VIEWS OF THE EARTH'S ORIGIN. 15 



sehel to probably amount to 23 F. A maximum eccentricity of the 

 earth's orbit is calculated to have been reached 210,000 years ago, 

 and a still greater eccentricity 850,000 years ago, when the winter 

 would have been forty-four days longer than now, and the meaii 

 temperature of the coldest month is stated at -7 and that of the 

 hottest month at 126. Mr. Croll has appealed to these principles as 

 evidence of a rotation of climates on the earth in past time, and 

 especially in explanation of the general phenomena of cold and glacia- 

 tion to which the northern regions of America and Europe were sub- 

 jected during the time which has been called the glacial period. The 

 hypothesis on astronomical grounds is speculative, but not impossible, 

 and deserves attention. 1 



Modern Speculations concerning the Origin of the Earth. The 

 sun and many of the distant fixed stars are formed of mineral sub- 

 stances, similar to those which make up the earth's surface. This 

 conclusion has been arrived at by studies with the spectroscope. This 

 knowledge being the foundation for all sound ideas concerning the 

 internal state of the earth, it is necessary to explain the evidence on 

 which it rests. When a ray of light enters a dark room through 

 a small slit in the shutter, and falls upon a triangular prism of glass 

 held near the slit, the light which passes through the glass is bent 

 and divided into the colours red, orange, yellow, green, blue, indigo, 

 and violet ; and this band like a rainbow is named the solar spectrum. 

 These colours are divided up into narrow strips by an immense number 

 of parallel delicate dark lines. Thousands of these lines have been 

 accurately drawn, and they are found to be always in the same 

 positions, some in each of the colours. They were first studied by 

 the Bavarian philosopher Fraunhofer, and are hence called Fraun- 

 hofer's lines. These lines indicate the existence of many substances 

 which are burning in the sun's atmosphere. And since the light 

 which those substances give is less brilliant than that of the blazing 

 atmosphere in which they burn, it follows that the rays from them 

 come to the earth relatively dark, and appear as dark lines in the 

 spectrum. The true meaning of Fraunhofer's lines was discovered 

 by experiments with the spectroscope upon minerals and metals which 

 form the earth's crust. The spectroscope, invented by Professors 

 Kirclioff and Bunsen of Heidelberg, is merely a telescope with 

 a triangular prism of glass at the end to break up the light into 

 a spectrum. And when a substance is burned in a flame like that of 

 a Bunsen's burner, and the light from it falls on the prism, the spec- 

 trum becomes more or less altered, and instead of a perfect spectrum 

 certain bright lines are seen, which are different for every metal or sub- 

 stance examined. Thus if a piece of clean platinum wire, which will not 

 burn, be touched with the finger and held in the flame, all the colours 

 of the spectrum disappear except a brilliant yellow line in the part of 

 the spectrum where the yellow colour should have been. This yellow 

 streak is caused by burning sodium, which passed through the skin 

 in the form of sodium chloride or common salt. The bright line thus 

 1 Croll, " Climate and Time." 



