Unless the still more perfect carbonic-acid vacua employed by 31r. Gassiot, and about to be described, should be obtainable in com paratively large vessels adapted for the observation of instruments, &&., —towards which result, indeed, 31r. Gassiot has made some approach, —Dr. Andrews's method appears to be the moat eligible tor the generality of exact researches for which a vacuum is required.
In a paper ' the Electrical Phenomena exhibited in Vaeue ' by Sir H. Davy, in the ' Philosophical Transactions' for 1822, lie relates some experiments which he made for the purpose of elucidating "the relations of electricity to space, as nearly void of matter ea it can bo made on the surface of the earth." He repeated the electrical experi ments with the Torricellian vacuum of Morgan and Walsh, and Instituted others with similar vacua above a difficultly fusible amalgam of mercury and tin, and above fused tin. With the results as bear ing on the theory of electricity we are not nt present concerned ; but these approximations to vacuous space were, of course, in reality, atmospheres of the vapours of tho metals employed, though of excessive rarity ; and a calculation made by Mr. liabbage for the author may be cited, as indicating how minute must have been the quantity of matter which they contained—how great mud have been its " attenuation," in the language of the present day—which is a point of Information important to the subject of this article. Considering the elastic force of vapour of water at 52' to be equal to raise by its pressure about '45 of an inch of mercury ; the relative strengths of vapour will be, reckoning the boiling points all from 52°, for mercury at 600°, .000015615, and for tin, at 5000', 37015, preceded by 48 zeros. Tho data on the diminution of the density of vapours by diminution of temperature supplied In this case by the chemist to the mathema tician were probably in some (levee erroneous, and the results would be affected by the limit to vaporisation for every substance at a certain temperature which Faraday, a few years after, rendered so highly probable, If not certain, but the latter would operate to diminish the density of the metallic vapours in question; and we may, all things considered, accept these numbers as fair expressions of the minute quantities of matter they are intended to represent.
Mr. Grove (` Phil. Trans.' and 'Phil. Mag.,' 1S52) having originally observed a peculiar striation in electric discharges taken in a well exhausted air-pump receiver, apparatus in some respects similar to that employed by Davy has been constructed by Mr. Gassiot, for the
further investigation of that phenomenon as observed in Torricellian vacua, and partly by means of a process devised by the late Mr. John Welsh (` Phil. Trans.' 1856, p. 507), he has produced more perfect Torricellian vacua than any before obtained. 'Phil. Trans.,' 1858, PP. 3, 5.
In the, continuation of the researches which these experiments of Mr. Grove and Mr. Gassiot initiated, the united science and ingenuity of several physicists, chemists, and mechanicians, have produced vacua still more perfect, that is, still more devoid of ordinary or ponderable matter ; more strictly again, spaces in which a smaller amount of such matter existed than in any obtained before, and probably greatly exceeding in this respect the most perfect Torricellian vacua previously experimented with ty the old electricians, and by Davy, as well as those first employe 1 by Mr. Gassiot himself. For the purpose of obtaining them, a method has been adopted, often indicated, and to a certain extent employed, in chemical and physical research, but now pursued with much greater care and refinement, and with all the resources to ensure accuracy which the most delicate operations of modern chemistry can supply. The vacuum tubes to which we now allude appear to have been first constructed about 1857, by M. Geissler, of Bono, and the vacua obtained by an application of the method alluded to, but which, it is understood, he has not precisely explained. Mr. Gassiot, desirous of knowing, during the progress of the experi mental research upon which he had entered, the exact conditions under which each particular vacuum had been obtained, and finding that there was some uncertainty in the description of those he had obtained from 3E. Geissler, employed Mr. Casella to construct above 100 new tubes, each of which, however, was charged and exhausted by himself or in his presence. Each tube was filled, in the first instance, with atmospheric air, hydrogen, oxygen, or nitrogen, then exhausted by a good air-punip ; :soother supply of air or of gas admitted, and the tube again exhausted ; after the repetition of this process two or three times, mercury was introduced, and the tube finally exhausted as a Torri cellian vacuum, and lastly hermetically sealed ; the attenuated medium within thus being mercurial vapour, plus the remains of air or of the gas with which the tuLe had been originally filled.