14 



matter was neither atoms of gas nor meteorites, but 

 something between the two. Our solar system, he says, 

 was formed from a spiral nebula, and the nebula con- 

 sisted of " planetesimals," or particles of liquid or solid 

 matter in a finely divided state. For our elementary 

 purpose this difference in the constitution of the nebula 

 matters little; but it involves some very important 

 differences at a later stage, which we will notice in due 

 course.* 



We start, therefore, with the material of our solar 

 system dissipated over several thousand million miles of 

 space. To-day it is collected into a great ball (the sun) 

 860,000 miles in diameter, and a few smaller spheres 

 from 3,000 to 87,000 miles in diameter. At one time it 

 was scattered loosely over the whole space occupied by 

 our solar system (5,500 million miles across) and far out 

 into adjoining space. Whether it was in the form of 

 gas, or planetesimals, or meteorites or any two of them 

 in turns we may leave open. No doubt it passed 

 through many phases, and the strongest probability is 

 that it consisted at first as all the great irregular nebulae 

 do of gas. In any case the general laws of its con- 

 densation into worlds remain. 



But many a reader will refuse to go further until 

 something is said of the origin of the nebula itself. 

 Here again the very magnitude of the universe comes to 

 our assistance. Nebulae have been born before our eyes 

 in the heavens within the last few years. On the night 

 of the 21st of February, 1901, a very bright new star 

 appeared in the constellation Perseus, where no star had 

 been visible the night before. When we learn that this 



* The planetesimal theory is fully worked out, and con- 

 trasted with the other theories, in Chamberlin & Salisbury's 

 Geology, vol. iu 



