The decomposition of the fixed alkalies and alkaline earths (1807-1808) / Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1901
    seemed probable that the inflammable matter arose from its decomposition. The residual potash was unaltered ; it contained indeed a number of dark grey metallic par- ticles, but these proved to be derived from the platina. I tried several experiments on the electrization of potash rendered fluid by heat, with the hopes of being able to collect the combustible matter, but without success; and I only attained my object, by employing electricity as the common agent for fusion and decom- position. Though potash, perfectly dried by ignition, is a non- conductor, yet it is rendered a conductor, by a very slight addition of moisture, which does not perceptibly destroy its aggregation; and in this state it readily fuses and decomposes by strong electrical powers. A small piece of pure potash, which had been exposed for a few seconds to the atmosphere, so as to give con- ducting power to the surface, was placed upon an insulated disc of platina, connected with the negative side of the battery of the power of 250 of 6 and 4, in a state of intense activity; and a platina wire, communicating with the positive side, was brought in contact with the upper surface of the alkali. The whole apparatus was in the open atmosphere. Under these circumstances a vivid action was soon observed to take place. The potash began to fuse at both its points of electrization. There was a violent effervescence at the upper surface; at the lower, or negative surface, there was no liberation of elastic fluid ; but small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver, appeared, some of v/hich burnt with explosion and bright flame, as soon as they were formed, and others remained, and were merely tarnished, and finally covered by a white film which formed on their surfaces.
    These globules, numerous experiments soon shewed to be the substance I was in search of, and a peculiar inflammable principle the basis of potash. I found that the platina was in no way connected with the result, except as the medium for exhibiting the electrical powers of decomposition ; and a substance of the same kind was produced when pieces of copper, silver, gold, plumbago, or even charcoal were employed for compleating the circuit. The phenomenon was independent of the presence of air; I found that it took place when the alkali was in the vacuum of an exhausted receiver. The substance was likewise produced from potash fused by means of a lamp, in glass tubes confined by mercury, and furnished with hermetically inserted platina wires by which the electrical action was transmitted. But this operation could not be carried on for any considerable time ; the glass was rapidly dissolved by the action of the alkali, and this substance soon penetrated through the body of the tube. Soda, when acted upon in the same manner as potash, exhibited an analogous result; but the decomposition demanded greater intensity of action in the batteries, or the alkali was required to be in much thinner and smaller pieces. With the battery of loo of 6 inches in full activity I obtained good results from pieces of potash weighing from 40 to 70 grains, and of a thickness which made the distance of the electrified metalHc surfaces nearly a quarter of an inch ; but with ji similar power it was impossible to produce the effects of decomposition on pieces of soda of more than 15 or 20 grains in weight, and that only when the distance between the wires was about i or of an inch. The substance produced from potash remained fluid at the temperature of the atmosphere at the time of its pro-
    duction; that from soda, which was fluid in the degree of heat of the alkali during its formation, became solid on cooling, and appeared having the lustre of silver. When the power of 250 was used, with a very high charge for the decomposition of soda, the globules often burnt at the moment of their formation, and sometimes violently exploded and separated into smaller globules, which flew with great velocity through the air in a state of vivid combustion, producing a beautiful effect of con- tinued jets of fire. III. Theory of the Decomposition of the fixed Alkalies ; their Composition, and Production. As in all decompositions of compound substances which I had previously examined, at the same time that combustible bases were developed at the negative surface in the electrical circuit, oxygene was produced, and evolved or carried into combination at the positive surface, it was reasonable to conclude that this substance was generated in a similar manner by the electrical action upon the alkalies ; and a number of experiments made above mer- cury, with the apparatus for excluding external air, proved that this was the case. When solid potash, or soda in its conducting state, was included in glass tubes furnished with electrified platina wires, the new substances were generated at the negative surfaces ; the gas given out at the other surface proved by the most delicate examination to be pure oxy- gene ; and unless an excess of water was present, no gas was evolved from the negative surface. In the synthetical experiments, a perfect coincidence likewise will be found. I mentioned that the metallic lustre of the substance from potash immediately became destroyed in the atmo- sphere, and that a white crust formed upon it. This crust
    I soon found to be pure potash, which immediately deliquesced, and new quantities were formed, which in their turn attracted moisture from the atmosphere till the whole globule disappeared, and assumed the form of a saturated solution of potash.* When globules were placed in appropriate tubes con- taining common air or oxygene gas confined by mercury, an absorption of oxygene took place; a crust of alkali instantly formed upon the globule ; but from the want of moisture for its solution, the process stopped, the interior being defended from the action of the gas. With the substance from soda, the appearances and effects were analogous. When the substances were strongly heated, confined in given portions of oxygene, a rapid combustion with a bril- liant white flame was produced, and the metallic globules were found converted into a white and solid mass, which in the case of the substance from potash was found to be potash, and in the case of that from soda, soda. Oxygene gas was absorbed in this operation, and nothing emitted which affected the purity of the residual air. The alkalies produced were apparently dry, or at least contained no more moisture than might well be conceived to exist in the oxygene gas absorbed; and their weights considerably exceeded those of the combustible matters consumed. * Water likewise is decomposed in the process. We shall here- after see that the bases of the fixed alkalies act upon this substance with greater energy than any other known bodies. The minute theory of the oxydation of the bases of the alkalies in the free air, is this:—oxygene gas is first attracted by them, and alkali formed. This alkali speedily absorbs water. This water is again decomposed. Hence, during the conversion of a globule into alkaline solution, there is a constant and rapid disengagement of small quantities of gas.
    The processes on which these conclusions are founded will be fully described hereafter, when the minute details which are necessary will be explained, and the proportions of oxygene, and of the respective inflammable substances which enter into union to form the fixed alkalies, will be given. It appears then, that in these facts there is the same evidence for the decomposition of potash and soda into oxygene and two peculiar substances, as there is for the decomposition of sulphuric and phosphoric acids and the metallic oxides into oxygene and their respective combustible bases. In the analytical experiments, no substances capable of decomposition are present but the alkalies and a minute portion of moisture ; which seems in no other way essen- tial to the result, than in rendering them conductors at the surface : for the new substances are not generated till the interior, which is dry, begins to be fused ; they explode when in rising through the fused alkali they come in con- tact with the heated moistened surface; they cannot be produced from crystallized alkalies, which contain much water; and the effect produced by the electrization of ignited potash, which contains no sensible quantity of water, confirms the opinion of their formation independ- ently of the presence of this substance. The combustible bases of the fixed alkalies seem to be repelled as other combustible substances, by positively electrified surfaces, and attracted by negatively electrified surfaces, and the oxygene follows the contrary order; * or the oxygene being naturally possessed of the negative energy, and the bases of the positive, do not remain in combination when either of them is brought into an electrical state opposite to its natural one. In the syn- thesis, on the contrary, the natural energies or attractions ♦ See Bakerian Lecture 1806, page 28 Phil. Trans, for 1807.