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both melting-point and atomic heat in inverted order. The same order is found to hold in their latent heats of fusion. Thus Person found that tin requires 14.25 cals., whilst, according to Rudberg, lead requires only 5.8. There are no data for comparing antimony and bismuth. In the zinc group we find that the latent heat of fusion varies directly as the melting-point.
The irregularities in group III.a as to melting-point cannot be explained in the above manner. The elements that follow Al, viz. Ga, In, and Tl, behave, as regards atomic heat and melting-point, like the non-metallic groups.
The inverse ratio between melting-point and atomic heat in the intermediate groups is well illustrated by carbon and boron. The former, which has never been melted, has the lowest atomic heat of all solid or liquid elements; whilst the latter, which is very infusible, has the next lowest.
If the above laws are true, they should enable meltingpoints not yet ascertained, as of thorium, molybdenum, &c., to be predicted with tolerable accuracy. Thus, Th should melt at about 700°, Mo at about 2000°. Buchholz found this latter to be imperfectly fusible at a white heat. Again, the melting-points and specific heats of new elements should be capable of being predicted with much greater accuracy than has been possible hitherto. Thus Brauner's Bohemium (see Nature' for October 11, 1894) has had the specific heat of 0.03 predicted for it from Dulong and Petit's constant, but no melting-point. By referring to group VI., where it will occur after tellurium, Bo=213, it is easy to see that this element will have a melting-point of about 650° and specific 6.1 heat of about =0.0286. 213
The following is a carefully calculated table of volumeheats, a further factor for comparing the elements, of value for their classification, introduced by Dr. Wm. Preyer, Das genetische System der chemischen Elemente (Berlin, 1893). Volume-heat is the atomic heat the atomic volume, and therefore CW÷ = = CD; where C is the specific heat, W the atomic weight, and D the specific gravity of the element.
The specific gravities are taken at 0°, and the specific heats at a constant temperature also, as near 15° as possible. The natural groups are those of Lothar Meyer, as modified by
W. Preyer. I have adopted Prof. Meyer's device of lettering the sub-groups, adding groups b and c, as in the preceding article. The values are from Landolt and Börnstein, Phys.Chemische Tabellen, or more recent determinations.
Phil. Mag. S. 5. Vol. 39. No. 236. Jan. 1895.
Crystalline Arsenic has the values C=083, D=5·73, CD=0·475 (Petten
dorff and Wüllner).
Group V.a. Vanadium, Niobium, and Tantalum have not had their specific heats determined.
Schütz, Deville and Debray.
Most observers find D for Pd as low as 115; Lowry's value seems reliable,
and gives concordant results.
Chlorine is neglected, as are Oxygen and Nitrogen, because we have only their specific heats as gases.
Manganese... 01217 7.39 0.899
It will be noted from the above that in group I. the atomic heats are nearly Li=4a, Na=2a, Ka, where a is 0.144. In group II., similarly, the ratio is Be=5a, Mg=3a,
In group VII. the volume-heats are constant. Several elements, of known atomic weight, offer no data for both C and D, and are altogether omitted.
The above tables make clear the following laws:
1. In each natural group of elements volume-heat varies inversely as atomic volume. (Group IV.a is an exception as far as available data go.)
2. The variations of volume-heat become less and less as valency for oxygen rises, until the seventh group is reached, when it becomes constant.
3. As atomic heat increases in some groups and decreases in others, with increase of atomic weight, whilst atomic volume regularly increases, it is evident that the increase in atomic volume proceeds at a higher ratio than the variation in atomic heat.
4. Atomic weight being a constant increment, it follows from the preceding law that in any natural group specific gravity varies more than specific heat.
VII. On a Suggestion by Professor J. J. Thomson in Connexion with the Luminescence of Glass due to KathodeRays. By JOHN BURKE, B.A., Lecturer in Physics, Mason College, Birmingham*.
AST August, at Oxford, I communicated a paper to the British Association on a strange luminous phenomenon which had been observed by Beccaria more than a hundred years ago. It was there pointed out that although the conclusions arrived at by the Italian physicist, if true, were likely to lead to results of an extremely interesting character, in connexion with Mr. Crookes' important researches on the luminescence of glass, &c., in vacuum-tubes, and although the mysterious nature of the phenomenon was likely to attract much attention, yet the subject was allowed to retain its obscurity. Beccaria (Artificial Electricity,' § 766) observed that when vacuum-bulbs were broken in the dark a light, consisting of a faint glow, was produced in the place where the bulb lay. He attributed an electrical origin to the phenomenon, owing to the manner in which certain substances were supposed to behave in yielding the glow. He mentioned, moreover, that the mere breaking of glass did not give rise to the phenomenon, but that the presence of air was essential to its production, and that when air was allowed to rush suddenly into a vacuum by the bursting of a bladder at the mouth of a * Communicated by the Physical Society: read November 9, 1894.
receiver, articles such as glass bottles, vessels of sealing-wax, &c., became luminous by, he supposed, the violent dashing of the external air on them; the luminosity was most conspicuous at the necks of the bottles or at the upper edges of the vessels. As has already been remarked, Beccaria's researches appear to have been forgotten for more than a century, and not until quite recently has attention been drawn to them. This has been done by Professor J. J. Thomson, who, in his Recent Researches in Electricity and Magnetism,' p. 119, recalls the fact and indicates its possible close relationship to Mr. Crookes' theory of the luminescence of the glass in Geissler's tubes : that the bombardment of the glass by the particles of gas projected from the kathode is intense enough to cause the glass to become luminous. Prof. Thomson quotes from Priestley's History of Electricity':
Signor Beccaria observed that hollow glass vessels, of a certain thinness, exhausted of air, gave a light when they were broken in the dark. By a beautiful series of experiments, he found, at length, that the luminous appearance was not occasioned by the breaking of the glass but by the dashing of the external air against the inside when they were broke. He covered one of those exhausted vessels with a receiver, and letting the air suddenly on the outside of it observed the very same light.'
That the light observed in both cases was the same, unless the exhausted vessel, which had been covered by the receiver, was broken by the dashing of the external air against it, is a circumstance which, from considerations that shall presently be adduced, I think we may be justified in questioning.
Through the kindness of Prof. FitzGerald in extending to me the facilities afforded in the Physical Laboratory of Trinity College, Dublin, I have been enabled to experiment upon this subject. It would be impossible for me to attempt to render in detail the acknowledgment of what has been due to his invaluable advice and suggestions.
It must be mentioned at the outset that the present investigation has by no means been completed, yet, thus far, it appears that some of Beccaria's results have not been wholly of that degree of exactness which we should have hoped for. It must, however, be borne in mind that scientific appliances in his time were less perfect than they are to-day, and such mperfection may account for much inaccuracy.
A number of incandescent lamps of various sizes with broken filaments were procured. An observer who had been fifteen or twenty minutes in the dark, whose sight had become