176 



KNOWLEDGE. 



[August, 1903. 



flat disc of gas or to one of meteors, no one can deny that in 

 its course round the sun, Venus passes through a very dense 

 portion of its substailce. If now we suppose this appear- 

 ance to be composed of meteoric swarms, pursuing their 

 way among each other in all directions, their orbits having 

 a general coincidence with the plane of the ecliptic, a view 

 which is gaining ground every day, we at once see how it 

 is readily possible for meteor showers like our November 

 displays to be comparatively common on Yenus. On 

 looking at the density of the Zodiacal Light at an angular 

 distance of 40'' from the sun, it seems to me as if it would 

 be a wonder, the ajipearance being meteoric, if Venus 

 were not illuminated at times in traversing these swarms. 

 My own opinion is that if the night side of Venus never 

 was illuminated, it would almost be prejudicial to the 

 hypothesis of the meteoric nature of the Zodiacal Light, 

 and in the same way the fact that this illumination can 

 be explained on this hypothesis and on no other, appears to 

 me a strong supplementary indication that this hypothesis 

 is the correct one. 



The above theory explains of course the intermittent 

 nature of the light, and although meteoric swarms can 

 only act on one side of a planet, yet in the nature of things, 

 as a little consideration will show, it will only be when a 

 swarm strikes a favourable half of the planet that the 

 light will be seen. Also the movements of the planet are 

 such as to cause the side thus illuminated to be the one most 

 readily noticed by the observer when the jilauet is in the 

 most favourable position for observation. 



One question there is which cannot yet be decided, viz., 

 Do these illuminations ever take place at the same point on 

 the jjlanet's orbit as previous illuminations? If it can be 

 proved that they do, I think the matter might almost be 

 regarded as permanently settled. 



THE CHEMISTRY OF THE STARS. 



IV.-STARS OF THE FIRST TYPE. 



By A. Fowler, f.r.a.s. 



Although Secchi seems to have clearly recognised that 

 the white stars of the "Orion" type were spectroscopically 

 different from those resembling Sirius, he was satisfied 

 to group together all the white stars under the general 

 name of Tyjie I. It has already been pointed out, how- 

 ever, that Secchi's classification is greatly improved by 

 restricting the definition of Type T. so as to include only 

 those stars in which the fine lines accompanying the 

 stronger ones of hydrogen are chiefly of metallic origin, 

 and giving the name of Type O to those stars in which the 

 chief lines, apart from those of hydrogen, are principally 

 due to helium and other gases. 



Taking the first type stars in this restricted sense, 

 Sirius may be regarded as the most typical example. 

 Here the dark lines of hydrogen are overwhelming, Imt 

 the additional lines seen in the best of modern photographs 

 are very numerous and sharply defined, and by no means 

 to be despised on account of feeble intensity. Their 

 chemical significance has lately been very fully investigated 

 by Sir Norman Lockyer, and "the following short extract 

 from his recent publication, to which reference has been 

 previously made, will indicate the present stale of our 

 knowledge in this direction. It is necessary to explain 

 that in this table the prefix " p " indicates that the line 

 is an "enhanced" one, or one that is stronger in the spark 

 than in the arc spectrum under ordinary conditions of 

 experiment, "p" being an abbreviation for "proto" ; the 

 idea underlying the use of this temi is that the enhanced 



lines appear under some condition of molecular simplifica- 

 tion. 



From the complete table it appears that the following 

 elements are responsible for the Sirian lines which have 

 so far been identified : — 



The significant outcome of the investigation is that the 

 chemical origins of the majority of the Sirian lines have 

 already been traced to terrestrial substances, and it is 

 certainly too early to invoke non-terrestrial matter in 

 order to explain the remainder. There is, in fact, no certain 

 evidence of the jn-esence of any substance in the Sirian 

 stars that has not already been found in the sun and upon 

 the earth. 



This uotaljle success in the interpretation of the Sirian 

 lines, as will be seen from the wave-length table above 

 given, is largely due to the comparatively recent investiga- 

 tions of enhanced lines ; but while the chemical signifi- 

 cance of such liues is beyond dispute, their physical 

 meaning is not yet generally agreed upon. Sir Norman 

 Lockyer, Dr. Scheiner, and others argue that they ai-e the 

 products of high temperature ; Sir William Huggins 

 seems inclined to account for them chiefly as an effect of 

 reduced density, and others incline to attribute them, in 

 laboratory experiments at least, to a particular electrical 

 condition. Special attention has lately been given to the 

 experimental study of the enhanced line of mague.^ium at 

 X 4481 {sei Knowledge, February, 1903, p. 32, Fig. 4), 

 which is very prominent in the Sirian stars. It has been 

 shown that this line may be made to appear in the arc 

 spectrum if the poles be surrounded by hydrogen or 

 certain other gases, or when the arc is passed under 

 water ; while, on the other hand, it almost disappears 

 from the spark spectrum when sufficient self-induction is 

 introduced into the secondary circuit. Similar complex 

 behaviour has also been recorded in the case of other 



