16 Sir William Thomson [Jan. 21, 



orbits, and if continued long enough will cause the two to revolve in 

 circular orbits round their centre of inertia with a distance between 

 their surfaces equal to 6 * 44 diameters of each. 



Suppose now, still choosing a particular case to fix the ideas, that 

 twenty-nine million cold solid globes, each of about the same mass as 

 the moon, and amounting in all to a total mass equal to the sun's, are 

 scattered as uniformly as possible on a spherical surface of radius 

 equal to one hundred times the radius of the earth's orbit, and that 

 they are left absolutely at rest in that position. They will all com- 

 mence falling towards the centre of the sphere, and will meet there 

 in two hundred and fifty years, and every one of the twenty-nine 

 million globes will then, in the course of half an hour, be melted, 

 and raised to a temperature of a few hundred thousand or a million 

 degrees Centigrade. The fluid mass thus formed will, by this pro- 

 digious heat, be exploded outwards in vapour or gas all round. Its 

 boundary will reach to a distance considerably less than one hundred 

 times the radius of the earth's orbit on first flying out to its extreme 

 limit. A diminishing series of out and in oscillations will follow, 

 and the incandescent globe thus contracting and expanding alternately, 

 in the course it may be of three or four hundred years, will settle to 

 a radius of forty* times the radius of the earth's orbit. The average 

 density of the gaseous nebula thus formed would be (215 x 40)~^, 

 or one six hundred and thirty-six thousand millionth of the sun's 

 mean density ; or one four hundred and fifty-four thousand millionth 

 of the density of water ; or one five hundred and seventy millionth of 

 that of common air at an ordinary temperature of 10° C. The density 

 in its central regions, sensibly uniform through several million 

 kilometres, is (see note on p. 11) one twenty thousand millionth of 

 that of water ; or one twenty-five millionth of that of air. 

 This exceedingly small density is nearly six times the density 

 of the oxygen and nitrogen left in some of the receivers 

 exhausted by Bottomley in his experimental measurements of the 

 amount of heat emitted by pure radiation from highly heated bodies. 

 If the substance were oxygen, or nitrogen, or other gas or mixture of 

 gases simple or compound, of specific density equal to the specific 

 density of our air, the central temperature would be 51,200° Cent., 

 and the average translational velocity of the molecules 6*66 kilo- 

 metres per second, being Vt ^^ 10*2, the velocity acquired by a 

 heavy body falling unresisted from the outer boundary (of 40 times 

 the radius of the earth's orbit) to the centre of the nebulous mass. 



The gaseous nebula thus constituted would in the course of a few 

 million years, by constantly radiating out heat, shrink to the size of 

 our present sun, when it would have exactly the same heating and 

 lighting efficiency. But no motion of rotation. 



* The radius of a steady globular gaseous nebula of any homogeneous gas is 

 40 per cent, of the radius of the spherical surface from which its ingredients 

 must fall to their actual positions in the nebula to have the same kinetic energy 

 as the nebula has. 



