528 Royal Astronomical Society. 



^2^{5N04}^=><'+2(N05, 3H0) = Cs^l J^jj'oj0jo+4(N05, HO). 



And this transformation may be reversed. 



Whilst completing my examination of this substance, my 

 attention M^as drawn to the communication of M. Payen in 

 the Comptes Rendus of Jan. 25th, where some properties of 

 " coton hypoazotique " are described. It is possibly the 

 same; yet, in order to express its distinctness from pyr- 

 oxyline, I would propose as the appellation of my substance 

 cotton-xyloidine. " 



Before concluding I would acknowledge my obligations to 

 several chemists whose published investigations on the same 

 subject have suggested many of my experiments, and more 

 particularly to Professor Fownes for the valuable advice with 

 which from time to time he has favoured me. 



LXXVII. Proceedings of Learned Societies. 



ROYAL ASTRONOMICAL SOCIETY. 



[Continued from p. 389.] 



June 11, /^N the Opinion of Copernicus with respect to the 



1847. ^^ Light of the Planets. By Professor De Morgan. 



The common story is, that Copernicus, on being opposed by the 

 argument that Mercury and Venus did not show phases, answered 

 that the phases would be discovered some day. The first place in 

 which I find this story is in Keill's Lectures, It is also given by 

 Dr. Smith, in his well-known Treatise on Optics, by BaiUi, and by 

 others. But I cannot find it mentioned either by Melchior Adam or 

 Gassendil, in their biographies of Copernicus ; nor by Rheticus, in his 

 celebrated Narratio, descriptive of the system of Copernicus ; nor 

 by Kepler, nor by Riccioli, in their collections of arguments for and 

 against the heliocentric theory ; nor by Galileo, when announcing 

 ^and commenting on the discovery of the phases ; and, what is most 

 to the purpose, Miiler, in his excellent edition of the great work of 

 Copernicus, when referring to the discovery of the phases of Venus, 

 as made since, and unknown to, Copernicus, does not say a word on 

 any prediction or opinion of the latter. 



This story may then be rejected, as the gossip of a time posterior 

 to Copernicus. If we try to examine what the opinion of Copernicus 

 on this matter really was, a point of some little curiosity arises. It 

 depends on one word, whether he did or did not assert his belief in 

 one or other of these two opinions, — that the planets shine by their 

 own light, or that they are saturated by the solar light, which, as it 

 were, soaks through them. I support the affirmative : that is to 

 say, I hold it sufficiently certain that Copernicus did express him- 

 self to the effect that one or the other of these suppositions was the 

 truth. 



If we take the first edition of the work Z)e Revolutionibus, which 



