30 REPORT — 1881. 



US so mucla, we have still even more to learn. Why should some 

 substances give few, and others many, lines ? Why should the same 

 substance give different lines at different temperatures ? What are 

 the relations between the lines and the physical or chemical pro- 

 perties ? 



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, perhaps, 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 substance. Brodie's 

 researches also naturally fell in with the supposition that the so-called 

 simple substances are in reality complex, and that their constituents occar 

 separately in the hottest regions of the solar atmosphere. Lockyer con- 

 siders that his researches lend great probability 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, 

 Eoscoe and Schuster in our own country, but also of many foreign 

 observers. 



When geology so greatly extended our ideas of past time, the con- 

 tinned 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 between its parts. It follows that the potential energy 

 of the sun is far from exhausted, and that with continued shrinking it 

 will continue to give out light and heat, with little, if any, diminution for 

 several millions of years. 



Like the sand of the sea, the stars of heaven have ever been used as 

 effective symbols of number, and the improvements in our methods of 

 observation have added fresh force to our original impressions. We now 

 know that our earth is but a fraction of one out of at least 75,000,000 

 worlds. 



But this is not all. In addition to the luminous heavenly bodies, we 

 cannot doubt that there are countless others, invisible to us from their 

 greater distance, smaller size, or feebler light ; indeed we know that 

 there are many dark bodies which now emit no light or comparatively 

 little. Thus in the case of Procyon, the existence of an invisible body 

 is proved by the movement of the visible star. Again I may refer to the 

 curious phenomena presented by Algol, a bright star in the head of 

 Medusa. This star shines without change for two days and thirteen 

 hours ; then, in three hours and a half, dwindles from a star of the 

 second to one of the fourth magnitude ; and then, in another three and 

 a half hours, reassumes its original brilliancy. These changes seem 

 certainly to indicate the presence of an opaque body, which intercepts at 

 regular intervals a part of the light emitted by Algol, 



