PART III— REAGENTS. 



The reagents that have been recommended for microscopical 

 work are quite numerous, and while nearly all of them may have 

 more or less special merit, the number of reagents actually 

 required in practice is fortunately quite small. 



It is important that the student recognize the necessity for a 

 thorough understanding of the structure of the material under 

 examination rather than place too much dependence upon the 

 effects produced by reagents ; in other words, the study of struc- 

 ture should precede the use of reagents, particularly stains, when 

 it will often be found that the latter can be dispensed with entirely. 



The chemicals that are employed in microscopical work, either 

 as reagents or for other purposes, may be classified as follows : 

 (i) Preservatives, (2) Fixing and Killing Agents, (3) Harden- 

 ing and Dehydrating Agents, (4) Clearing Agents, (5) Stains 

 and (6) Special Reagents. 



Preservathes are substances used to preserve material which 

 is to be examined. The most important of these are alcohol 

 (from 40 to 95 per cent.) and formalin [2 to 6 per cent, aqueous 

 or alcoholic (60 per cent._ alcohol) solution], the latter of which 

 is considered advantageous in the preservation of specimens con- 

 taining coloring substances, as leaves, flowers, etc. Almost any 

 antiseptic of the proper strength may be used as a preservative. 



Fixing or Killing Agents are more especially employed in 

 the study of the protoplasmic cell-contents, where by their use 

 the life-processes of the cell are brought to a sudden termination, 

 the object being to fix the contents in a condition approaching as 

 nearly as possible the normal living state. In order to carry out 

 this operation successfully, the living specimen must be placed in 

 the fixing or killing agent as soon as collected, and if the specimen 

 is large it should be cut into small pieces. The following are some 

 of the common fixing agents : Chromic acid in 0.5 to i per cent, 

 aqueous solution ; osmic acid in i to 2 per cent, aqueous solution ; 

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