Prof. A. W. Bickerton on Cosmic Evolution. 217 



always independent atoms, and probably play an important 

 part in laying the foundation of an incipient cosmic system, 

 helium being- the most important. Possibly these elements 

 have no other function than this, as deductions from their 

 properties suggest that they must largely pass out of cosmic 

 systems before the system matures. Hydrogen plays the 

 same initial part, but it is more than a cosmic pioneer; it has 

 important functions in cosmic systems of all orders. 



This paper is chiefly devoted to grazing and whirling 

 collisions of celestial bodies. 



Grazing collisions of stars were discussed before the Royal 

 Society by Dr. Johnstone Stoney more than thirty years ago. 

 The formation of double stars and new stars by such an 

 occurrence was suggested by him, as well as the probability 

 of the existence of dead suns in countless hosts. 



The especial point I wish to forward is that a grazing 

 impact will generally result in the formation of a new body, 

 whilst the two struck stars proceed on their journey ; as it 

 were, flint and steel have struck and have cut off a part from 

 each other, that results in an intensely heated spark. 



Because the non-colliding parts are but little affected by 

 the collision I call such a phenomenon a " partial impact.'' 



With stars of the same order of dimensions that our sun 

 has, the velocity developed by mutual attraction will be 

 hundreds of miles a second. When by impact this motion is 

 converted into heat in the coalesced parts, the temperature 

 will be practically the same, whatever the amount struck off; 

 if the graze be small, the attractive power of the new body will 

 also be small, and it is evident that the velocity of the molecules 

 may be great enough for every molecule to have more than 

 the critical velocity; each molecule as it reaches the surface 

 will consequently leave the body, never to return. 



The temperature may easily be from ten to a hundred 

 million centigrade. It will of course vary with the chemical 

 composition. 



Thus is produced in less than an hour a fiery mass 

 expanding about a million miles an hour, and this increase in 

 size will cause for a time an increase in brilliancy. Presently, 

 however, the radial direction of the molecules will tend to 

 cause their motion to become parallel, and will lessen the 

 number of molecular encounters, and consequently the 

 amount of radiation, so that after a time our bright star 

 becomes a planetary nebula. In a year or so each molecule 

 is wandering alone ; the brilliant body and the nebula are 

 gone. The body has lost its light, not by cooling, but by 

 being too hot to hold together. 



