1907.] Chitin by its Physical Constants. 481 



which I have determined in Littorina, Patella, and Buccinum, lie between 

 1*550 and 1*554. 



The shell of Sepia has been shown by chemists to contain chitin, and the 

 percentage yield of chitosamin has been measured by them. After decalci- 

 fication with 10-per-cent. hydrochloric acid and boiling in 5-per-cent. potash 

 it readily breaks up, the thick outer wall of the shell with the rostrum and 

 lateral expansions becoming freed from the mass of delicate septa. This outer 

 layer quickly becomes transparent and colourless in parts, the rostrum and 

 thickened region in its neighbourhood being yellowish. The septa remain 

 opaque, they adhere to one another by a maze of low sinuous ridges, the 

 capillary spaces between which readily become filled with air, unless special 

 care is taken to avoid this. The specific gravity of the outer layer is not 

 a constant quantity, but ranges from 1*385 to 1*393. Alternate boiling in 

 various strengths of potash higher than 5 per cent, and in 50-per-cent. 

 hydrochloric acid do not reduce this range. The septa, on the other hand, 

 form a zone in the diffusion column at the level of chitin. The refractive 

 index of the inner laminae lies between 1*550 and 1*554 ; that of the outer 

 is variable, lying sometimes between 1*550 and 1*544, sometimes between 

 1*550 and 1*554. The variable specific gravity of the outer layer of the 

 Sepia shell points to the presence of more than one substance. As the 

 change in specific gravity goes hand in hand with a corresponding change in 

 refractive index, the suggestion naturally arises that there may be a series of 

 chitins, of which a lighter and a heavier member are present in the Sepia shell. 



Eosenheim* has given some reason to suppose that the fossil carapace of 

 Pterygotus osiliensis contains chitin. It would be interesting to apply the 

 method described in this paper in confirmation of his results, and to extend 

 it to the study of other organic substances. 



Summary. 



The determination of the physical constants of chitin forms a useful 

 method of identifying it. The specific gravity of chitin from various sources 

 approximates to the value 1*398, a number which represents the specific 

 gravity of chitin precipitated from its solution in strong acid. The refractive 

 index lies between the limits 1*550 and 1*557. 



The bristles of Lumbricus, the pupal skin of Pieris and other Lepidoptera, 

 the radula of Mollusca and the shell of Sepia, when freed from mineral 

 matter and easily soluble organic substances, have specific gravities and 

 refractive indices which lie between the same limits as those of chitin from 

 various sources. 



* 'Boy. Soc. Proc.,' B, vol. 76, 1905, pp. 398—400. 



