54 Mr. G. J.. Stoney on the Physical [Recess, 



last occasions upon which their atmospheres brush against one another, 

 the outer constituent of their atmospheres, and the outer constituent alone, 

 would be raised by the friction to brilliant incandescence, which would re- 

 veal itself by the temporary substitution of four intensely bright for four 

 dark hydrogen lines in a spectrum which everywhere else continues to be 

 filled with dark lines. And, moreover, these dark lines would for a while 

 be rendered faint by the fierce heat radiated upon the outer parts of the 

 atmosphere of each star by its companion*. It will be a matter of great 

 interest to watch this star when sufficient time shall have elapsed to give a 

 hope of seeing it double. 



92. When a body of moderate dimensions enters the atmosphere of a 

 great star, the resistance to which it is subjected will be very nearly the 

 same per square metre over the whole of its front surface ; but if it be of 

 sufficient size to occupy a considerable height of the atmosphere through 

 which it passes, it will be exposed to much more resistance beneath than 

 above ; and those conditions will have arisen which may terminate in a 

 double star. The cases must be rare in which two stars that clash together 

 happen to be of nearly equal mass. But when this does occur, the cir- 

 cumstances which are the most favourable to the formation of a double 

 star have taken place. This seems to account for the very remarkable pro- 

 portion of double stars which have nearly equal constituents. It would 

 appear, too, that in this class we should expect to find those instances in 

 which the perihelion distance is greatest, since it will be nearly the sum of 

 the radii of the distended atmospheres of the two stars. 



93. If two stars which are undergoing the process of formation into a 

 double star, be of very unequal mass, the smaller one will be stripped at 

 each perihelion passage of some of its atmosphere. All those parts which 

 by the friction are brought into a state of rest relatively to the parts of the 

 atmosphere of the larger star with which they come in contact, will, after 

 the stars have been separated, settle down upon the larger star. They will, 

 before the next perihelion passage, be replaced upon the smaller star by a 

 fresh supply of the same gases diffusing upwards from beneath, and almost 

 to the same height. When the stars come together again, this, in its turn, 

 will be stripped off ; and by a sufficient repetition of the process at succes- 

 sive perihelion passages several of the lighter constituents of the atmo- 

 sphere of the smaller star will be transferred over to the larger. Upon 

 the larger star this will not have any visible effect ; the acquisition will 



* y Cassiopeiae may perhaps be a similar system, in which the elliptic orbit has degraded 

 either quite, or nearly into, a circular orbit, so as for the present to subject the outer 

 layer of hydrogen to such a friction as keeps it constantly alight. If so, and if the stars 

 are of materially unequal size, this must terminate, either by the atmosphere of the large 

 star shrinking away from the companion, or by the companion's settling gradually down 

 by a spiral motion through the atmosphere of its primary. If the latter be what is to 

 occur, we shall have a splendid conflagration, the star first becoming intensely white, and 

 afterwards deep red. 



