6 INTRODUCTION. 



given as to the preparation and employment of some of the 

 most useful reagents. More detailed directions must be sought 

 in special treatises upon micro-chemistry. 1 The list and the 

 general rules here given will serve for most investigations. 



19. It is best to try first a very small amount of the reagent, 

 and carefully note its effect before adding more. If it is neces- 

 sary to increase the amount, draw a little through by means of 

 bibulous paper, as previously directed. Many reagents are slow 

 in producing their effects. Hence some time must be allowed to 

 elapse before one reagent is replaced by another, and it is well 

 in some cases to apply slight heat to accelerate or increase 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. For 

 instance, small dark crystals of iodine separate from an alcoholic 

 solution when this is brought into contact with water. Removal 

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

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

 is to be immediately followed by another, provided the specimen 

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

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

 duction of several reagents in these operations is conducted 

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

 both these operations ma}' 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 ht/dmte, Caustic 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 S3'rupy 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. Hilfsbueli by Behrens (Schwetschke, 

 Braunschweig), 1884. 



