196 ANNUAL OF SCIENTIFIC DISCOVERY. 



an intense blackness is produced where they are superimposed ; if, 

 however, they overlap, in the direction of their long diameters the 

 light passes through unaltered ; when again, a third crystal happens 

 between two others, at an angle of 45, it depolarizes the light and 

 appears colored, like a plate of selenite between two tourmaline plates ; 

 the color depending upon the thickness. When the solution of disul- 

 phate of quinine is strong, and heated so that no precipitate occurs in 

 adding the tincture of iodine, the crystals are formed as soon as the 

 liquid cools, like a bright green film on the surface, and glittering like 

 so many spangles ; a few may be removed cautiously with as little of 

 the fluid as possible by means of a loop of thread, and deposited on a 

 glass plate, and if any liquid remains on the plate it must be cautiously 

 removed by touching it with the corner of a bit of bibulous paper ; 

 being well dried, the glass may be cautiously warmed, for the crystals 

 dissolve at a low heat, and a small drop of Canada balsam placed upon 

 them, and the thin glass cover in the usual way of mounting microsco- 

 pic objects. Or a drop of the hot solution may be placed on a cold 

 glass slip and as soon as the crystals have formed sufficiently, the thin 

 glass cover applied so as to exclude bubbles of air, and the edges 

 sealed with the gold size, or other cement. 



When the crystals are placed upon a glass slide in an excess of the 

 mother liquid, tufts of radiating sulphate of quinidine are formed 

 below them, these when placed in a beam of polarized light with a 

 film of selenite or mica intervening, furnish a most gorgeous exhibition 

 of colors, varying as the polarizer is turned on its axis, and unsurpassed 

 by anything we have ever witnessed. 



By very careful manipulation Dr. Herapath has succeeded in 

 obtaining the crystals sufficiently large to answer instead of the tour- 

 maline plates and Kicols' prism ; these are, however, exceeding rare 

 at present, indeed the largest the Dr. has yet produced is only one-fourth 

 of an inch in diameter. Upon submitting these artificial plates to micro- 

 metrical measurement, it was found that those which possessed suffi- 

 cient thickness to adhere together in clusters, and to raise themselves 

 upon their edges so as to exhibit their thickness, were none of them 

 more than l-300th of an inch, many of them l-600th or l-900th of an 

 inch only ; but even these were much thicker than any of the broad, 

 thin plates, so readily broken, some of which were successfully 

 mounted; these could not have been more than 1-lOOOth of an inch ; 

 and when it is recollected, that tourmalines, as sold for optical pur- 

 poses, are generally from 1-lOOth to l-50th of an inch, it is at once 

 apparent that this newly discovered salt of the vegetable alkaloid 

 quinine, is the most powerful substance known as a polarizing 



crystal. 



\ 



DETECTION OF SODA BY POLARIZED LIGHT. 



THE double chloride of potassium and platinum, crystallizing in 

 regular octahedrons, exercises, when placed in a dark field of the 



