438 Crystals for the Micro-Polariscope. 



difficult. The usual method is to place upon the centre of the 

 slide a few drops of the solution of the salt which it is desired 

 to obtain in crystals, and either to apply heat until the crystal- 

 lization begins, or to set the slide aside for spontaneous evapo- 

 ration. In the first case, although good crystals may frequently 

 be obtained in the centre, the margin is nearly always a con- 

 fused, almost amorphous mass, which is not only useless as a 

 polariscope object, but very much injures the appearance of 

 the slide. In the second case, the crystals are generally much 

 too large and opaque. 



By the adoption of the method which I am about to de- 

 scribe, both these extremes are avoided. The principle is this. 

 The glass being first made chemically clean, it is completely 

 immersed in a hot solution of the salt of which it is desired to 

 mount specimens. On being removed, both surfaces of the 

 slide soon become covered with a fine network of crystals. 

 The whole of one surface, and all but the central portion of the 

 other, being then washed away, a disc of crystalline deposit is 

 obtained, which is perfectly uniform from edge to centre. But 

 few special appliances, and these of the very simplest cha- 

 racter, are required, but it is nevertheless well to have them 

 systematically arranged before going to work. The following- 

 list comprises all that is necessary — 



1. A small Berlin ware evaporating dish, with tripod or 



other stand for supporting it over a spirit or gas 

 lamp. 



2. A stout glass rod about six inches long, having an 



inch of one end covered with a piece of vulcanized 

 india-rubber tubing. 



3. Solution of caustic soda or potash. 



4. Distilled water. 



5. A small funnel and filtering paper. 



6. A pair of horn or wood forceps. 



7. The solution of the salt to be crystallized. 



The evaporating dish must be of just such dimensions that 

 an ordinary microscopic slide three inches long will rest by 

 its short edges on the sides, so as to be entirely immersed in 

 the liquid contained in the dish. Capsules of this kind are 

 commonly used in chemical operations, and may bo readily 

 obtained. 



The solution of potash or soda is to be used in removing all 

 trace of grease and organic matter from the glass, and must 

 be strong — in fact, the stronger the better. An ounce of fused 

 potash dissolved in four ounces of water will do capitally. This 

 solution is best kept in a bottle, the mouth of which is coveivd 

 by a piece of glass, and which is wide enough to admit the 

 india-rubber covered end of the glass rod. 



