CELESTIAL CHEMISTRY. 377 



monoxide and dioxide, nitrogen, and hydrocarbons, ordinary 

 chemical analysis thus confirming the discoveries of the spectro- 

 scope. 



The pressure of solar rays — not of necessity rays of light as 

 we understand it — acting on the carbon and other fine particles 

 of the cometary tails, would naturally cause them to radiate 

 cone-wise from the nucleus, and so the tail of a comet usually 

 acquires the shape of a cone or funnel : hence, to the observer 

 who sees it lengthwise the edges of the tail may seem denser 

 than the intervening space. This circumstance, clearly illus- 

 trated in the great comet of 1811, whose tail was 130 million 

 miles long, aids in explaining the exceeding transparency of 

 comets' tails. 



Other spectroscopic observations of comets led Sir Norman 

 Lockyer to regard them for the most part as agglomerations of 

 meteorites and meteoric dust; whether this be so or not, it is 

 known that there is occasionally a close connection between 

 comets and meteor swarms, and the reason why the dwellers 

 on the earth experience little more than a luminous haze when 

 our planet passes through the tail of a comet may lie in the 

 fact that our atmosphere acts as a shield, catching the falling- 

 meteorites, and dissipating them by the heat of their friction 

 through the air. Halley's comet is an instance of a comet ac- 

 companied by a train of meteorites. 



After this great comet has crossed our path during the first 

 half of next year we shall no doubt know much more of what 

 the spectroscope can tell us respecting comets, but, in view of 

 its imminent return a word or two regarding its history may be 

 of some interest. 



This comet has attracted perhaps more attention than any 

 other during the last 400 years; it is not too much to say that 

 from a popular view it is the most famous comet on record, and 

 our interest in it should be all the greater from the circum- 

 stance that it will be in the southern hemisphere that it will be 

 best seen when it visits our neighbourhood next year. 



Professor Clerk-Maxwell, with the instinct born of what 

 Lord Kelvin once called his deeply penetrating genius, as- 

 cribed to the radiation pressure of the sun's light the production 

 of the tails of comets. He was a quarter of a century in ad- 

 vance of his time, for only now is science awakening- to a 

 realisation thereof. If this be regarded as an established truth, 

 it carries along with it another, namely, that comets lose some of 

 their substance every time they approach the sun, and may 

 therefore be expected to diminish in prominence at each appear- 

 ance. So Halley's comet was less imposing in 1835 than when 

 it appeared in 1759, and stretched across 60 degrees of the 

 heavens, that is to say from the horizon two-thirds the distance 

 to the zenith. It appeared shortly before the destruction of 

 Jerusalem by Titus in a.d. 70 : it made its appearance before the 

 death of Agrippa in b.c. 12, and before that of Attila in a.d. 

 451, but its two most famous appearances are connected with 

 the Norman Conquest in 1066 and with the capture of Constanti- 

 nople bv the Turks in 1456. To the former occasion the 

 Chroniclers of the time make reference; indeed the phenomenon 

 is commemorated in the Bayeux tapestry. 



