The Origin of Bock-Salt. 27 



as a result of their being subjected to a temperature which 

 was very high in its earlier stages, and which gradually 

 declined, as a consequence of radiation, as time went on. It 

 is further assumed, on sufficiently good grounds, that the 

 entire nebula moved with a rotatory motion as it progressed 

 through space, the direction of motion being from left to 

 right, looking toward the centre of the mass. 



As the temperature declined, the nebulous mass gradually 

 divided into several portions, each of which eventually con- 

 stituted the nuclei around whose centres, as the cooling 

 process advanced, the gaseous elements passed into the solid 

 state, and gave rise to the several planets and their satellites 

 as they exist to-day. 



In regard to the cause of the initial high temperature in 

 question, it is perhaps hardly going too far to say that 

 Dr Croll's " Impact Theory " is the one that best accords 

 with all the known facts. The fundamental principle upon 

 which this theory is based is that motion is eternal, and is 

 the normal state of things in the universe ; and that it was 

 the arrest of motion caused by the impact of two stellar 

 bodies approaching each other at a high velocity which, by 

 transmuting the energy of motion into the energy of heat, 

 gave rise to the high temperature whose origin is here under 

 consideration. Whether this mode of accounting for the 

 initial high temperature of the nebula from which our Solar 

 System has arisen be accepted as the true one or not, is 

 at present immaterial, seeing that the fact itself is no longer 

 questioned. Nor is it necessary to consider the details of the 

 cooling process, further than to bear in mind that the order 

 of consolidation of the various elements and compounds out 

 of which our Earth is built was regulated by the temperature 

 requisite to maintain each of them in a state of gas. We 

 may perhaps assume that it would be most likely that those 

 substances, such as many of the metals, which required the 

 highest temperature to keep them in a gaseous state, would 

 be the first to cool and form the core around which, as the 

 cooling proceeded, the remaining constituents of the litho- 

 sphere would arrange themselves one over another in accord- 

 ance with the principle just referred to. 



