394 ADDITIONS. 



high power to see that the layer of jelly is not too 

 thick. If it is, warm the specimen and gently press 

 the cover-slip, let cool, scrap away the pressed out 

 jelly, and surround the edges of the cover-slip with 

 cement. 



Mounting in Farrant's solution. 



Purp gum arabic 40 grams. 



"Water 40 c.c. 



Glycerine 20 c.c. 



A little thymol or other preservative. 

 The mixture may be made thus : — Take lumps of gum 

 arabic which are nearly free from colour, weigh 

 out 40 grams, grind to powder, place in about 

 150 c.c. water, warm or boil to dissolve. Add a 

 few drops of a strong alcoholic solution of thymol. 

 Filter through a hot filter, changing the filter when 

 clogged. Evaporate the filtrate until it is about 

 80 c.c, then add the glycerine. The mixture is best 

 kept in a bottle with a glass cap. 

 When mounting in this, let a section soak for a minute 

 in glycerine, then in a drop of Farrant's solution on 

 the slide. Before cementing, the solution should be 

 allowed to become firm at the edges. 



Staining. 



After treatment with osmic acid. When it is re- 

 quired to harden a tissue in osmic acid and after- 

 wards to stain it, the tissue should be left for half- 

 an-hour to two hours only in the acid ; then washed 

 well with water and kept in alcohol for a few days. 



Nitrate of silver preparations. The reduction of 



silver in these may be greatly retarded by placing 

 them for a few minutes in a 2 to 10 p.c. solution of 



