February 3, 19 10] 



NA TURE 



401 



heliocentric longitude is 30° on the enrlicr occasion 

 and 225° on the later. The earth reaches these helio- 

 centric longitudes in October and JMay res])ectively. 



At the return of 1835 perihelion passage was on 

 November ib. There was consequently a close 

 approach between the earth and the comet about a 

 month earlier. In igio the perihelion passage will 

 be on April 20; a month earlier than this, when the 

 comet is close to the earth's orbit, the earth will be 

 at the diametrically opposite point. A month after 

 perihelion, however, there w-ill be a very close 

 .ipproach. 



The most unfortunate date for perihelion passage 

 for vielding a close approach to the earth is January. 



The comet would then be behind the sun at 



perihelion, and more than an astronomical unit 

 uwav when crossing the earth's orbit. 



On the present return the approach after 

 perihelion will be unusually close. The follow- 

 ing table gives the ecliptic coordinates of the 

 earth for the annexed dales : — 



produced will be one-thousandth part of the least 

 measurable quantity, but the speculation is most 

 interesting in view of the fact that there are un- 

 explained phenomena in planetary movements. 



P. H. COWELL. 



JUBILEE OF THE THEORY OF ELECTRO- 

 LYTIC DISSOCIATION.' 

 T N his address to the British Association in 1884, 

 •'■ the president. Lord Rayleigh, said, "from the 

 further study of electrolysis we may expect to gain 

 improved views as to the nature of chemical reactions, 

 and of the forces concerned in bringing them about. 



When, therefore, the comet crosses the plane 

 of the ecliptic twenty-eight days after perihelion 

 passage (iVIay iS) it \\\\\ be almost exactly be- 

 tween the earth and the sun, and the earth will 

 probably be in the tail of the comet. 



The closest approach at this return takes place 

 a dav or two later. 



Tlie closest approach possible would corre- 

 spond to a perihelion passage about a week and 

 a half earlier in the year than the present one. 



It appears, therefore, that the date of 

 perihelion passage at this return is most fortu- 

 nately timed, and a fine display may be expected. 



The comet's history has been traced back to 

 240 B.C., and it has very seldom returned to 

 perihelion unrecorded; so seldom, in fact, as to 

 suggest that in the exceptional cases the records 



• -52 



• -48 



• -40 



»-3Z 

 • -28 



• -/6 

 » - 12 



• -4 



• O P£ni;iEUON 



The diagram gives the position of the Earth (o 

 in days. The line of sight is dra 



ix days in May ; also the position of the comtl 

 1 for May 18, twenty-eight days after perihelia 



have perished rather than that the comet in any 

 circumstances can pass by unseen. 



A tail twenty or thirty degrees in length is expected 

 on the present occasion. It will be best seen at the 

 end of Mav, and in England it will, unfortunately, be 

 lower in the sky than in more southern latitudes. 

 There will, however, be no difficulty whatever in 

 seeing it in England, unless there is a prolonged spell 

 of bad weather. 



The approach to the earth is so close that an 

 American astronomer has conceived the idea of weigh- 

 ing the comet by the deviation it produces in the orbit 

 of the earth. We can hardly believe that the effect 

 NO. 2I0I, VOL. 82] 



. . . I cannot help thinking that the next great 

 advance, of which we have already seen some fore- 

 snadowing, will come on this side." 



The first step of the advance spoken of by Lord 

 Rayleigh had already been made, for in that same 

 year the young Swedish physicist, Arrhenius, pre- 

 sented as his doctor's dissertation to the University 

 of Upsala a memoir with the title " Recherches sur 

 la Conductibilit^ galvanique des E lectrolytes. Premiere 

 Partie : La Conductibilit^ des Solutions aqueuses ex- 



^ Zcilschri/t fiir fhysiknlhche ChcinU: Bd. 6g, Jubelband. Svante 

 Arrhenius zur Feter des 25-jiihrigen Bestandes seiner Theorie der elektrolyt- 

 ischen Dissociation gewidmet von seinen Freunden und Schiilern. Miteiner 

 Einleitung von W. Ostwald. Pp. xxix+68s. (Leipzig : W. Engelmann, 1909.) 



