350 



NA TURE 



[Febkuauv 9, 1899 



OUR ASTRONOMICAL COLUMN. 

 CoMF.r 1S98 VII. (ConniNCTON-PAiM.vl — Mr. C. \. 

 Merneld, of Sydney Observatory, gives an eplicmeris of this 

 comet, in .Ulronomische Nachrichlcn (Bd. 148, No. 3542), as he 

 considers it likely that it will be possible to observe the comet 

 from northern observatories. 



1899. a(.ipp.) «(app.) 



The comet is rapidly movinj; northwards, passing from near 

 <p Kornacis into Eridanus. It shoidd be looked for soon after 

 sunset, almost due south from Mira Ceti. It is said to be easily 

 visible with an instrument of six inches aperture. 



Eros (433). — In Harvard Co/lef;e Ob^en'alory Circular, 

 No. 37, Prof. E. C. Pickering describes the finding of trails of 

 the planet on thirteen more plates, in addition to those men- 

 tioned in the last Circular. These plates were exposed during 

 the period 1893 6, and the estimated photographic magnitude 

 varied from 8'2 to I2'5. lie lays great stress on the fact that 

 all the photographs which have been found showing the planet 

 have been taken with douhlet objectives, giving a large field and 

 large relative aperture. The difficulty is enhanced by the 

 variation in the brightness of the planet, as during the last 

 eleven years it has only been brighter than the ninth magnitude 

 for two months. 



In Popular .Is/rotwmy, January 1899, Mr. W. W. Payne 

 brings together the information given in various disconnected 

 articles by several authors. This will prove convenient for 

 many interested in the planet, and unable to find access to the 

 individual papers. 



The Su.n's Heat. — In the .-Xstronomische Nachrichtcn (Bd. 

 148, No 3540), IJr. T. J. J. See introduces a new law bearing 

 on the thermodynamics of a contracting gas, and discusses its 

 bearing on the question of the heat of the sun, and also its 

 application to estimating the relative ages of the stars and 

 nebula-. 



The modern theory of the sun's heat is primarily due to 

 Helmholtz, and its conclusions are based on the supposition that 

 the sun's mass is of homogeneous density. This Prof. See 

 doubts, and the result of a series of computations for the heat 

 given out by the contraction of a liderogeitcous mass was the 

 law of temperature he now brings forward. The effect of this 

 unequal density is to lengthen considerably the period heretofore 

 advanced for the duration of the sun as a light- and heat-giving 

 source, llelmholtz's theory indicates that the output of heal for 

 a given change in radius of the contracting mass increases very 

 rapidly as the radius itself becomes small. From this it appears 

 that the greatest amount of heat is produced when the mass has 

 reached its least dimensions and contraction is about to cease. 



Discussing the resulting temperature of a mass contracting 

 under itsosvn gravity, the law ileduced is: "The abs<ilute tem- 

 perature of a g.iseous star or nebula condensing under its own 

 gravitation varies inversely as the radius of the contracting mass." 

 The curve representing this condition will be recognised as a 

 rectangular hyperbola referred to its asymptotes. 



With this idea it would follow that at first when the nebula 

 is infinitely expanded, its temperature is the absolute zero of 

 space, and that this gradually rises to a maximum when the 

 mass has contracted tu the smallest radius consistent with the 

 laws of gases. After liquefaciiun has set in, free contraction is 

 obstructed and finally cea^e^ : the temperature falls, and the 

 body becomes finally invisible. 



This is in accord with the idea of the nebulic being low 

 temperature Ixidies. As it contracts the production of heat 

 exceeds the radiation, and the temperature rises inversely as 

 the radius decreases. 



In this connection Prof. See mentions the curves of Lane 

 for the laws of internal tem|K;raturc and density of gaseous 

 masses. (./«;. Jour. Set'., July 1870.) He illustrates these 

 by the case of the sun, and infers that il is increasing in 

 temperature still. The presence of hydrogen in the white 



NO. 1528, VOL. 59] 



stars he rea.sons in the same manner. While the nebula is 

 yet considerably extended, gravity is small, and all the ele- 

 ments float in the atmosphere without reganl to relative atomic 

 weight, and such produce spectra with many substances, as we 

 see in such solar stars as Capella, Arclurus, iVc. When the mass 

 is further condensed, the heavier elements are kept relatively 

 lower by the increased gravitation, and hydrogen, the lightest 

 of the elements, is present as the exterior envelope, and hence 

 the simplified spectrum of the Sirian stars. 



The jihenomenon of variable stars with dark companions is 

 accounted for on this assumption, the two being of the same 

 age but of greatly different masses. In the case of coloured 

 double stars the companion is generally blue or purple, and the 

 large star yellow or red, which again is in accord with this 

 theory. 



Taking the present temperature of the sun to be 8000° C, 

 he calculates that the temperature of the central nebula at the 

 time of formation of the earth was less than 40° C. ; the earth 

 beginning at this, contracted until it rose to about 2000° C, 

 which is high enough to account for all known geological 

 phenomena. Jupiter and Saturn are considered, on similar 

 grounds, to be .still gaseous and increasing in temperature, and 

 though not now self-luminous, may eventually become so. Incon- 

 clusion. Prof. See suggests that as the nebul.e are at low tempera- 

 tures many of them may be invisible, although existent. Many 

 nebula' have been photographed by the ultra-violet light they 

 emit, which are quite invisible in the most powerful telescopes. 

 If this be true, the numerical predominance of stars over nebula;, 

 visually, is explained, <is according to the nebular hypothesis, 

 the two classes of bodies should exist in approximately equal 

 numbers. 



w 



THE CONSTITUTION OF THE ELECTRIC 

 SPARK.^ 

 HEN an electric spark passes between metallic electrodes, 

 the spectrum of the metal appears, not only in immediate 

 contact with the electrodes, but stretches often across, from pole 

 to pole. Il follows that during the short lime of the duration >>f 

 the spark, the metal vapours must be able to difi'use through 

 measurable distances. 



The following investigation was undertaken primarily to 

 measure this velocity of diffusion with the special view of com- 

 paring different metals, and different lines of the s.ame metal. 



Feddersen published, in the year 1862, an interesting re- 

 search, in which photographs of sparks passing betweei^ 

 different metal poles are taken after reflection from a rotating 

 mirror. He could from his experiments draw some conclusions 

 which have a bearing on the subject, but it was necessary for 

 our purpose that the light should also be sent through a spec- 

 troscope, so as to distinguish between the luminous particles of 

 air and those nf the melal |)oles. 



The metliod of the rotating mirror tried during the course of 

 several years in various forms by one of us, did not prove suc- 

 cessful. On the other hand, good results were obtained at once 

 on trying the method used by Prof. Dixon, in his researches tin 

 explosive waves. This method consists in fixing a photographic 

 film round the rim of a rotating wheel. All that is necessary 

 for its success is to have sparks so powerful that each single one 

 gives a good impression of its spectrum on the film. Were the 

 sparks absolutely instantaneous, the im.ages taken on the rotating 

 wheel would be identical with tho.se developed on a stationary 

 plate, but on trial this is found not to Ix- the case. The metal 

 lines are found to be inclined and curved when the wheel ro- 

 tates, and their inclination serves to me.-isure the rate of ditiusion 

 of the metallic particles. The air lines, on the other hand, 

 remain straight, though slightly widened. 



To avoi.l the tendency of the film to fly otT the wheel when 

 fixed round its 'rim, as in the original form of the apparatus, a 

 spinning disc was constructed for us by the Cambridge Scientific 

 Instrument Company. The film is placed flat against the disc, 

 and is kepi in place by a second smallir disc, which can lie 

 screwed lightly to the first. The diameters of the two discs are 

 33 and 222 cm., the photographs being taken in the annular 

 .space of loS cm., left uncovered by the sn>aller disc. An 

 electric motor drives. the disc, and we have obtained velocities 

 of 170 turns per second, though in our exjuriments the number 

 of revolutions was generally about 120, giving a linear velocity 



' By I'rof. Anliiir Schuster, K.R.S., and G. Hcnis.nlccli. Read before ihc 

 Royal Society Fcbruir>' 2. 



