t6 PLEASANT WAYS IN SCIENCE. 



like the light of Betelgeux, no spectroscopic evidence what- 

 ever of the presence of hydrogen. Manifestly, if the whole 

 surface of the sun were in the condition of the portions 

 which give bright hydrogen lines, the spectrum of the sun 

 would resemble that of Gamma Cassiopeiae ; while if the 

 whole surface were in the condition of those parts which 

 show no lines of hydrogen, the spectrum of the sun would 

 resemble that of Betelgeux. Now if there were any reason 

 for supposing that the parts of the sun which give no lines 

 of hydrogen are those from which the hydrogen has been 

 temporarily removed in some way, we might reasonably 

 infer that in the stars whose spectra show no hydrogen lines 

 there is no hydrogen. But the fact that the hydrogen lines 

 are sometimes seen bright renders this supposition unten- 

 able. For we cannot suppose that the lines of hydrogen 

 change from dark to bright or from bright to dark (both 

 which changes certainly take place) without passing through 

 a stage in which they are neither bright nor dark ; in other 

 words, we are compelled to assume that there is an inter- 

 mediate condition in which the hydrogen lines, though 

 really existent, are invisible because they are of precisely 

 the same lustre as the adjacent parts of the spectrum. 

 Hence the evanescence of the hydrogen lines affords no 

 reason for supposing that hydrogen has become even re- 

 duced in quantity where the lines are not seen. And there- 

 fore it follows that the invisibility of the hydrogen lines in 

 the spectrum of Betelgeux is no proof that hydrogen does 

 not exist in that star in quantities resembling those in which 

 it is present in the sun. And this, being demonstrated in 

 the case of one gas, must be regarded as at least probable 

 in the case of other gases. "Wherefore the absence of the 

 lines of oxygen from the spectrum of any star affords no 

 sufficient reason for believing that oxygen is not present in 

 that star, or that it may not be as plentifully present as 

 hydrogen, or even far more plentifully present 



There are other considerations which have to be taken 

 into account, as well in dealing with the difficulty arising 



