506 Mr. Warington on Preserving Salts for the Microscope. 



vations made at any period. This was rendered the more de- 

 sirable from the difficulty of obtaining perfect and isolated 

 subjects, and the rapidity with which many of these undergo 

 alteration by exposure to the air. With some few salts this 

 operation was comparatively easy, as the Canada balsam 

 offered an excellent medium for the purpose, but in the greater 

 number of cases I have examined it proved totally indifferent ; 

 and this arises from two causes, the first from the action of 

 the turpentine contained in the balsam rendering the sur- 

 faces of the crystals opalescent ; the second, from the heat, 

 which it was necessary to apply to make the balsam suffi- 

 ciently fluid to displace the atmospheric air, fusing the salt in 

 its water of crystallization, or rendering it opake from the 

 loss of water. Olive oil on trial appeared a good medium for 

 all cases, but was objectionable from its fluidity, and from its 

 depositing its stearine in cold weather. Castor oil was then 

 tried, and this I have adopted with great satisfaction. 



The method to be adopted in mounting these specimens is 

 as follows : — A warm saturated solution of the salt required is 

 to be prepared, and a drop of it placed upon the glass slider, 

 on which it is intended to be permanently mounted, and al- 

 lowed to crystallize; when a good group of crystals is ob- 

 tained the uncrystallized portion is to be cautiously removed; 

 this is best effected by drawing it gradually away in a small 

 stream along the edge of the slider, having previously broken 

 through that part of the crystalline ring adjacent to the edge; 

 the salt is to be allowed to drain itself quite dry by placing the 

 slider on its end in a vertical position. It should next be exa- 

 mined under the microscope, to ascertain the fitness of the 

 crystals for the purposes required, because many salts separate 

 from their solutions in crystals too thin to exhibit any pris- 

 matic colours when viewed by polarized light, appearing only 

 of a pearly or silvery aspect, while others form in the oppo- 

 site extreme, and are totally unfit, from their thickness, for 

 investigation. Presuming, however, that the crystals are such 

 as the investigator requires, the next step is to drop on a 

 small quantity of castor oil; that which has been filtered cold 

 must be employed, as otherwise it is liable to the same ob- 

 jection as olive oil, and care must be taken that it covers the 

 whole of the salt, and has displaced all the particles of atmo- 

 spheric air that may have been adhering to the crystals. This 

 having been done, a small piece of very thin glass is to be 

 carefully placed on the surface of the oil, and any excess which 

 may by this means have been pressed out, cautiously removed 

 by bibulous paper from the edges. Two or three coats 

 of a strong varnish of shell -lac, as ordinary sealing-wax in 



