130 METHODS IN PLANT HISTOLOGY 



CLEARING 



After the sections have been dehydrated, they must be cleared, or 

 made transparent by some clearing agent. The clearing agent must be 

 a solvent of balsam, but it is not at all necessary that the balsam shall 

 be dissolved in the particular clearing agent which has been used. 

 Xylol balsam is used, not only when preparations have been cleared in 

 xylol, but also when they have been cleared in clove oil, cedar oil, 

 bergamot oil, or other clearing agents. 



Xylol is the most generally useful clearing agent. Place the sHde in 

 equal parts of xylol and absolute alcohol and then in pure xylol, allow- 

 ing each to act for about 2-5 minutes. 



Clove oil is also an excellent clearing agent. The clove oil should 

 follow the absolute alcohol, without any mixtures. Pour on a few 

 drops of clove oil, and drain them off at once in such a way as to carry 

 with them whatever alcohol may still remain. Then flood the slide 

 repeatedly with clove oil, draining the clove" oil back into the bottle. 

 If judiciously used, 50 c.c. of clove oil is enough to clear 50 prepara- 

 tions. Sections are usually cleared in a few seconds. The only objec- 

 tion to clove oil is that mounts harden slowly. To overcome this diffi- 

 culty, the shde should be dipped in xylol for a minute before mounting 

 in balsam. 



Synthetic oil of wintergreen is much less expensive and some claim 

 that it is just as good as clove oil. We prefer clove oil. 



For clearing sections on the slide, other clearing agents are hardly 



worth mentioning. 



MOUNTING IN BALSAM 



After the sections are cleared, wipe the slide on the side which does 

 not bear the sections. Put on a drop of Canada balsam and add a 

 clean,! thin cover. Before the cover is put on, pass it through, the 

 flame of an alcohol lamp to remove moisture, for it would be a pity 



' Slides and covers should be treated with hydrochloric acid, or equal parts of 

 hydrochloric acid and water, for several hours. They should then be thoroughly 

 rinsed in water and wiped with a cloth perfectly free from hnt. After rinsing in 

 water, they may be kept in 95 per cent alcohol. When a cover is needed for use, 

 it is Dr. Land's practice to rest the corner of the cover on a piece of filter paper to 

 remove the drop of alcohol; then pass the cover through the flame of a Bunsen or 

 alcohol lamp. The film of alcohol will burn and the cover may warp, but it will 

 usually straighten, and it will be clean and dry. 



The mixture of sulphuric acid and dichromate of potash, used for cleaning labo- 

 ratory glassware, is equally good for slides and covers. 



