348 THE AMERICAN MONTHLY [Nov., 



chroic, but not very strongly so. But it is interesting to 

 observe, with the polarizer alone, the color given by the 

 thicker crystals. They are blue in one position and faint 

 red or yellowish according to their thickness when 

 turned half way round. The salt crystallizes best when 

 allowed to dry spontaneously and is best mounted in 

 castor oil. Balsam affects slightly the brilliancy of the 

 preparation. As some of the crystals are pretty thick, 

 ihe same precautions are necessary as in mounting the 

 di-acetate of copper. 



It may be useful to give the details for preparing 

 plates of the iodo-sulphate of quinine called Herapathite, 

 which can be used to replace a Nicoll's prism or ana- 

 lyzer. This description is taken from Dr. Grolding Bird's 

 "Elements of Natural Philosophy," 1860. 



To prepare this salt for the formation of polarizing 

 laminae, the following plan may be adopted : — Dissolve 

 50 grains of di-sulphate of quinine in two fluid ounces 

 of acetic acid, and two of proof spirit, warmed to 100 

 degrees F. in a very wide-mouthed flask or glass beaker. 

 Then slowly add 50 drops of a solution of 40 grains of 

 Iodine in an ounce of rectified spirit. Agitate the mix- 

 ture, and then set it carefully aside for six hours, in an 

 apartment maintained at a temperature of about 50''F. 

 The utmost care must be taken to avoid any motion of 

 the vessel. Indeed, all accidental vibration should be 

 guarded against by suspending the vessel by a string, or 

 by allowing it to rest on a mass of cotton wool. 



If, after six hours, the large laminse of the salt have 

 not been formed, warm the fluid with a spirit lamp, and 

 when it has become clear, add a few drops of the solu- 

 tion of iodine in spirit. The large laminaB form on the 

 top of the fluid, and should be removed carefully by 

 gliding under one them a circular piece of thin glass. 



The specimen should be drained by resting the edge 

 of the glass on a piece of bibulous paper, but it must not 



