474 



On the Identification of Chitin by its Physical Constants. 

 By Igerna B. J. Sollas, Fellow of Newnham College, Cambridge. 



(Communicated by Professor W. J. Sollas, F.K.S. Keceived May 27,— 



Bead June 13, 1907.) 



The determination of the distribution of chitin in the animal kingdom is 

 hampered by the absence of any test or positive means of identifying it. 

 Gamgee, in his 'Text-book of Physiological Chemistry,' gives a list of 

 structures of invertebrate animals in which chitin has been described. But 

 when those cases are eliminated in which the identification has been based 

 solely on the negative character of insolubility in caustic alkalis or weak 

 acid, the revised list, as it appears, for instance, in von Fiirth's ' Vergleichende 

 chemische Physiologie der niederen Tiere,' is greatly curtailed. It is true 

 that chitin yields a characteristic decomposition product, the amido- 

 derivative of sugar known as chitosamin, in definite proportions, but the 

 amount of material available is not always sufficient to allow of the prepara- 

 tion of this product. In cases, however, where we have other reasons for 

 suspecting the presence of chitin, the reducing action of the chitosamin 

 resulting from the treatment of the original substance with sulphuric acid is 

 a valuable confirmatory test. 



Under these circumstances it seemed that it might be worth while to make 

 a determination of the specific gravity of chitin by the well-known method 

 of a diffusion column. The specific gravity of any substance as thus 

 determined stands in real and intimate relation to its chemical constitution, 

 for the mass dealt with is always small and can therefore be rendered 

 homogeneous by various cleansing processes. It is readily permeated by the 

 suspending fluid and the absence of any other sources of error may be 

 ensured without difficulty. 



In addition to the determination of the specific gravity, that of the 

 refractive index should be made, for two substances may possess the same 

 specific gravity, but not the same chemical composition, though when we have 

 other reasons for suspecting their identity this is unlikely. 



The refractive index of any substance has been shown by Gladstone and 

 others to be connected with the specific gravity by a relation which, whether 



71 — 1 01 "— 1 ♦ 



expressed as — — or more correctly as — - — — 7 , is constant for that sub- 

 ^ d (n 2 +2)d 



stance and is independent of the temperature. Therefore the demonstration 



