THE PREPARATION OF MICROSCOPICAL OBJECTS. 9 



transferred to 70, and then to 90 per cent, alcohol, in which 

 latter they may be left till required. The time of immersion in 

 acid alcohol will vary, according to the nature and size of the 

 specimen, from a quarter of an hour up to a day or more. 



3. Picro-carmine is a very useful, and to a certain extent a 

 differential stain, as it colours the several tissues different 

 tints. It may be prepared thus. Dissolve 1 gramme of carmine 

 in 4 cc. of liquid ammonia and 200 cc. of distilled water. Add 

 5 grammes of picric acid; shake the mixture well for some 

 minutes, and then decant from the excess of acid. Leave the 

 decanted liquid for some days, stirring it occasionally : then 

 evaporate it to dryness, and to every 2 grammes of the dried 

 residue add 100 cc. of distilled water. 



Picro-carmine answers best with specimens preserved in 70 per 

 cent, alcohol. They should be left in the stain for a day, and 

 then placed in 70, and afterwards in 90 per cent, alcohol. Some 

 specimens give better results when w^ashed freely with water 

 on removal from the picro-carmine, and then placed in 1 per 

 cent, acetic acid for an hour before transferring to alcohol. 



4. Magenta stains very rapidly but diffusely : the colour 

 also is not permanent. 



5. Silver Nitrate. A h per cent, solution in water stains the 

 intercellular substance, which binds together the several cells of 

 a tissue, much more strongly than the cells themselves, and is 

 therefore chiefly used when we wish to render prominent the 

 outlines of the individual cells. The specimens should be placed 

 fresh in the silver solution for from two minutes to a quarter 

 of an hour, then washed thoroughly with distilled water, and 

 exposed to the light imtil stained sufficiently deeply, when they 

 may be mounted in glycerine. Such preparations are rarely 

 permanent, as the reduction of the silver, to which the staining 

 is due, continues until the specimens ultimately become too 

 -dark to be of any use. 



6. Osmic Acid. A 1 per cent, solution of osmic acid in water 

 forms an extremely useful staining reagent. It is especially vise- 

 ful for the detection of fat, which is stained by it a dark brown 

 or black colour. Specimens, which must be quite fresh, should 

 only be left in it a few minutes, and may then be mounted in 

 glycerine, or else washed, dehydrated, and mounted in balsam. 



7. Acetic Acid. Although not strictly a staining agent 



