56 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



between its parts. It follows that the potential energy 

 of the sun is far from exhausted, and that with con- 

 tinued 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 count- 

 less 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. The 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. 



Thus the floor of heaven is not only ' thick inlaid 

 with patines of bright gold,' but studded also with 

 extinct stars, once probably as brilliant as our own 

 sun, but now dead and cold, as Helmholtz tells us that 



