THE HEAT OF THE SUN. 55 



It is hardly necessary to point out that, while the 

 spectrum has taught us so much, we have still even 

 more to learn. Why should some substances give few, 

 and others many, lines? Why should the same sub- 

 stance give different lines at different temperatures? 

 What are the relations between the lines and the 

 physical or chemical properties? 



We may certainly look for much new knowledge of 

 the hidden actions of atoms and molecules from future 

 researches with the spectroscope. It may even, per- 

 haps, teach us to modify our views of the so-called 

 simple substances. Prout, long ago, struck by the 

 remarkable fact that nearly all atomic weights are 

 simple multiples of the atomic weight of hydrogen, 

 suggested that hydrogen must be the primordial sub- 

 stance. Brodie's researches also naturally fell in with 

 the supposition that the so-called simple substances are 

 in reality complex, and that their constituents occur 

 separately in the hottest regions of the solar atmosphere. 

 Lockyer considers that his researches lend great proba- 

 bility to this view. The whole subject is one of intense 

 interest, and we may rejoice that it is occupying the 

 attention, not only of such men as Abney, Dewar, 

 Hartley, Liveing, Roscoe, and Schuster in our own 

 country, but also of many foreign observers. 



When geology so greatly extended our ideas of 

 past time, the continued heat of the sun became a 

 question of greater interest than ever. Helmholtz has 

 shown that, while adopting the nebular hypothesis, we 

 need not assume that the nebulous matter was originally 

 incandescent ; but that its present high temperature 

 may be, and probably is, mainly due to gravitation 



