ON SECTION CUTTING. 11 



4. Chromic Acid. A 0-25 to 0-5 per cent, solution of chromic 

 acid in water is a useful hardening reagent; it acts much more 

 slowly 'than osmic acid, but penetrates to greater depths. 

 Specimens should usually be left in the solution for one or more 

 days. 



5. A Mixture of chromic acid with a few drops of osmic acid 

 is often very useful, as it combines the advantages of both 

 reagents. 



6. Picric Acid is a very valuable hardening reagent, of which 

 the best preparation is Kleinenberg's. Specimens should be left 

 in it from 12 to 24 hours. It is prepared thus : with 100 cc. 

 of water make a cold saturated solution of picric acid : add 2 

 cc. of concentrated sulphuric acid : filter, and add to the filtrate 

 three times its volume of water. 



B. Dehydration. 



Specimens that have been hardened in any of the preceding 

 reagents, except alcohol, should, on removal, be placed for a few 

 hours in 30 per cent, alcohol, and then transferred to 50 per 

 cent, alcohol : on the following day they should be transferred 

 to 70 per cent, alcohol, which should be changed daily until the 

 specimens are free from the hardening reagent : they may then 

 be left in 90 per cent, alcohol until required. 



C. Staining. 



The hardened specimens, if not too large, may now be stained 

 with either hsematoxylin, borax-carmine, or picro-carmine ; they 

 should then be replaced in 90 per cent, alcohol. If the 

 specimen is too large to stain whole, the sections must be stained 

 after they are cut. 



D. Imbedding^. 



The preparation of sections is greatly facilitated by imbedding 

 the specimen in some waxy substance. For this purpose various 

 materials have been employed, but by far the most useful is 

 paraffin, which is used in the following manner : 



The stained S23ecimen is placed in absolute alcohol for an 

 hour or two in order to completely dehydrate it. It is then 

 transferred to turpentine, in which it is left for half an hour or 

 more until completely saturated. From the turpentine it is 

 transferred to melted paraffin, which is kept by means of a 

 water bath at a temperature just above its melting j^oint. In 



