354 Dr. Herapath on the Process for 



a central point, and then tlie foliaceous laminae (still consisting 

 of adherent componeut prisms or sections of rhombic plates) 

 radiate like the petals of a flower ; under these circumstances 

 the co?o//a-shaped disc would be useless ; but either petal, if 

 large enough, would be serviceable, as in the latter all the atoms 

 would be ill the same relative plane with regard to polarized 

 light ; consequently it is necessary to select for mounting the 

 most uniform plates, and not always the largest clusters, for 

 these would have a " macled " appearance, and produce a beam 

 of light polarized in more than one plane, and therefore creating 

 confused results in the polariscope. 



Having obtained these crystals by one of the above formulae, 

 and caught one upon the glass circle as before described, it is 

 better to submit it to a process of washing (before drying) so as 

 to remove the mother-liquid and get rid of the interference of 

 crystals of sulphate of quinine deposited either beneath or upon 

 the plate ; in my last paper, " cold distilled water imbued with 

 iodine " was recommended for this purpose. 



However, as the effect I then ascribed to iodine is by no 

 means certain, and as it sometimes also injures the plate by de- 

 positing the brown iodide of quinine upon its surface, it is far 

 better to employ a saucer or evaporating basin full of a satm'ated 

 solution of Herapathite in a mixture of distilled water, and one- 

 eighth of its bulk of acetic acid for this purpose. The acetic 

 acid greatly hinders the tangential flying off of the crystalline 

 plates from the glass discs upon our sloivly and very cautiously 

 immersing them in a horizontal position beneath the surface of the 

 water ; and if this should occur, the plate is frequently broken ; 

 should it float off, it must be recaught upon the glass disc as 

 before, and then be cautiously dried by absorbing all moisture 

 by bibulous paper as before, and drying it as rapidly as may be 

 consistent with safety by exposure to air at 50° or 60°. 



If it be considered desirable, it may be exposed for a short 

 time to the vapour of iodine ; this is best accomplished by means 

 of solid iodine — the tincture was once recommended, but the 

 alcoholic vapour renders it brittle, a defect which is not found 

 with the solid substance. 



I have been recently occupied, in conjunction with my friend 

 Mr. Thwaites, in making some experiments upon different media 

 for mounting these crystals. Those principally employed have 

 been iodized almond oil and pure elaine, also slightly charged 

 with iodine, as well as glycerine similarly iodized. The mode of 

 using these is to prepare a saturated solution of iodine in either 

 medium, then dilute it with four times the quantity of pure 

 medium for use. Having placed the circular disc on the table, 

 with a very minute di-op of either substance on its centre, invert 



