1 Dr. J. Oroll on the Origin of Nebulae. 



whether they were eternal or were created, are questions wholly 

 beyond the domain of the physicist. The theory takes for a 

 fact the existence of stellar masses in a state of motion ; and its 

 advocate is not required, as a physicist, to account for the ex- 

 istence either of those masses or of their motions. Neither is 

 it necessary for him to advance any hypothesis to show how 

 the 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 contrary to known facts), then 

 collisions must occasionally take place. The chances are that 

 stellar masses are of all sizes, moving at random in all direc- 

 tions 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 colli- 

 sion would transform the whole of the motion into heat afford- 

 ing 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 less than 100,000,000,000 

 foot-pounds of energy transformed into heat, or as much heat 

 as would suffice to melt 90 tons of iron or raise 264,000 tons 

 1° C. The whole mass would be converted into an incandes- 

 cent 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 ichy Nebulce occupy so much Space. — It may be ob- 

 jected that enormous as would be such a temperature, it would 

 nevertheless be insufficient to expand the mass against gravity 

 so as to occupy the entire space included within the orbit of 

 Neptune. To this objection it might be replied, that if the 

 temperature in question were not sufficient to produce the 

 required expansion, it might readily have been so if the two 

 bodies before encounter be assumed to possess a higher ve- 

 locity, which of course might have been the case. But 

 without making any such assumption, the necessary expan- 

 sion of the mass can be accounted for on very simple prin- 

 ciples. It follows in fact from the theory, that the expansion 

 of the gaseous mass must have been far greater than could 

 have resulted simply from the temperature produced by the 

 concussion. This will be obvious by considering what must 

 take place immediately after the encounter of the two bodies, 



