252 FROM NEBULA TO NEBULA 



an outlet for itself, upheaving the crust into mountain 

 ranges and loading the atmosphere with immense quanti- 

 ties of carbonic dioxide. Now, carbon dioxide is an effi- 

 cient absorber of heat, hence, in the periods when the at- 

 mosphere was rich with it that is to say, during the in- 

 terims between the mountain-making cataclysms the 

 earth experienced her genial climates. On the other 

 hand, however, from the moment of the subsiding of these 

 convulsive movements, the carbon dioxide began to be 

 gradually extracted from the atmosphere, by plants to 

 form coals and other carbonaceous deposits, and by pre- 

 cipitation in the rains and subsequent absorption in the 

 rocks ; a process which eventually brought about periods 

 of low temperature immediately preceding the upheaval 

 next in order of events. But let me quote from his own 

 words (Origin of the Earth, p. 103, et seq.) : 



In the sun, there is a persistent eruptive tendency of great 

 power. At short intervals, great bolts of sun-substance are shot 

 forth at high velocities. This takes place without any obvious 

 outside stimulus ; or, if there be such stimulus, it is not declared. 

 Beyond question if suitable strong stimulus from without were 

 brought to bear on the sun, such as the differential attraction of 

 a passing star, it would respond with eruptions of much greater 

 intensity and mass. 



It thus appears that from so simple a cause as the differential 

 gravity called into action by the close approach of one massive 

 body to another, there may arise a graded series of eruptions 

 ranging from fractional ejections to profound disruption and 

 dispersion, according to the closeness of approach, the relative 

 masses of the bodies, and their internal state. The ejected parts 

 will pursue such courses as may be imposed on them by the new 

 forces of attraction brought into play by the changing relations 

 of the two bodies, both of which are necessarily in swift curving 

 motion, while one or both are losing mass by disruptive action. * * 



For an illustrative case, selected to suit our problem, let our 

 sun, in its ancestral state, be the body approached. For its 

 partner in action, let a more massive star be chosen and, for con- 

 venience, let it be so dense and inert that its response to the re- 

 action of the sun upon it may be neglected. In addition it will 

 be convenient to speak of the relative changes of position of the 

 two as if the whole motion were made by the passing star. * * * 



In selecting the closeness of approach, let us observe that 

 only 1/745 of the sun's substance was required to form our whole 



