76 Lord Rayleigh. Electromagnetic Rotation of Light. [Jan. 15, 



2. This residue, which when washed and dried is obtainable in a 

 white amorphous condition, is insoluble in weak acids ; but in con- 

 centrated mineral acids it is soluble in the cold. 



3. On boiling the solution in sulphuric acid, a body which has the 

 power of reducing cupric salts is formed. 



4. On boiling the solution in hydrochloric acid it turns brown, and 

 on evaporating this solution to dryness a body crystallises out which 

 has all the properties of hydrochlorate of glycosamine. 



I prepared some of this body from the chitin contained in the 

 exoskeleton of cockroaches, and also obtained from Professor Lan- 

 kester some crystals of the same body which Professor Gamgee had 

 kindly sent him. 



I was thus enabled to compare the crystalline body I had obtained 

 from the invertebrate cartilage with that of the pure hydrochlorate 

 of glycosamine, and they were found to agree in the following 

 points : — 



a. Crystalline form : rhombic prisms of the monoclinic system ; 



measurement of the angles gave the same result in all cases. 



b. Action of polarised light : nil. 



c. Solubilities : easily soluble in water, soluble with difficulty in 



alcohol. 



These results are especially interesting as showing that chitin is 

 not a body which is exclusively epiblastic in origin, but in these three 

 instances at least occurs in mesoblastic structures. 



II. " On the Constant of Electromagnetic Rotation of Light 

 in Bisulphide of Carbon." By Lord Rayleigh, F.R.S. 

 Received December 29, 1884. 



(Abstract.) 



A complete account is here given of the experiments briefly referred 

 to in the Preliminary Note,* and of others on the same plan of more 

 recent date. As regards the method, it may be sufficient to add to 

 what was there said, that the electric currents were estimated by com- 

 paring the difference of potential generated by the current in travers- 

 ing a known resistance with that of a standard Clark cell, the value 

 of the cell being known by converse operations, in which the current 

 was measured by a special electromagnetic apparatus.f Allowance 



* « Proc. Roy. Soc," vol. 37, p. 146, June, 1884. 



+ " On the Electrochemical Equivalent of Silver, and on the Absolute Electro- 

 motive Force of Clrrk Cells." " Proc. Eoy. Soc," vol. 37, p. 142, June, 1884. 



