed for explaining the phenomena; perhaps the probability is in favour of Sir H. Davy's; but the arguments, by which it is supported, cannot yet be considered as finally conclusive. We see ten or twelve different substances agreeing with the muriatic acid in a very great majority of their properties, and depending for these properties on the oxygen which they contain, and one anomalous substance only, which possesses these properties in a very slight degree, that is, sulphurated hydrogen, and which is found to contain little or no oxygen: it does not therefore appear to us to have been unphilosophical to suppose that the muriatic acid also contained oxygen. It is true that this presumption is weakened by the failure of the newly acquired powers of chemical electricity to obtain oxygen from it; but however great and wonderful those powers may be, they are not altogether irresistible, since some of the metals of the earth have been more easily exhibited by chemical than by electrical means. The oxymuriatic acid gas approaches much more nearly to the combinations of oxygen than to oxygen itself, in the facility with which it unites with metals, and in some other respects; nor do the combinations of this
gas appear to resemble those of oxygen by any means so closely, as might be expected from the analogy of two elementary principles belonging to the same class. We are willing to allow, that the necessity of supposing a portion of water inseparable from the muriatic acid gas militates in some measure against the common opinion; but it must be remembered that neither the sulphuric nor the nitric acid has ever been obtained without admixture, either of water or of some other substance. On the other hand, the theory of simple proportions affords an objection of considerable weight to the doctrine advanced by our author; for the quantity of muriatic acid contained in some of the supersalts and subsalts bears a regular relation to the oxygen of the earths or oxyds on the common supposition, and not on that of the elementary nature of chlorine; the patent yellow, for example, if we mistake not, is a substance which appears to be produced by a regular process in a constant manner, and which must, upon this hypothesis, be supposed to be a mixture of two distinct combinations, governed by no intelligible. law, while, if we consider it as one of the submuriates of lead, it exhibits a strict analogy with other substances.
But even if we grant the existence of chlorine as an elementary principle, we cannot approve of distinguishing its combinations by terminations only, much less by terminations so simple as ane, ana, anée, which our author has proposed for the different combinations of chlorine with any other simple substance. According to the Linnean precept, Phil. bot. § 287, a specific name must not be united to the generic as a termination,' and Sir H. Davy has himself confessed that for calomel and corrosive sublimate the terms mercurane and mercurana would be an insufficient distinction; to say nothing of the inelegance of a French vowel in an English word, and of the difficulty of preserving the terms distinct in translations into other languages, which ought to induce us to refer all scientific nomenclature to some common form, that of the Latin language, for instance, whence they might be again derived for the use of each modern language according to its characteristic genius. We do not apprehend that the word chlorid, following the analogy of oxyd,' (p. vii,) would have been a more theoretical expression? than the termination ane, and we might add to it, if necessary, dichlorid and trichlorid. In the case of the earths and alkalis, there is a manifest reason for using single words; these substances, unlike the chlorids,' occurring continually in combination, it would require great circumlocution to express the most familiar compounds, unless some such abbreviation were permitted.
With regard to the fourth principal subject on which the present work throws many new lights, that is, the simplicity of the proportions of chemical combinations, the proofs are so numerous and satis