GENERAL REAGENTS 189 



tests for starch (Sec. 12), coloring the grains blue, or a deep brown if the 

 solution be too strong. Cellulo.se (Sec. 12) is generally stained yellow 

 or brown, which changes to blue if a strong solution of sulphuric acid 

 be applied after the iodine. 



B. Chlorzinc iodine. This is a troublesome reagent to prepare, but the best 

 test for cellulose. Dissolve zinc in pure hydrochloric acid and evapo- 

 rate the solution (with metallic zinc present in it during the pr()ces.s) to 

 the density of sulphuric acid. Add as much potassium iodide as the 

 solution will dissolve, and finally as much metallic iodine as it will 

 take up. The solution will keep better away from the light. Chlorzinc 

 iodine stains pure cellulose a clear blue or violet. It reacts best on 

 preparations in water. 



C. Potash solution. A 6% solution of potassium hydrate, or caustic potash, 

 in water is an excellent clearing and softening agent. A 15'; solution 

 is necessary for some subjects, as firm leaf sections. The potash 

 solution may be neutralized by washing the sections in commercial 

 acetic acid and then mounting them in the latter. 



Potash solutions must be kept tightly stoppered. Rubber stoppers 

 answer well; glass ones should be covered with paraffin, otherwise 

 they are likely shortly to become stuck beyond the power of removal. 



D. Acetic acid. A 1% solution of glacial acetic acid in water will fix and 

 frequently bring out clearly the nucleus and other protoplasmic struc- 

 tures of a cell. Beautiful temporary preparations may then be made 

 by staining with gentian violet (see F) or methyl green (Sec. 186, B). 



E. Eosin. A strong solution of eosin in water is the most useful. Alco- 

 holic solutions may be employed when the preparation is in alcohol. 

 This stain has the peculiar advantage of coloring protoplasm alone, 

 leaving the cell wall unaffected. 



F. Gentian violet. A deep violet solution in 1% acetic acid is a good 

 strength for temporary staining of fresh material, or after fixing with 

 1% acetic acid (see D). 



G. Alcohol. Alcohol has its chief value for temporary preparations in 

 driving out air bubbles from material which will not wet easily in 

 water, as, for example, the mycelium of fungi. 



H. Distilled water. Temporary preparations of living plants are mounted 

 in tap water, or that in which they live, if aquatic. Distilled water is 

 used when the preparations are made from preserved material. 



I. Glycerin. A solution one third glycerin and two thirds distilled water 

 is very useful in preserving temporary preparations. A drop or two 

 placed at the edge of the cover glass will prevent the preparation from 

 drying up. This .solution, or one considerably stronger, is also used 

 for permanent preparations when inclosed in a cement ring (Sec. 188). 



