PROBABLE ORIGIN OF NEBULJ-. 301 



in a state of motion ; and its advocate is not required, 

 as a physicist, to account for the existence either of 

 those masses or of their motions. Neither is it neces- 

 sary for him to advance any hypothesis to show hos- 

 tile masses came into collision ; for unless we are to 

 assume that all stellar masses are moving in one 

 direction and with uniform velocity (a supposition con- 

 trary to known facts), then collisions must occasionally 

 take place. The chances are that stellar masses are 

 of all sizes, moving in all directions and with all 

 velocities. We have here therefore, without any 

 hypothesis, all the conditions necessary for the origin 

 of nebulae. Take the case of the origin of the nebulous 

 mass out of which our sun is believed to have been 

 formed. Suppose two bodies, each one half the mass 

 of the sun, approaching each other directly at the rate 

 of 476 miles per second (and there is nothing at all 

 improbable in such a supposition), their collision would 

 transform the whole of the motion into heat affording 

 an amount sufficient to supply the present rate of 

 radiation for 50 million years. Each pound of the 

 mass would, by the stoppage of the motion, possess not 

 les's than 100,000,000,000 foot-pounds of energy trans- 

 formed into heat, or as much heat as would suffice to 

 melt 90 tons of iron or raise 204,000 tons 1° C. The 

 whole mass would be converted into an incandescent 

 gas, with a temperature of which we can form no 

 adequate conception. If we assume the specific heat 

 of the gaseous mass to be equal to that of air (viz. 

 •2374), the mass would have a temperature of about 

 300,000,000° C, or more than 140,000 times that of 

 the voltaic arc. 



Reason why Nebulce occupy so much space. — It may 

 be objected that enormous as would be such a tem- 

 perature, it would nevertheless be insufficient to 



