THE PREPARATION OF MICROSCOPICAL OBJECTS 9 



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-Carmiae is a very useful, and to a certain extent a 

 ditierential stain, and it colours the several tissues different 

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

 in 4 c.c. of liquid ammonia and 200 c.c. 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 c.c. of distilled water. 



Picro carmine answers best with specimens preserved in 70 

 per cent, alcohol. They should be brought through weak spirit 

 to water, and left in the stain for a day ; then brought up to 70, 

 and afterwards to 90 per cent, alcohol. Some specimens give 

 better results when washed 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 ^ 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 until 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 use- 

 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, in- 

 asmuch as it does not colour the specimens, acetic acid may 



