CELESTIAL CHEMISTRY. 375 



which visits the vicinity of the earth and sun once every three 

 quarters of a century, and paid its last visit in 1836. The earhest 

 historical records show that comets have always been looked 

 upon as portents of evil and viewed with alarm because of their 

 peculiar and rapidly changing appearence, and of the mystery 

 which envelopes their sudden approach. A comet is popularly 

 regarded as possessing" two characteristic features — a more or 

 less brilliant head or nucleus, and a shadowy or dimly luminous 

 tail. The nucleus has in several cases shown a spectrum of 

 bright lines, indicating that it is self-luminous, while the coma 

 — or tail — gives the continuous spectrum afforded by reflected 

 sunlight. The tail is frequently of vast length, and in nearly 

 every case points away from the sun, no matter whether the 

 comet itself is approaching that orb or receding from it. Some- 

 times there are two or three tails, and the comet of 1744 had as 

 many as six, spread out like a fan across the sky. 



Bv far the most of the comets that approach our neighbour- 

 hood are invisible — or practically so — to the unaided eye. 

 About four or five of these celestial visitants aire detected by 

 astronomers every year, but most people see a comparatively 

 small proportion of them. Notwithstanding' a comet's vast 

 size, its weight is by no means considerable when compared 

 with the weights of other heavenly bodies : how extremely thin 

 their substance is may be gathered from the fact that when, 

 in 1858, the tail of i3onati's comet passed in front of the 

 bright star Arcturus, the star was seen with undiminished 

 brightness right through a tail 50,000 miles thick. Do not 

 misunderstand : I am not now speaking of the length, but of the 

 tliickncss of the tail; its length in this case was more than 50 

 million miles. As we all know, the brightest star is often 

 obscured completely by a xery slight fog. I therefore leave you 

 to imagine the extremelv gossamer-like tenuity of the comet's 

 tail. 



Now of what do comets consist? Meteorites and meteoric 

 dust, say some: and this has usually been the accepted opinion. 

 But let us see what information the spectroscope has to impart. 

 As I observed just now, there has hitherto been but little 

 opportunity of acquiring information regarding these curious 

 bodies, but such as has been already acquired is certainly 

 interesting and wonderful. As long ago as 1868 Sir William 

 Huggins found the nuclei of comets to consist, for the most 

 part, of carbon — " litminous carbon vapour " as Sir William 

 described it. Daniel's comet of 1907 showed absorption bands 

 indicating the presence of what are usually known to the 

 chemist as organic compounds, that is to say. hydrocarbons 

 and cyanogen, and considerable difference was perceptible 

 between the chemical composition of the head and that of the 

 tail. In addition, there were several strange bands which 

 seemed to represent an unknown substance or substances. The 

 presence of cvanogen in comets has often been observed, and 

 Morehouse's comet, which appeared last year, also showed 

 manv bands due to that gas. but the indications of hydrocarbons, 

 although present, were rather faint. The unknown substances 

 detected in the comet of the preceding year were also found in 



