October 1, 1804.] 



KNOWLEDGE 



217 



y.^* AN ILLUSTRATED «<^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: OCTOBER 1, 1894. 



CONTENTS. 



Experiments on the Radiation of certain Heated 



Gases and Vapours. By J. Evershed. F.R.A..S. ... 217 



The Ancient Mammals of Britain.— III. BvR.Ltdeekee, 



B.A.Cantab., F.R.S 221 



Insect Secretions.— VI. By E. A. Butleb, B.A., B.Sc. ... 223 



Photographs of the Milky Way and Nebulae. By A. C. 



Kaxv.ird 226 



Notices of Books 22() 



Letters :— S. W. I'.rRxit.iJr ; T. W. Backhouse ; J. Logax 



LoBi.EY ; TV. T. Ltn-x . 228 



Cellulose and some of its more Recent Applications. 



By Chas. A. SiLisERRAD, B.A., B.So 230 



Sympathetic Explosions. By C. A. Mitchell, B.A.Oxon. 231 



Globular Star Clusters. By J. E. Gore, F.R.A.S. ... 232 



On the Inhabitants of some Common Galls. By the 



Rev. Alex. S. Wilson, ILA., B.Se " ... 235 



The Face of the Sky for October. By Hebbebt 



Sadleb, F.R.A.S 237 



Chess Column. By C. D. Looock, B.A.Oxou 239 



EXPERIMENTS ON THE RADIATION OF 

 CERTAIN HEATED GASES AND VAPOURS. 



By J. Ea-ershed, F.R.A.S. 



THE recently published researches of Dr. Pringsheim 

 {]V,iiliiuiii}i's Annahn, 1802, No. 3) and Prof. 

 Smithells (Philo>ii)pliical Maiia:iiie, March, 1894) 

 on the radiation of gases, raises an important 

 question in connection with solar physics, namely, 

 do tlio various gases forming the chromosphere and 

 prominences, and the metallic vapours in which the photo- 

 sphere is bathed, emit their special light radiations merely 

 by reason of the high temperature to which they are 

 exposed, or is there some chemical process involved ; the 

 luminosity being caused by what, from sheer ignorance as 

 to the actual relations existing between matter and the 

 ether, has been called " chemical luminescence," or the 

 direct transformation of chemical energy into light'? The 

 latter view appears to have been adopted by A. Brester, 

 and in his theory of solar phenomena he supposes a 

 continual recombination of dissociated matter in the outer 

 cooling layers, where other matter, condensed by the loss 

 of heat into mist, no longer forms an obstacle to the free 

 play of the chemical forces involved. 



Or again, is the gaseous luminosity the result of 

 electrical excitation, as other physicists have supposed, 

 notably M. Deslandres, of the Paris Observatory ? On this 

 view the solar atmosphere is believed to be the seat of 



violent electrical disturbances, somewhat analogous to our 

 terrestrial thunderstorms and aurora but on a colossal 

 scale, the prominences and those curious patches of bright 

 calcium vapour which have been found to be so inti- 

 mately associated with the facul* on the solar disc, being 

 regarded as local manifestations of electrical energy. 



I propose in this article to bring forward evidence to 

 show that whilst both chemical and electrical phenomena 

 may, and very probably do, at times produce luminous 

 effects on the sun, yet there is no necessity whatever to 

 invoke the aid of either of these forces to account for the 

 general illumination of the chromospheric gases and the 

 ordinary prominences or the metallic vapours forming the 

 " reversing layer " at the base of the chromosphere, and 

 that there is strong reason for believing that these gases 

 shine simply because they are excessively hot. 



This, of course, was Kirchoffs original idea, but it seems 

 to have been called in question of late years on account 

 of the ditficulty or impossibility experienced by experi- 

 mentalists in inducing any gas or vapour to emit its own 

 characteristic light radiation, or indeed any radiation, 

 except when under the influence of the electrical dischirge 

 as in Geissler tubes, or of chemical forces as in ordinary 

 flames ; and it has always hitherto been through one or 

 other of these agencies that the emission spectra of bodies 

 has been studied in the laboratory. Quite recently, 

 however. Prof. Smithells has carefully studied the glow 

 produced when iodine vapour is heated, and has come to 

 the conclusion that the light is simply the result of the 

 high temperature. The vapour is in fact "red hot" just 

 like any heated solid body, and it is this observation that 

 has led me to make a series of experiments both with 

 iodine and several other elementary gases and vapours, 

 with the view of determining, first, whether the glowing 

 iodine obeys Kirchoff's law, according to which the radia- 

 tions of the heated vapour should have the same wave- 

 lengths as are absorbed when white light passes through 

 it, and secondly, whether this property of glowing is 

 peculiar to iodine or is shared by other elementary bodies 

 in a greater or less degree. After this I proposed to 

 attack the problem of heated metallic vapours, and find an 

 answer to the question whether they can be made to emit 

 their characteristic light radiations by heat alone, the 

 results of the researches of Dr. Pringsheim in this 

 direction (above quoted) not appearing to me to give a 

 decided verdict one way or the other. 



The first experiments which I shall describe answer the 

 first two questions quite satisfactorily, and as these are 

 comparatively simple in their nature, and easily verified, 

 it will be unnecessary for me to give an exact and detailed 

 description of the apparatus actually employed ; the only 

 essential parts being a spectroscope, a Bunsen burner, and 

 a piece of hard glass tube. I will, however, ask the 

 indulgence of the readers of Knowledge when we come to 

 consider the more critical experiments with glowing sodium, 

 when it will be necessary to describe at some length the 

 disposition of the various parts of the apparatus used, and 

 the precautious taken to insure a valid result. 



To determine, then, the first point — that is, the nature 

 of the light emitted by iodine — a piece of hard glass tube, 

 closed at one end, was strongly heated about the middle, 

 and an image of the hot part was thrown across the slit 

 of a spectroscope by means of a lens. A faint streaky 

 spectrum, due to glowing opaque particles in the glass, 

 was all that could be seen. A little iodine was then pushed 

 into the tube, and immediately a bright glow filled the 

 central bore. The spectrum showed no lines or bands, 

 but appeared perfectly continuous, and similar to that pro- 

 duced by a red-hot iron wire, with which it was directly 



