METHODS. 13 



To the detailed instructions given above, the follow- 

 ing general remarks may be added. All the above- 

 mentioned staining-fluids may be used for protoplasm 

 and nuclei. 



The stain produced by aniline-colours is apt to fade, so 

 that they are not to be recommended for preparations 

 which are to be kept for a long time. The staining of 

 haematoxylin also fades, but more slowly. In order to 

 prevent fading, the preparations should be kept in the 

 dark. 



Clearing the preparations. If it is not desired to 

 observe the details of structure of the protoplasm or of 

 the nucleus, the best clearing agent is a solution of 

 potash, either in water or alcohol. The most generally 

 useful is the 5 per cent, solution made by dissolving 

 five grammes of solid caustic potash in 100 c.c. of dis- 

 tilled water. The alcoholic solution is made by adding 

 strong alcohol (ordinary methylated alcohol will do) to 

 a quantity of a concentrated solution in water until a 

 precipitate begins to be formed. The mixture must 

 then be well shaken, and allowed to stand and settle 

 for twenty-four hours; the clear fluid is then poured 

 off. For use a mixture of equal parts of this solution 

 and of distilled water may be made. 



The clearing action of potash is due to the swelling 

 of the cells and their contents, so that they become 

 more transparent ; at the same time it dissolves many 

 of the granules in the protoplasm, and saponifies the 

 oil-drops. The swelling caused by the action of the 

 solution in water is often too great, especially when it is 

 desired to see the cell- walls distinctly; this difficulty 

 may be got over by the use of the alcoholic solution. 



