On the Identification of Chitin by its Physical Constants. 475 



of common values for the specific gravity and the refractive index of two 

 substances affords strong evidence of their identity.* 



Unfortunately, I do not possess a reading microscope, so that I have taken 

 my readings direct from a scale at the back of the experimental tube. But 

 I have appreciated the advantage of the use of glass beads : when these are 

 made and suitable fluids chosen the process need occupy but little time. 



The fluids I have used are chloroform, of which the specific gravity is 

 roughly 1*48 at 21° C, and " absolute " alcohol of specific gravity 0*791 at 

 21° C. The specific gravity of chitin from the integuments of Astacus 

 having been found roughly to lie somewhere between 1*39 and 1*41, mixtures 

 of chloroform and alcohol of specific gravity 1*36 and 1*44 were made with 

 the help of a Westphal's balance, and these mixtures formed the two fluids of 

 the column. In using light fluids it is not possible to float glass beads, such 

 as are used for dealing with minerals in heavy fluid columns, and which are 

 described in ' Nature ' :*f it is necessary in the present case to blow the beads 

 dumbbell-shaped, but with both ends hollow. Hence it is possible to obtain 

 beads which float with their long axis horizontal and this is an advantage, for 

 the nearer the axis of the bead approaches to the horizontal position the less 

 is likely to be the error which arises in estimating its centre of gravity. 



Chitin from various sources was prepared in the usual way by soaking in 

 10-per-cent. hydrochloric acid, boiling in 5-per-cent. caustic potash^ for many 

 hours with frequent changes of the liquid, washing in water, very dilute 

 hydrochloric acid, and extracting in alcohol and ether. In some cases this 

 treatment did not render the chitin quite colourless : the last traces of 

 pigment were removed by Mayer's method of bleaching, in which chlorine is 

 the active agent. 



The specific gravity of chitin from integuments of Astacus cleaned in this 

 way was found to range between 1*400 and 1*404. The chitin of a cater- 

 pillar, Bombyx cynthia, was slightly heavier, 1*404 to 1*408, of the cockroach 

 wing 1*402, of the pupa case of the buff- tip moth 1*404 : of Oniscus and 

 spiders, again, the value agreed very nearly with that of Astacus. Two 

 species of Myriapoda gave the value 1*397 to 1*399 ; one of these was the 

 common English Lithobius, the other a large millepede which I picked up in 

 the Karroo, South Africa, but have not identified. 



* This method was originally published in 1885 by Professor Sollas in the 'Roy. Soc. 

 Dublin Proc.,' new ser., vol. 4, p. 378, and subsequently in 'Nature,' 1891, vol. 43, p. 404, 

 and 'Quart. Journ. Geol. Soc., 5 vol. 58, p. 163, 1902. 



+ Loc. cit. 



% The use of potash of this strength, which is weaker than that employed by Kruken- 

 berg, was recommended to me by Dr. F. Gowland Hopkins, to whom I am indebted for 

 much kindly interest in this investigation. 



VOL. LXXIX. — B. 2 M 



