378 CELESTIAL CHEMISTRY. 



One of the most remarkable appearances of Halley's comet 

 was during' the Ttu"kish invasion of Europe in 1456; just when 

 the Turks had mastered Constantinople, the comet is said to have 

 hung like a blazing scimitar in the sky, and to have struck 

 terror into the hearts of all Europe. 



In many respects Halley's comet resembles IVIorehouse's; 

 both undergo rapid changes from day to day ; both show a num- 

 ber of side brushes, making various angles with the main tail; 

 both have a way of dispensing altogether with their tails for a 

 while, and then donning them again; in both cases the nucleus 

 seems to be constantly revolving and to show signs of almost 

 explosive activity. 



Halley's is the only great comet whose orbit is well-known. 

 It visits our little plot in space on an average once every 'jj 

 years; the last visit was in 1835-6, when Sir John Herschel made 

 a series of ol)servations of the comet at the Cape Observatory. 

 Since tlie destruction of Jerusalem all its appearances, with one 

 exception — that of 912 — have been traced, and it will again ap- 

 pear to millions of beholders for the 24th time since that 

 epoch-marking event early in 1910, when it will doubtless once 

 more be a striking object. 



The following is a list of its previous appearances as far as 

 known : — 



iVrt. Perihelion. 



A.D. 



15. Feb. 25, "i},'. 



16. July 19, 912 



17. Oct. 9, 989. 



18. Mar. 27, 1066. 



19. ApL 6, 1 145. 



20. Sept. 10, 1222. 



21. Oct. 22, 1301. 



22. Nov. 8, 1378. 



23. June 8, 1456. 



24. Aug. 25, 1 53 I. 



25. Oct. 26, 1607. 



26. Sept. 14, 1682. 

 zj. Mar. 12, 1759. 



28. Nov. 15, 1835. 



29. ApL 19, 1910. 



I should like to say something about the fixed stars and 

 nebulae before I conclude. That the former are suns, like ours, 

 but many of them enormously larger, was known long before 

 the invention of the spectroscope; but it has remained for the 

 spectroscope to tell us what those distant suns are made of. 

 It has told us that many of them differ greatly from our sun in 

 chemical composition, and differ as greatly from one another. 

 Those heavenly bodies which shine by the reflected light of our 

 sun — the moon and Venus, for example — naturally show- 

 exactly the same dark lines in their spectra which we find in 

 the solar spectrum, l^ut the dark lines in the spectra of the 

 fixed stars are of quite a different order, and indicate a vast 

 range of chemical condition, approaching, it is true, that of our 

 sun in some cases, but in others differing as widely therefrom 

 as the condition of our sun differs from that of this earth. For 

 instance, the atmosphere of that brilliant star Aldebaran, or the 



