l888.] NEW- YORK MICROSCOPIC AI, SOCIETY. 117 



make the liquid to circulate so as to keep the solution of uni- 

 form temperature and concentration. Remove from over the 

 lamp, and, after one or two seconds, run off the liquid from the 

 corner of the slip into the flask. Then hold the slide over the 

 lamp until rosettes begin to form, and, when these are almost as 

 large as desired for the central discs, place the slide at once on 

 the cold iron. This will stop the process of crystallization, and 

 after the slide is cooled, crystallization can be started again by 

 heating, when a rim will be formed on the rosettes of any size, 

 depending on the length of time during which the heat is ap- 

 plied. The growth of the rim can be stopped by cooling. 



When the crystals are satisfactory, and just sufficiently cold to 

 stop crystallization, immediately put a drop of balsam on a thin 

 cover, which, held by the forceps, can be heated over the lamp, 

 and applied to the slip. The balsam can be made to spread by 

 warming the slide, cover side down, over the lamp. 



If properly prepared, the amorphous deposit of Salicin will 

 remain in that condition, and cause the rosettes to appear on a 

 black back-ground, when viewed with the prisms at right angles 

 in the microscope. 



Another substance, which can be made to crystallize in very 

 beautiful forms with much less trouble than Salicin, is Hippuric 

 Acid. 



Of this substance a concentrated but not saturated solution in 

 hot, absolute alcohol is prepared in the little flask ; the glass 

 slip heated over the lamp ; the hot solution poured on the end 

 of the slip, and flowed over the centre, with some ebullition, 

 and immediately run off into the flask. Wait a few seconds un- 

 til the rosettes begin to form on the slide, and then quickly 

 place on the cold iron, until the slide is cool. Now examine 

 with the microscope, when presently the rosettes will begin to 

 radiate a fringe around their circumference. This can be accel- 

 erated by allowing the breath to come in contact with the slip. 

 When the discs are nearly of the size desired the process will be 

 stopped, or rather changed in character, by slightly warming over 

 the lamp for one or two seconds, and then cooling, and proceed- 

 ing as before, if further developments are desired. When the 

 crystals cover the surface seal on the thin glass cover. 



If properly prepared these slides are very vivid in color in 

 the polariscope, and exhibit also very pleasing forms. 



