412 POPULAR LECTURES AND ADDRESSES. 



radius of the earth's orbit. The average density- 

 of the gaseous nebula thus formed would be 

 (215 x 4o)~ 3 , 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. 399) 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 ex- 

 perimental 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 com- 

 pound, of specific density equal to the specific 

 density of our air, the central temperature would 



from which its ingredients must fall to their actual positions in the 

 nebula to have the same kinetic energy as the nebula has. 



