But he was desirous of still farther refining his experiments, so as to exclude, as far as it was possible, the presence of any volatile matter; and in this part of the enquiry he displayed, in a very masterly manner, that happy talent in which he so far surpassed his contemporaries, of suggesting expedients and contriving new apparatus in order to vanquish practical difficulties.
To get rid of a portion of mercurial vapour, he employed a difficultly fusible amalgam of mercury and tin, which was made to crystallize by cooling in the tube; but, in this case, the results were precisely the same as when pure mercury had been used. He then attempted to make a vacuum above the fusible alloy of bismuth, but he found it so liable to oxidate and soil the tube, that he soon renounced farther attempts of this kind. Nothing discouraged, he determined to try the effects of a comparatively fixed metal in fusion. By melting freshly cut pieces of grain tin, in a tube made void after having been filled with hydrogen, and by long-continued heat and agitation, he obtained a column of fixed metal which appeared to be entirely free from gas; and yet the vacuum made above this exhibited the same phenomena as the mercurial vacuum, except that they were not perceptibly increased by heat: a fact which Davy must have anticipated, as he attributed the greater display of electrical light, at high temperatures, to the effect of increased density of vapour; it is therefore a matter of surprise that he did not give more importance to the phenomenon.
He made two experiments on electrical and magnetic repulsions and attractions in the mercurial vacuum, and he found that two balls, the one of platinum, the other of steel, properly arranged for the purpose, repelled each other, when the conducting wire to which they were attached was electrified in the most perfect mercurial vacuum, as they would have done in the usual cases: and that the steel globules were as obedient to the magnet as in the air; which last result, he observes, it was easy to have anticipated.
He also made some comparative experiments, with the view of ascertaining, whether below the freezing point of water the diminution of the temperature of the Torricellian vacuum diminished its power of transmitting electricity, or of being rendered luminous by it. To about twenty degrees, this appeared to be the case; but between twenty degrees above, and twenty degrees below zero, the lowest temperature he could produce by pounded ice and muriate of lime, it seemed stationary; and, as well as he could determine, the electrical phenomena were very nearly of the same intensity as those produced in the vacuum above tin.
"It is evident," he says, "from these general results, that the light (and probably the heat) generated in electrical discharges depends principally on some properties or substances belonging to the ponderable matter through which it passes: but they prove likewise that space, where there is no appreciable quantity of this matter, is still capable of exhibiting electric phenomena—viz. those of attraction and repulsion, &c.: a fact unquestionably favourable to the idea of the phenomena of electricity being produced by a highly subtile fluid or fluids, of which the particles are repulsive with respect to each other, and attractive of the particles of other matter."
However much we may admire the experimental address displayed in this paper, we must confess that its results are very far from being satisfactory. His having assumed, without proof, and even without examination, the theory that a perfect vacuum cannot be produced in the Torricellian tube, and made it the foundation of his reasonings, appears to me to have vitiated all his conclusions. Mr. Faraday has rendered it extremely probable, that a limit does actually exist to the production of vapour by bodies placed in vacuo,[76] beneath which they are perfectly fixed; and if this be true, it is evident that, at low temperatures, a perfect vacuum may be produced in the Torricellian tube; and it is highly probable that Davy did thus actually produce one in several of his experiments; especially in those where he found that, by a farther reduction of temperature, no farther diminution of electrical effect was perceptible: he had in fact arrived at this limit to vaporization, and therefore a farther reduction of temperature could not possibly influence the phenomena. In this point of view, the electrical light would seem to be primary, or independent of foreign matter.—But though the premises be granted, let the reader pause before he hastens to any conclusion; for the cloud of mystery has not been dissipated, it has only changed its place. At the termination of his paper, Davy indulges in a conjecture subversive of every conclusion deduced from experiments in vacuo. "When the intense heat," says he, "produced by electricity, and the strong attractive powers of differently electrified surfaces, and the rapidity of the changes of state, are considered, it does not seem at all improbable, that the superficial particles of bodies, which, when detached by the repulsive power of heat, form vapour, may be likewise detached by electrical powers, and that they may produce luminous appearances in a vacuum free from all other matter, by the annihilation of their opposite electrical states."
During the course of the enquiry, Davy is led to suppose that air may exist in mercury, in the same invisible state as it does in water, that is, distributed through its pores; and that absorption of air may, therefore, explain the difference of the heights of the mercury in different barometers. This, it must be confessed, if true, is a most disheartening fact, as it at once precludes the possibility of any thing like accuracy in our barometers; but Mr. Daniell, to whom on all subjects of meteorology we are bound to pay the greatest deference, differs altogether from our philosopher upon this point, and he adduces a single observation which he thinks nearly disproves the supposition. "All fluids," says he, "which are known to absorb air into their pores, invariably emit it when the pressure of the atmosphere is removed; but, upon an extensive examination of large bodies of mercury, variously heated in the vacuum of an air-pump, I never saw a bubble of air given off from the surface of the metal." Davy, it must be stated, obtained a far different result; but an observation of Mr. Daniell explains the cause of it. "Air," he continues, "will rise from the contact of the mercury with the glass in which it is contained, in exact inverse proportion to the care with which it has been filled, but it never rises from the surface of the mercury alone. The difficulty of properly filling a barometer tube, I attribute to the attraction between the glass and the air—not to that between the mercury and the air."[77]
On the 13th of June 1822, a memoir was read before the Royal Society, "On the state of Water and Aëriform matter in cavities found in certain Crystals. By Sir Humphry Davy, Bart. P.R.S."
It is generally admitted by Geologists, that the greater number of the crystalline substances of the mineral kingdom must have been previously in a liquid state; but different schools have assumed different causes for their solution; some attributing the effect principally to the agency of water, others to that of heat.
In the paper under consideration, the author very freely avows himself as the champion of the latter doctrine.