210 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



the same time strongly influenced by the electrical repulsion. 

 The second type of tails he considered to be made of hydro-, 

 carbons, since hydrocarbons have a specific weight such that the 

 repellent and attractive forces of the sun upon their particles may be 

 nearly equal. Iron, on the other hand, would be more subject to the 

 action of gravity, on account of its greater weight, and was therefore 

 taken as adapted to tails of the third type. 



The observations on meteorites recorded in the Bakerian Lecture, 

 and the discussion of cometary observation contained in this Appendix, 

 show that the vapours which are given out by the meteorites as the 

 sun is approached, are in an approximate order : 



Slight hydrogen. 

 Slight carbon compounds. 

 Magnesium. 

 Sodium- 

 Manganese. 

 Lead. 

 Iron. 



Now of these the hydrogen and carbon compounds are alone per- 

 manent gases, and the idea is that they have been occluded as such 

 by the meteorites. They are given out as the temperature of the 

 meteorite again increases. 



Tails extending 10,000,000 miles through the cold of space, cannot, 

 as Bredichin supposes, I suggest, be composed of iron vapour, bat 

 they may well be, and doubtless are, of the hydrogen and various 

 carbon compounds. 



The magnesium and iron vapours will condense soon after their 

 repulse from the meteorite, the volatilisation of which produced thein, 

 and here, as Beichenbach with marvellous prescience suggested in 

 pre-spectroscopic times, we have the chondroi of the exact chemical 

 nature which he postulated. 



There is nothing extravagant in these suppositions, for we now 

 know that all the substances in question do exist in comets, and 

 it is evident that much is to be learnt from a continuation of the 

 inquiry. 



We know that the short-period comets get less brilliant with every 

 approach to perihelion, and that some do not even throw out a tail, 

 and we can easily ascribe both these results to the fact that after 

 several such appulses the vapours liable to be driven out of the 

 meteorites by temperature get less and less. 



If this be so, we may regard the comet with many tails as one 

 which for the first time undergoes perihelion conditions. We are in 

 presence of the " unperihelioned matter" glimpsed by Sir William 

 Herschel. 



