1889.] On the Spectra of Meteor-swarms. 383 



fact that on the ascending side of the curve the varying volatilities of 

 the meteoritic constituents of the swarms brought out by successively 

 higher temperatures are in question, whilst on the descending side of 

 the curve we have to deal with successive chemical combinations, 

 brought about by a fall of temperature in a gaseous mass. 



III. Relation between the Early Species of Group III and the Later 

 Species of Group II. 



Since bodies of Group III are produced by the further condensation 

 of the condensing swarms which I have included in Group II, there 

 must be a close relation between the earlier species of Group III and 

 the later species of Group II ; that is, if there be anything like the 

 continuity which my hypothesis demands. We know, for instance, 

 that in the later species of Group II, there are flutings both dark and 

 bright, and dark lines, amongst the latter being 6, D, and E. As the 

 lines are produced, so to speak, at the expense of the flutings, we 

 should expect to find that lines of magnesium, sodium, manganese, 

 and iron are the most prominent, especially in the earlier species of 

 Group III. In a. Orionis we have associated with the metallic flutings 

 the lines b and D, and both are well developed, E is also present, but 

 it is not nearly so strong as b or- D. The F line of hydrogen is shown 

 as a thin line in a photograph of the spectrum taken by Professor 

 Pickering, although, as far as I know, it had not been previously 

 recorded. With ara increase in temperature, a condensing swarm 

 like a, Orionis would give a spectrum without flutings ; the magne- 

 sium flutings would be replaced by b, and the iron fluting would be 

 replaced by iron lines, of which E and the line at 579 would be the 

 most prominent. F is absent in most of the stars of Group II, 

 because the radiation of hydrogen from the interspaces is just suffi- 

 cient to balance the absorption; but in bodies of Group III, the 

 interspacial radiation will have almost disappeared, and absorption 

 will be predominant. We shall thus have F appearing thin in the 

 early stages of Group III, and gradually thickening until it becomes 

 as thick as in a Lyrse. 



In the earliest stages of Group III we should therefore expect to find 

 F and E thin and b and D thick. As yet we have no evidence as to 

 the first appearances of dark b and D in Group II, but future obser- 

 vations made with special reference to this point will at once indicate 

 in what species they first make their appearance as" absorption lines. 



With the next increase of temperature F and E will thicken, but b 

 and D will show no marked difference. With a further increase b and 

 D will lose their supremacy, and will be only of about the same thick- 

 ness as F and E, because most of the magnesium and sodium would 

 have been driven out with the first rise in temperature. Afterwards 

 all the lines, except those of hydrogen, will gradually thin out on 



