264 PRACTICAL MICROSCOPY. 



changing this at the expiration of an hour, and allowing 

 the sections to remain in the second solution for about the 

 same period. 



The logwood stained sections, after being well washed, 

 are soaked in alcohol for an hour, then removed to oil of 

 cajeput, and allowed to remain in this for a couple of 

 hours; at the end of this time transfer them to oil of 

 turpentine. In less than an hour they will be ready for 

 mounting in balsam or dammar. The sections should not 

 be allowed to remain long in the turpentine or else they 

 become brittle. 



In the case of picro-carmine stained sections they should 

 be removed from the alcoholic solution of picrate of 

 ammonia into alcohol for about a minute, then into oil of 

 cajeput. 



The object of employing an alcoholic solution of picrate 

 of ammonia is to avoid the loss of colour which attends the 

 use of alcohol only, the yellow stain of picric acid being 

 readily removed from the tissue by that liquid. Picrate of 

 ammonia may be easily made by adding a slight excess of 

 liquid ammonia to a solution of picric acid, and evaporating 

 the mixture to dryness at a gentle heat.* The residue is 

 dissolved in alcohol and filtered. 



Wood sections stained in picro-carmine are very beauti- 

 ful and permanent. The staining being done at one 

 operation, and the colours being remarkably selective, 

 there is an absence of secondary tints, as in the case of 

 most other double stains, especially where one tint is 

 partially washed out to make way for another. 



Regarding permanence, some stained sections mounted 

 nearly five years ago appear to have retained their 

 brilliancy unimpaired. 



* This salt had better be purchased by the student, as the picrates explode 

 violently when over -heated. 



