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On the Change in the Absorption-Spectrum of produced by Heat

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THE PHYSICAL- SOCIETY

OF LONDON.

FEBRUBBY1891. XI. On the Chnye in the Ab8orpt&m=8’ectmm of Cohalt Glass produced by Heat. By Sir JOHN CONROY,Bt., M.A., Fellow and Bedfwd Lecturer of Ballwl Colkge, and Millard Lecturer of Trinity College, Oxford*. SIB DAVIDBREWSTER made some experiments on the influence of heat on the absorbing power of coloured media, and states in his ‘ Treatise on ’ (edit. 1853, p. 174) that he ‘‘ was surprised to observe that it produced opposite effects upon different , diminishing the ab- sorbing power in some, and increasing it in others.” He found that the transparency of a piece of purple glass was much increased on heating, whilst that of a yellowish-green glass and of a red glass was diminished ; the purple glass recovered its colour on cooling, the other two did not do so completely. Feussner (Fortschritte der Physik, 1867, p. 237) mads some observations on the effect of heat on the absorption- spectra of substances in solution. No observations on the effect of heat on the transparency of solid substances for rays of different refrangibilities except

* Read February 13,1801. 104 SIR JOHN CONROY ON THE CHANGE IN THE thosesof Sir David Brewster appear to have been published, although, of course, the change of colour which borax blow- pipe-beads containing certain metallic oxides undergo on cooling is well known. I therefore venture to communicate to the Physical Society some determinations which I have recently made on the changes produced by heat in the absorption-spectrum of cobalt glass. The absorition-spectra of coloured glasses are not, as a rule, very Characteristic, and merely show a continuous absorption extending through a considerable psrtion of tbe spectrum. Cobalt glass, however, as is well-known, has a characteristic absorption-spectrum consisting. of three dark bands, in the red, yellow, and green, with a considerable amount of absorp tion between the first two; so that with a rather deeply coloured glass the transmitted light consists merely of the extreme red, some yellowish-green, and the and violet rays. A small piece of this glass was heated by means of 8 medium-sized Bunsen (15 millim. tube), the glass being supported on, and nearly surrounded by, combustion-furnace tiles. It was found that in this way the glass could be heated till the edges began to soften and were visibly red, without much risk of its cracking either whilst being heated or during its subsequent cooling. An ordinary gas-jet was used as the source of. light, and the light transmitted by the glass examined with a spectroscope ; a small direct-vision one being used, as it was found that the changes in the spec- trum were less distinctly seen with a spectroscope of greater dispersive power. The absorption-spectrum of the cold glass is represented in fig. 1, which is drawn to an arbitrary scale, and not to one of wave-lengths. On gradually heating the glass, the absorption between the two first dark bands, those in the red and yellow, diminishes. The band in the red moves towards the least refrangible ettd of' the spectrum, whilst those in the yellow and green retain their position, but become less distinct, Fig. 2 represents the spectrum of the hot glass. As the glass is heated the intensity of its colour decreases ; as it cools it recovers its original colour, and the absorption- spectrum changes back into that represented by fig. 1. ARSORPTION-SPECTRUM OF COBALT QLASS BY HEAT. 105

Tho position of the bands was measured, and the table gives the results, reduced to a scale of wave-lengths. Cold Glm. Hot Glees. Dr. Russell’e Vsluea. 712 6G5 Band I.( 636700 655 632 608 608 605 Band 11.i 580 583 588 565 558 555 ’“nd ‘1’. { indistinct indistinct Dr. W.J. Russell, in a paper on the “Absorption Spectra of Cobalt Salts ” (Proc. Roy. Soc. xxxii. p. 258) gives a map of the spectrum of cobalt glass drawn to a scale of wave- lengths. The position of the bands, as shown in Dr. Russell’s map, is given in the third column of the above table. The agreement between the values obtained with the cobalt glass when cold and those from Dr. Russell’s map, except for the least refrangible edge of the first band, are as close as, perhaps, could well be expected, considering the small dis- persion of the spectroscope nsed for the measurements, and the difficulty of determining the exact position of an absorp tion band. There is considerable difference with regard to the position of the least refrangible edge of band 1 ; that the position assigned to it for the particular sample of glass used in these experiments is, at least, approximately correct, is shown by the fact that the red lithium line of wave-length K2 H 106 MR. T. H. BLAEESLEY’S FURTHER 670 lies within the band, both when the glass is cold and when it is hot. These observations and those of Feussner show that the absorption-spectra of some substances vary with the tempera- ture, as indeed might have been anticipated from the behaviour of the blowpipe-beads already referred to. In the case of solutions this may be due to the formation of different hy- drates, or to the partial dissociation of the substances ; but in the case of a solid substance, like cobalt glass, an actual change in the chemical constitution of the glass at a tempe- rature considerably below its fusing-point does not seem very probable, although the well-known effects to light in causing glass which has been decolorized with manganese dioxide to become purple seems to show that such a change is not im- possible.

XII. Further Contributions to Dynamometry, or theeNeasure- ment of Power. By T. H. BLAKESLEY,MA., M. Inst. C.E.* Now that the advantage of using the split dynamometer to measure power, as first proposed by me more than half a decade since, seems to be at length receiving some attention through the generalization of the method when applied to transformers, proved independently by Prof. W. E. Ayrton and myself t, it may be well to point out clearly the principles upon which such an instrument must be inserted into an electrical system to effect the measurement of a phy- sical quantity, and the nature of the quantities which admit of such measurement. In the first place, an exact idea must be formed of the nature of the physical quantity indicated by the reading of a dynamometer, or the angle through which the torsion-head is turned to bring the coils into a standard relative position, which is usually, but not necessarily, one in which the coils are at right angles one to the other. That position has the advantage of introducing no mutiial induction in the instrument itself. * Read February 27,1891. f To Professor Ayrton and Mr. Taylor belonga undoubtedly the credit of priority in this generalization, Professor Apton informed me of the fact, but left me to diacover the p0qJ-T. H. B.