Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

ral substances, which had never been separated into any other forms of matter in the common processes of experiment, were susceptible of analysis by electrical powers: in consequence of these circumstances, the fixed alkalis, and several of the earths have been shown to be metals combined with oxygen; various new agents have been furnished to chemistry, and many novel results obtained by their application, which, at the same time that they have strengthened some of the doctrines of the school of Lavoisier, bave overturned others, and have proved that the generalisations of the antiphlogistic philosophers were far from having anticipated the whole progress of discovery.

· Certain bodies, which attract each other chemically, and combine when their particles have freedom of motion, when brought into contact, still preserving their aggregation, exhibit what may be called electrical polarities; and by certain combinations these polarities may be highly exalted; and in this case they become subservient to chemical decompositions; and by means of electrical arrangements, the constituent parts of bodies are separated in a uniform order, and in definite proportions. Bodies combine with a force, which in many cases is corTespondent to their power of exhibiting electrical polarity by contact; and heat, or heat and light, are produced in proportion to the energy of their combination. Vivid inflammation occurs in a number of cases in which gaseous matter is not fixed; and this phenomenon happens, in various instances, without the interference of free or combined oxygen.

Crystals of oxalic acid,' for example, (p. 159) touched by dry quicklime, exhibit electrical powers; and the acid is negative, the lime positive. All the acid crystals, upon which I have experimented, when touched by a plate of metal, render it positive. And in Voltaic combinations with single plates or arcs of metal, the metal is negative on the side opposed to the acid, and positive on the side or pole opposed to the alkali.

* Bodies that exhibit electrical effects previous to their chemical action on each other, lose this power during combination. Thus, if a polished plate of zinc is made to touch a surface of dry mercury, and quickly separated, it is found positively electrical, and the effect is increased by heat; but if it be so heated as to amalgamate with the surface of the mercury, it no longer exhibits any marks of electricity.When any conducting substance, capable of combining with oxygen, has its positive electricity increased, it will attract oxygen with more energy from any imperfect conducting mediun; and metallic bodies, that in their common state have no action upon water, such as silver, attract oxygen from it easily, when connected with the positive pole in the Voltaic circuit ; and bodies that act upon wateí, such as zinc and iron, so as to decompose it slowly, refuse to attract oxygen from it, when they are negatively electrified in the Voltaic circuit.

• Acids, which are negative with respect to alkalis, metals, and earths, are separated from these bodies in the Volinic circuit at the positive surface; and alkalis, metals, and earths, are separated from acids at the negative surface: and such are the attracting powers of these surfaces, that acids are transferred through alkaline solutions, and alkalis

through

through acid solutions, to the surfaces where they have their points of rest. It is easy to show this by making a combination of three agate cups, one containing sulfate of potassa, one weak nitric acid, and the third distilled water, and connecting them by asbestus moistened in pure water, in such a manner, that the surface of the acid is lower than the surface of the fluid in the other two cups. When two wires of platina, from a powerful Voltaic apparatus, are introduced into the two extreme cups, the solution of the salt being positively electrified, a decomposition will take place, and in a certain time a portion of potassa will be found dissolved in the cup in contact with the negative wire, though the fluid in the middle cup will still be sensibly acid.'

We must here take the liberty of remarking, that several of these singular effects had been observed by Hisinger and Berzelius in Sweden a year or two before the date of Sir Humphry Davy's discoveries: but they had neither led those chemists to entertain any suspicion of the true laws by which they are governed, nor to apply them to the production of any unknown substances. The first of the remarkable decompositions that our author effected, by means of his newly established principles, was that of potass, or the vegetable fixed alkali, from which he obtained the new metal potassium in October 1807. When a thin piece of pure or caustic potass, in its usual state of a dry hydret, or combination with water, is placed between two discs of platina connected with the extremities of a Voltaic apparatus of 200 double plates; it will soon undergo fusion, osygen will separate at the positive surface, and small metallic globules will appear at the negative surface, which consist of potassium.' It may also be procured by heating iron filings to whiteness in a gunbarrel, and suffering melted potass to come slowly into contact with them, as MM. Gay Lussac and Thénard discovered; and even by strongly igniting potass with charcoal, as Mr. Curaudau has shown. This metal is about one seventh specifically lighter than water; it fuses at about 150° of Fahrenheit, and becomes gaseous below a red heat. It intiames violently when moistened, or when gently heated in contact with the air, affording alkaline fumes. Its powerful attraction for oxygen renders it a very useful agent in chemical analyses : naphtha seems to be almost the only substance in which it can be kept with convenience.

Soda, the mineral alkali, affords, when similarly treated, though not quite so easily, a metal much resembling potassium, but a little heavier, though still lighter than water; fusible at about 200°, and evaporating at a strong red heat: our author has very properly named it sodium; it agrees with potassium in most of its properties. Barium was obtained in May 1808, in the form of a dark

grey mass, with little lustre, by means of a process suggested by MM.

E 4

Berzelius

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

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 ingemuity 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 a 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 1:1ajority 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

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 bas 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' that 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 throw's many new lights, that is, the simplicity of the proportions of chemical combinations, the proofs are so numerous and satis

factory,

« ΠροηγούμενηΣυνέχεια »