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Origin of the Continuous Spectrum. There remains to be considered the continuous light seen when the sodium is first volatilized, and before the broad D emission-line appears (fig. 3, a). After a number of experiments, made under various conditions as to the purity of the coal-gas or nitrogen employed and density of the sodium vapour, I conclude that this radiation is really due to free sodium, and that it always appears when the vapour is above a certain density. It is well known that dense sodium vapour gives a remarkable banded absorption-spectrum *. In the experiment alluded to, I have studied the continuous emissionspectrum in relation to this banded absorption, and find that they are intimately connected: thus, when the sodium is first volatilized and a bright glow fills the tube, the vapour appears a splendid violet colour by transmitted light; it is in fact opaque to all rays except the violet; gradually, however, green rays begin to be transmitted (fig. 4, a), then red (fig. 4,1); the dark space between the


and the violet is now seen to be made up of a large number of black lines (close together with the low dispersion employed), these rapidly decrease in intensity as the vapour diffuses along the tube and becomes less dense, while the black band blotting out the yellow assumes the usual appearance of the wide D absorption-band f. The continuous emission glow persists through these progressive changes in the absorption, but becomes gradually fainter, and as the last traces of the absorption-lines in the blue disappear it fades away almost entirely, leaving behind, so to speak, the broad bright emission-band at D, the exact reverse of the absorption-band, except for the black line filling up the centre,due to relatively cool vapour in the nearer part of the tube.

It seems, therefore, that dense sodium vapour, like iodine and other coloured vapours giving bandel absorption-spectra, emits when in this state light of all wave-lengths, and that the change to discontinuous emission is determined by a

ction of density. There are indications, however, that at higher temperatures the continuous light would give place to bands corresponding with the absorption-bands, just as iodine is said to give a banded emission spectrum when heated

* Vide Roscoe & Schuster, Proc. Roy Soc. xxii. p. 362.

† A curious phenomenon may be seen when a current or gust of gas is allowed to drive the dense vapour along the tube. The colour of the vapour seen by transmitted light suddenly changes from green to a splendid ruby-red. This is not due to a change in the character of the absorption-spectrum, but way be explained as an effect of refraction : the sodium vapour, being blown out more in the centre than along the sides of the tube, acts like a prism, refracting the blue and green rays radially from the centre, the red alone-being least deviated-passing out at the end of the tubé. This effect is more obvious with hydrogen than with nitrogen.

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externally to a high temperature (Salét, Traité sur Spectroscopie, p. 174) *. . For if the continuous light is watched at the highest temperature attainable with this apparatus, it is seen not to fade equally in all parts of the spectrum,-a stage is reached (see fig. 3,6) when, in addition to the D'emissionline, a green line appears in the place of a conspicuous absorption-line, and a faint glow remains in the blue corresponding to the absorption-bands in that region (figs. 3–4,6).


IN CRYSTALS. BY M. CHARLES SORET. The method used has been pointed out in a previous note (Archives de Genève, 1893, xxix. p. 355).

Heating by the method of Jannettaz a point on a face not perpendicular to the axis of rotation in a crystal in which the coefficients in question are not zero, isotherms should be obtained which are not symmetrical in reference to that diameter which coincides with the projection of the axis of rotation.

The crystals were mounted and centred at the end of a vertical axis provided with screw motions for setting the face horizontal. A small platinum sphere heated by an electrical current is applied against this face at a point A on the prolongation of the vertical axis.

Isotherms were obtained in the usual way, taking care to turn the crystal steadily and regularly during the heating so that this was symmetrical about the point A. This point, the intersection of the face investigated and the axis of rotation, was thus without any possible error in the centre of heating.

After obtaining the isotherm it was examined, without touching the adjustment, by means of an eyepiece with a micrometric scale. The crystal being first placed with its rotational axis in the plane of incidence, a rotation of 180° about A should modify the points of intersection of the micrometer and the isotherm, if this was symmetrical as regards the centre of heating.

Now no appreciable disymmetry could be observed any more than by the other methods previously employed. Small variations, evidently accidental, alter the direction from one observation to another on the same crystal, amounting to go to zb of the diameter of the curves. Irregularities of the same order were observed on isotropic plates and on faces of crystals, in which no constant deformation of the isotherms could be foreseen.

The researches were made on crystals of dolomite of Binn (face of the prism), and of Traverselle (face of the rhombobedron): on crystals of erythrite and on apatite of the Pfitschthal (face of the prism). The existence of coefficients of rotation in the crystals appears therefore more and more improbable.-From the Archives de Genève, communicated by the Author.

* If iodine is heated in a hard glass tube in a furnace until the glass begins to fuse, the colour of the glowing vapour changes from yellow to pale greenish white. This probably indicates the change to a discontinuous emission.







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Fig. 4.
The continuwus spectrum at the edges comes from the glowing walls of the tube.

Minterni Brog.lith.


Fig. 3







JUNE 1895.

XLVIII. On the Scale-Value of the late Dr. Joule's Thermometers. By ARTHUR SCHUSTER, F.R.S.*

[Plates V. & VI.] N order to bring the results of Joule's researches on the

modern experiments on the same subject, it is necessary to determine the scale-value of Joule's thermometers in terms of some easily reproducible standard.

We possess already a comparison by Joule himself of his thermometer with one used by Rowland, who has corrected Joule's result to the scale of his own air-thermometer.

Some doubt may still exist, however, as to the true scalevalue of these instruments, partly owing to the fact that we have no information how the comparison between Joule's and Rowland's thermometers was conducted, and partly because we do not know to what degree of accuracy Rowland's airthermometer would agree with that of the Bureau International des Poids et Mesures, which for the present must be considered as the standard.

The historical importance of the instruments used by Joule seemed to make it desirable therefore to subject them to a more extended investigation. The request which I made to Mr. B. A. Joule to allow me the use for a short time of his late father's thermometers was inet by a most ready compliance,

* Communicated by the Author. Phil. Mag. S. 5. Vol. 39. No. 241. June 1895. 2 K

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