On the Manufacture of Artificial Tourmalines. 353 



second quantity of iodine solution, so that the liquid may retain 

 its dark sherry colour during the cooling. 



After making the addition of the tincture of iodine in either 

 of the previous formulse, it is necessary to continue the heat of 

 the spirit lamp for a short time, to dissolve all the cinnamon- 

 brown iodide of quinine first deposited, so that the solution shall 

 become perfectly clear dark-sherry wine colour, and then to 

 filter the solution rapidly through white porous paper into a 

 perfectly clean wide-mouthed Florence flask, matrass, or beaker, 

 and to attend to the following conditions : — 



1st. It is well to have at least six flasks half-filled with solu- 

 tions made by the above formulae, so as to ensure the production 

 of foliaceous plates in some one or more of them j according to 

 the state of the atmospheric temperature, as previously explained. 



2ndly. It is more convenient to set these at work as nearly 

 simultaneously as possible, and to watch them well during the 

 next three or four hours, especially if formula No. 2 be employed, 

 so that the produce may be caught at the most favourable 

 moment. If any foreign bodies, as hair, dust, or filaments of 

 paper, are floating in the solutions, the broad plates do not form, 

 as those particles act as nidi for the crystals, and fatally hasten 

 the process of deposition, so that the produce is small and con- 

 fused. 



3rdly. It rarely or never happens that we obtain these large 

 plates at the first attempt; it is often necessary to redissolve 

 and ciystallize two, three or four times before we find any large 

 enough for optical purposes ; the reason is that the process of 

 crystallization is at first much too confused and rapid to allow of 

 the requisite arrangement of the particles or prisms, so as to 

 constitute the broad compound optical leaflets. 



4thly. It is always desirable not to raise the temperature to 

 the boiling-point when redissolving, as spirit and iodine are both 

 then lost, and the formative power of the mother-liquid becomes 

 impaired and at length completely lost, in consequence of the 

 relative proportions of acid and spirit being so altered by con- 

 tinued distillation of the latter. 



5thly. It is always better to add four or five drops of the 

 tincture of iodine after redissolving, in order to produce an 

 atmosphere of iodine vapour above the liquid ; this in condensing 

 starts an early surface crystallization, whilst the mother-liquid 

 is sufficiently charged with the compound. 



6thly. These compound foliaceous plates having " crenated 

 edges " consist of prisms arranged side by side in various forms, 

 but frequently producing discs of considerable size, the com- 

 ponent atoms of which are all arranged in the same optical 

 direction ; but at other times crystallization appears to start from 



Phil. Mag. S. 4. Vol. 7. No. 46. Maij 1854. 2 B 



