iNTBODtJOTlON. 



sa^srt;afS3t 



in special treatises upon ic ^^"^ ^ t 5 



it, .1 verv small amount of the 



action ; but this must be very cautiously done. 



20 If one reagent is to be followed by another, attention 

 must be given to the effects which the reagents have upon each 

 other, or upon the medium, as well as upon the specimen. Fo* 

 instance, small dark crystals of iodine separate from an alcoholic 

 solution when this is brought into contact with water. 1 

 of the cover-glass is advised in all cases where one reagent if 

 be washed out before the application of a second, or where < 

 is to be immediately followed by another, provided the specimen 

 is not so delicate as to be disturbed by it. Some parts of 

 specimen are apt to escape action, if the washing or the intro 

 duction of several reagents in these operations is comUicU 

 without lifting the cover; but by the exercise of great car 

 both these operations may be carried on successfully by the use 

 of bibulous paper without removing the cover-glass. 



21. Owing to their importance, potash and iodine are de- 

 scribed first. The other reagents are given in alphabetical 

 order, for convenience of reference. 



22. Potash, Potassic hydrate, Caiistic potassa, are names 

 interchangeably given to white solid potassa and to its solutions. 

 This substance absorbs carbonic acid so eagerly from the air, 

 that it must be kept in glass-stoppered bottles. To prevent the 

 stoppers from becoming fastened by the action of the alkali on 

 the glass, it is well to smear them with vaseline or paraffin. 



23. Solutions of two strengths are used. I. Concentrated. 

 Solid potassa is dissolved in the smallest amount of water (not 

 far from half its own weight) by which it will become liquid. 

 This dense syrupy liquid is too strong for ordinary use. II. A 

 common solution made with one part of solid potassa in three, 



1 Consult the following : Botanical Micro-Chemistry, by Poulsen, translated 

 by Trelease (Cassino, Boston), 1884. Hilfsbuch by Behrens (Schwetschke, 



Braunschweig), 1884. 



