INTRODtrCTIOtf. 5 



which microscopic specimens are preserved are termed mounting- 

 media. 



15. MEDIA. In all ordinar}* cases pure water is the best 

 medium in which to place the object for examination. If dis- 

 tilled water cannot be procured, filtered rain-water or melted ice 

 will answer perfectly. In some instances water produces an 

 immediate change either in the cell-wall or in the contents of 

 the cells. For instance, the superficial cells of the coats of 

 man}- seeds swell up at once when they are placed in water, and 

 lose their former shape ; on the other hand, important contents 

 in the seeds of man}- plants are dissolved immediately when the 

 sections are moistened. Hence, other media must be sometimes 

 substituted for water. Absolute alcohol (see 40) is the most 

 useful for meeting the cases above referred to. Thus, if a sec- 

 tion of a seed-coat be first examined in absolute alcohol, and the 

 alcohol be gradually replaced by water as directed in 17, the 

 changes due to water will take place slowly, and can be watched 

 throughout. For the cases in which the cell contents are sus- 

 pected of undergoing change from water, castor-oil is a useful 

 medium. If thought best, this can be removed subsequently 

 from the specimen by alcohol or ether, and the latter in turn 

 ma}* be made to give place to water, and the changes can be 

 followed with certainty. 



16. Glycerin (see 60), either concentrated or somewhat 

 diluted with water, is a bighty useful medium, imparting a good 

 degree of transparenc}- to most specimens. It withdraws a part 

 of the water of the cell-sap, and in the case of thin-walled cells 

 this is followed bj- some change of form. The remarkable effects 

 produced upon some of the contents of cells by the action of 

 glycerin and similar agents will be referred to under Protoplasm. 



17. One medium may be replaced by another by the careful 

 use of bibulous paper. Good filtering paper is the best for this 

 purpose. If a little of the liquid which it is desired to place 

 under the cover-glass be put at the edge of the cover, and the 

 opposite edge be then touched lightly with the paper, the liquid 

 will be at once drawn through. B}- successive applications of 

 the same liquid, the specimen can be thoroughly washed without 

 removal of the cover-glass. 



18. REAGENTS. Four reagents are in very common use in 

 nearly all histological examinations ; namel}*, caustic potash, a 

 solution of iodine, an acid, and a staining agent. Even in ordi- 

 nary cases, however, it is desirable to have a somewhat wider 

 choice than this, and therefore the following brief hints are 



