340 



radical nature of the hydrocarbons, and to negative the assumption 

 of their being homologues of marsh-gas. 



The paper concludes with a description of a method, by which, 

 where numerous vapour-density determinations are to be made, the 

 necessity is avoided of refilling the balloon with water or mercury 

 in order to determine the residual air. 



II. "On the Optical Characters of certain Alkaloids associated 

 with Quinine, and of the Sulphates of their lodo-com- 

 pounds." By WILLIAM BIRD HERAPATH, M.D., in a 

 Letter to Professor STOKES, Sec. R.S. Communicated by 

 Professor STOKES. Received January 8 ; 1857. 



You will probably recollect that I sent you some time since a 

 small portion of an alkaloid, which at that time was called quinidin 

 in Germany, but it has since been distinguished from it and named 

 cinch onidin. You then examined it for epipblism or fluorescence, 

 and you pronounced the opinion that it possessed this property only 

 in a minor degree, and you imagined that this arose from the presence 

 of a small per-centage of a-quinine. 



I have since obtained, through the kindness of Mr. J. E. Howard, 

 specimens of the perfectly pure alkaloids quinidin and cinchonidin, 

 and find that quinidin, which I can now identify as the /3-quinine of 

 Von Heijningen, possesses the phenomenon of fluorescence or epi- 

 polic dispersion as powerfully as a-quinine ; whilst cinchonidin, if per- 

 fectly pure, is devoid of it altogether ; and recent experiments have 

 shown me that a small per-centage of quinidin was the cause of the 

 epipolic dispersion found by you in the specimen of cinchonidin sent 

 by me. 



Tt may be as well to state that the cinchonidin tested by water 

 of chlorine and ammonia gave no evidence of green tint, which it 

 would have done if only -g^L-o^th part of either a-quinine or quinidin 

 had been present, according to some recent experiments of my own. 



I have also found that 1 gr. of pure quinine or quinidin in 35,000 

 of water will give an evident " epipolic" appearance ; whilst when 

 diluted with 70,000 grs. of water we have still very evident appear- 



