Dr. J. Croll on the Origin of Nebulae. . 3 



gists take cognizance. Measuring the rate of the subaerial 

 denudation by a method which I pointed out several years ago*, 

 we are able to determine roughly the time required for the 

 removal of the rock. But the 60 million years thus obtained, 

 be it observed, are only the inferior limit. We know that a 

 certain amount of rock has been removed ; but how much more 

 may have been carried away we cannot tell. Consequently, 

 although we have good grounds for believing that 60 million 

 years have elapsed since life began on the globe, yet the lapse 

 of time may really have been very much longer. We are jus- 

 tified, therefore, in concluding that our globe has been receiv- 

 ing from the sun for the past 60 million years an amount of 

 light and heat daily not very sensibly less than at present. 

 This shows that gravitation alone will not explain the origin 

 of the sun's heat, and that a far more effective cause must be 

 found. Now the only other conceivable cause exceeding that 

 of gravity is, of course, motion in space. 



If the gravitation theory fails to explain the origin of the 

 sun, it fails yet more decidedly to account for the nebulas. In 

 fact it does not attempt any explanation of the origin of the 

 latter ; for it begins by assuming their existence, and not only 

 so, but that they are in process of condensation. This must be 

 the case, because the theory in question assumes that the par- 

 ticles of a nebulous mass have, in virtue of gravity, a mutual 

 tendency to approach one another ; and it cannot tell us how 

 this tendency could exist without producing its effect. The 

 advocates of the theory are not at liberty to call in the aid of 

 heat in order to explain why the particles are not mutally 

 approaching ; because it is this mutual approach which, ac- 

 cording to the theory, produces the heat, and of course with- 

 out such approach no heat could be generated. A nebulous 

 mass with a tendency to condensation could not have existed 

 from eternity as such ; but what the previous condition of a 

 nebula was, and how it came to assume its present state, the 

 gravitation theory cannot say. It begins with a star or sun 

 in process of formation, but does not help us to understand 

 how the process of formation commenced. 



It is quite otherwise, however, with the other theory. This 

 latter does not, like the former, begin by assuming the exist- 

 ence of a nebulous mass ; on the contrary, it goes back to the 

 very commencement of physical inquiry, to the very point 

 where physical investigation takes its rise, and beyond which 

 we cannot penetrate. The only assumption it makes is that 

 of the existence of matter and motion — if indeed this can be 

 called an assumption. How matter and motion began to be, 

 * Phil. Map:. May 1868, and February 1867. 

 B 2 



