44 FORMATION OF THE EARTH 



seasons. In the twenty million years during which life has 

 existed on the earth, such climatic changes must actually have 

 occurred. 1 The computations of Blandet have led us to assume 

 that during these twenty million years the diameter of the sun 

 has decreased by one-half. When life began on earth the sun 

 was so large that there was hardly any night at all. The work 

 of J. Bosler 2 suggests that through radiation alone the sun 

 must have lost in thirty million years a mass equivalent to 

 that of the earth. This diminution, together with modifications 

 in the longitude of the earth, would bring about a slowing down 

 of the earth's motion equivalent by the end of the same time 

 to a retardation of 36 hours for the seasons. The temperature 

 of the sun, therefore, must have correspondingly diminished 

 since the commencement of life. Scientists are not agreed as 

 to its temperature at the present moment. M. Violle reduces 

 it to 2,500 degrees, Lord Kelvin places it at 14,000, and 

 M. Le Chatelier stops at an intermediate figure, 7,500 degrees. 

 6,000 degrees is regarded as the most probable figure. 



Whatever the facts may have been, the diminution of the 

 solar temperature must have changed the nature of the light 

 emitted. To-day the sun is, in fact, a yellow star. We know 

 that there are white and blue stars in the firmament which 

 are hotter. The sun must once have belonged to one of these 

 categories. The light which it then sent the earth was richer 

 in chemically active rays — in blue or violet, or invisible ultra- 

 violet rays. The energy of such light was then much greater 

 and quite different from the solar light of to-day. It must 

 consequently have produced on the earth chemical phenomena 

 now impossible. We shall have recourse to this important fact 

 later on ; for the present, however, we may conclude that at 

 a period not far distant relatively to the duration of the 

 geological epochs, the sun was of sufficiently great dimensions 

 to render the climate of those regions of the earth, corresponding 

 to those now above water, quite different from what it is to-day. 

 As to that, however, there are reasons nearer at hand thanks to 

 which frequent changes of climate must have occurred in the 

 same place. What determines climatic characters to-day is 

 the altitude above sea-level, the proximity of high mountain 

 chains covered with eternal snow and glaciers, and the position 

 relative to the sea of these mountain chains whence hot or cold 

 1 IX, 122. 2 XI. 



