190 CLEARING AGENTS. 



preparations cleared quickly, celloidin more slowly, anilin 

 colours unaffected. 



Very useful ; its worst fault is its high price. 



337. Turpentine. Generally used for treating sections that 

 have been cut in paraffin, as it has the property of dissolving 

 out the paraffin and clearing the sections at the same time, 

 but many other reagents (naphtha, for instance) are prefer- 

 able for this jDurpose (see ante, 276). If used for alcohol 

 objects it causes considerable shrinkage and alters the struc- 

 ture of cells more than any other clearing agent known to 

 me, unless used in the thickened state, a method which is 

 much liked for some purposes in Germany. Thickened tur- 

 pentine ("Verhartzes Terpentinol " of German writers) is 

 prepared by exposing rectified turpentine in thin layers for 

 some days to the air. All that is necessary is to pour some 

 turpentine into a plate, cover it lightly so as to protect it from 

 dust without excluding the air, and leave it until it has attained 

 a syrupy consistency. Turpentine has, I believe, the lowest 

 index of refraction of all the usual clearing agents, except 

 bergamot oil ; it clears objects less than balsam. 



338. Carbolic Acid. Best used in concentrated solution in 

 alcohol. Clears instantaneously, even very watery preparations. 

 This is a very good medium, but it is generally better avoided 

 for preparations of soft parts which it is intended to mount in 

 balsam, as they generally shrink by exosmosis when placed in 

 the latter medium. It is, however, a good medium for celloidin 

 sections (see above, 298). 



339. Kreasote. Much the same properties as carbolic acid. 

 Beech-wood kreasote is the sort that should Be preferred for 

 many purposes, for clearing celloidin sections (for which it 

 is a very good medium) amongst others. 



340. Xylol, Benzol, Toluol, Naphtha, Chloroform. Too vola- 

 tile to be recommendable as general clearing agents, but may 

 be used for celloidin sections or for paraffin sections. For 

 these, I greatly recommend naphtha. 



341. Absolute Alcohol (SEILEK, Journ. Boy. Mic. Soc., 1882, p. 126). 

 Absolute alcohol is recommended by Seiler for preparing objects for mount- 

 ing in balsam, the balsam being in this case dissolved in warm absolute 

 alcohol (see No. 398). The method is said by Seiler to give very good results 



