100 THE MICROSCOPIST. 



to dryness. Many salts may be preserved in balsam, but 

 some are injured by it, and need glycerin or castor oil as 

 a preserving fluid. 



The method of crystallization may be modified in vari- 

 ous ways so as to obtain special results. Thus if a solu- 

 tion of sulphate of iron is suffered to dry on a slide, the 

 crystals will be arborescent and fern-like, but if the liquid 

 is stirred with a glass rod or needle while evaporating, 

 separate rhombic prisms will form, which give beautiful 

 colors in the polariscope. Pyrogallic acid also crystallizes 

 in long needles, but a little dust, etc., as a nucleus, brings 

 about a change of arrangement resembling the "eye" of 

 the peacock's tail. 



A saturated solution dropped into alcohol, if the salt is 

 insoluble in alcohol, will produce instantaneous crystals. 



To obtain the best results, some crystals, as salicin, 

 should be fused on a slide over the lamp, and the matter 

 spread evenly over the surface. This may be done with 

 a hot needle. The temperature greatly affects the char- 

 acter of the crystallization. If very hot, the crystals run 

 in lines from a common centre. A medium temperature 

 produces concentric waves. 



Many new forms result from uniting different salts in 

 different proportions. The knowledge of these different 

 effects can only be attained by experience. 



Sections of crystals, as nitrate of potash, etc., to show 

 the rings and cross in the polariscope, are difficult to 

 make. After cutting a plate with a knife to about one- 

 fourth of an inch thick, it may be filed with a wet file to 

 one-sixth of an inch, smoothed on wet glass with fine 

 emery, and polished on silk strained over a piece of glass, 

 and rubbed with a mixture of rouge and tallow. The 

 nitre must be rubbed till quite dry, and the vapor of the 

 fingers prevented by the use of gloves. 



For a general account of the use of polarized light, see 

 Chapter VI. 



