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LIII. On the Manufacture of large available Crystals of Sul- 

 phate of Iodo-quinine (Herapathite) for Optical Purposes as 

 Artificial Tourmalines. By William Bird Herapath, M.D., 

 Bristol*. 



HAVING been repeatedly applied to by various parties for 

 the details of my process for the manufacture of these 

 useful crystals, I have been induced to enter into numerous 

 experiments to obtain greater certainty in the results, and to 

 study the conditions necessary for the production of broad folia- 

 ceous plates ; permit me to make the formula known to science, 

 together with the precautions necessary for adoption to secure 

 the crystals when obtained, and to mount them so as to be avail- 

 able as polarisers or analysers for the microscope, or even to 

 enable us to perform all the experiments in the polariscope. 



The success which I have obtained is so great, that there is 

 no doubt tourmalines and Nicholas prisms will be soon com- 

 pletely superseded by these new crystals, since the scarcity of 

 the one and the difficulty in manufacturing the others render 

 them very costly apparatus. But a little practice in the follow- 

 ing process will soon enable any one to make them large enough 

 for every purpose ; and so superior are they in power to the best 

 tourmaline, that two plates scarcely thicker than gold-leaf may 

 (by a slight modification of my formerly published method) be 

 rendered totally impervious to light when they are crossed at 

 right angles. 



[Herewith are enclosed two marvellously thin plates of consi- 

 derable size ; one being six-tenths of an inch long and three- 

 tenths of an inch broad, the other the same length but one-tenth 

 of an inch broader. Upon crossing them you will perceive that 

 they are optically perfect as polarisers, and as useful as plates of 

 tourmaline for which you would be charged four guineas each 

 plate. I have succeeded in getting much larger ones by the 

 same process and equally good.] 



The materials employed are the same as before, the chief 

 modification being in the proportions of the ingredients, and the 

 care taken in the method of crystallization. 



It is necessary to procure pure disulphate of quinine, and for 

 this purpose none approaches so thoroughly to the standard of 

 absolute purity as that manufactured by Messrs. Howard and 

 Kent. 



I dissolve it in pyroligneous acid having a specific gravity of 

 1*042, and dilute the solution with an equal quantity of proof- 

 spirit made by adding rectified spirit of wine, spec. grav. 0*837, 

 to equal bulks of distilled water. 



* Communicated by the Author. 



