G9 



system of sun and planets to another. Encke's comet, on tlie 

 other hand, is getting nearer and nearer the sun with each 

 revohition, and in the course of time must be absorbed in its 

 fiery mass. The orbit of a comet is vastly more elliptical than 

 that of any planet, at one time approaching the sun, perhaps, 

 till the heat it receives from it, as calculated by Sir I. Newton, 

 must render it 2,000 times hotter than red hot iron, and then 

 passing away into frigid depths of space to an unimaginable 

 distance. It will not sui'prise you, then, to learn that while the 

 sun's power, as the comet approaches it, initiates such marvellous 

 changes in its form, as it recedes its light diminishes, and its 

 luminous ti'ain is absorbed. It becomes again that faintly 

 luminous spherical body that it first appeared to us. And now 

 the substance of Avhich these strange wanderers are composed — 

 is it gaseous or of solid particles 1 Do comets shine by their own 

 light, or like the planets, by the light reflected i'rom the sun? 

 The spectroscope in the hands of Dr. Huggins solved that 

 problem some years ago. It was found that the nucleus was a 

 glowing incandescent gas. Nay, he can even tell us what gas 

 was then glowing. 



The spectra obtained from it was almost identical with that 

 given by a hydro-carlion. Other observers differ slightly in their 

 views with regard to the spectra of comets ; but all concur in the 

 conclusion that carbon is present in the bright central portion of 

 the head of the comet. By the same means it was proved that 

 the tail shone by reason of the light reflected from the sun. 

 That it was composed therefore of comjiaratively small particles, 

 which were illuminated by the sun's rays as a cloud of chalk dust 

 on a summer's day. It must not be understoorl, however, that 

 the particles of which the tail of a comet is formed are as fine as 

 such particles of dust. Bodies as large as a billiard ball or a brick 

 would be but as fine dust compared with the enormous dimensions 

 •of such a congeries of them as the tail of Donati's comet. 

 What would happen if the earth were to encounter such atoms in 

 its course ? ^Moving at the enormous velocity that these move 



