We proceed to notice some modern and comparatively recent expe rimental investigations, involving the production of nearer and nearer approximations to a physical vacuum, or the more and more complete removal of ponderable matter from an inclosed spnce. It is matter of this kind only, the quantity of which, in a given space, can at all be diminished ; that of the matter of a higher order, the ether which manifests and transmits heat and light and perhaps magnetism, accord ing to our present experimental means, cannot be affected ; though apparently acted upon in a certain mannner by the molecules of ordinary matter, it cannot be confined or diminished in amount, any more than it can be measured or weighed. Or,—to express this in terms independent of theory,—a vacuum transmits light and heat, diminished only by the imperfect transparency and tmnscaleseence of the including vessel. In the investigation in question, more or less perfect vacua have been obtained by the air-pump ; others on the prin ciple of the space void of air left above the mercury in the barometer, called the Torricellian vacuum •, some by the combination of both these means; and others again by the union of one or both with chemical agency, by which apparently the most perfect vacua have been produced.
Dr. Thomas Andrews, Vice-President of Queen's College, Belfast, whose refined physicochemical researches havo required the use of the nearest approach to a perfect vacuum in which certain instruments could be observed, has devised a method of obtaining pro- • bably a more perfect air-pump vacuum than had before been produced. He characterises the Torricellian as the nearest approach to a perfect vacuum which at the time when his method was devised had been obtained "It is true," he remarks, " that it contains a little mer curial vapour at the ordinary temperature of our summers, and probably also nt lower temperatures, but the quantity is exceedingly small, and its influence in depressing the barometric column must be altogether inappreciable. Besides the mercurial vapour, a trace of air may gene rally be detected." Dr. Andrews shows that it is easy to calculate approximatively the depression of the column produced by this ; and he finds that if the diameter of the bubble be inch, the pre.ssuro of mercury under which it has been !immured 2 inches, and the volume of the space above the mercury when the tube is vertical cubic inch, the depression of the mercurial column is nearly ; " or the depression of she mercury, In cousequence of the vacuum not being absolutely perfect (with respect to air), amounts only to of an inch. It is easy in actual practice," Dr. Andrews continues, " to realise • Fee a critical notice of Faraday's ' Exp. Iles? in ' Phil. ]tag.,' June, 1839, ser. lit vol ate., p. 469.
t 'Second Liam:nation on the Progress of idathematleal and Physical Science s' supplement to Loe)clopiedia BrItanoica,' vol. iv., p. 83, note ; Locy. Brit.,' 8th edit., rot ti. 684, note.
j Dow impoitant to the progress of tiro science of physical forces this calling the attention of philosopher. to Newtun's view. has realty been, will appear from the following extract from the ' Theory of the Force of Gravity,' by Profeseor Challis, published in the ' Philosophical Magazine' for December, 1589, p. 442. "The aerie in dishing has been so long and so extensively re,. girded as an nIUmale principle, and not its a temporary hypothesis admitting eventually of explanation, that It requites acme degree of moral cuur..ge to maintain a different theory. Science, to my opinion, Is much Indebted to Pro fessor Faraday for having recently directed attention to tha opposite views entertained by Newton on this point, and tor giving ex; ression to analogous ideas of his own. (see the Lecture on the Conservation of Force, In the • Hill. Mae.' fur April, 1327, vol. alit., p. 232.)" 'the lecture refuted to was delivered In the preceding February ; tut Faraday, as II e bane seen above, had originally directed attention to Sesotho'. eft WI In 1883 ; and he regarded their impotence
as so great, that be had recalled them also to hie discourses of the two following years, this close approximation to a perfect vacuum," and the quantities here stated, he says, apply to a barometric tube employed in an experiment he aubeequently describes. Observing, however, that the Torricellian vacuum is unfortunately applicable to very few physical investigations, as no instrument of any kind can be introduced into it, nor even any aubstanco which is acted upon by mercury, and noticing the imper. fection of the ordinary air-pump and of 31. Regnault's method of 'othing the exhaustion further after the valves have ceased to act, he proceeds to describe, in the following terms, a process by which, with very little trouble, a vacuum may be obtained in the ordinary receiver of an air-pump, so perfect that the residual air exerts no appreciable elastic force.
" Into the receiver of an ordinary air-pump, which is not required to exhaust further than to 0•3 inch, or even 0.5 inch, but which tuust retain the exhaustion perfectly for any length of time, two open vessels are Introduced, one of which may be conveniently placed above the other; the lower vessel containing concentrated sulphuric acid, the upper a thin layer of a solution of caught potash, which has been recently concentrated by ebullition. The precise quantities of these liquids is not a matter of importance, provided they are so adjusted that the acid is capable of desiccating completely the potash ikaution, without becoming itself notably diminished in strength, but at the same time does not expose so large a surface as to convert the potash into a dry mass in less than five or six hours at the least. The pump is in the first place worked till the air in the receiver has an elastic forco of 01 or 0'4 inch, and the stopcock below the plate is then closed. A communication is now established between the tube for admitting air below the valves, and a gasholder containing car• bottle acid, which has been carefully prepared so as to exclude the presence of atmospheric air. After all the air has been completely removed from the connecting tubes by alternately exhausting and admitting carbonic acid, the stop-cock below the plate is opened. and the carbonic acid allowed to pass into the receiver. The exhaustion is again quickly performed to about the extent of half an inch or leas. If a very perfect vacuum is desired, this operation may be again repeated ; and if extreme accuracy is required, it may be performed a third time It is not likely that anything would be gained carry ing the process further. On leaving the apparatus to itself, the car bonic acid which has displaced the residual sir is absorbed by the alkaline solution, and the aqueous vapour is afterwards removed by the sulphuric acid. The vacuum thus obtained is so perfect, that even after two operations it exercises no appreciable tension." Even after this limit has been reached, the exhaustion may be pushed still further, "till it must become at last not less complete than the Torricellian vacuum ; while at the same time by suppressing the manometer, the existence of mercurial vapour may be altogether prevented. The manipulation required to arrive at this result will not interfere with the presence of even the most delicate instruments in the receiver." In an experiment which Dr. Andrews describes, the theoretical residue of air would be of the entire quantity in the receiver, which would cause a depression of of an inch only, and this, he says, must have been nearly realised. Such a vacuum has remained without the slightest change for fourteen days. ' PhiL Mag.,' Feb , 1852 ; ' Quart. Journ. of Chem. Soc.,' vol. v., p. 189-192.