224 CLEARING AGENTS. 



the alcohol. They will sink down to the level of separation 

 of the two liquids at once; and after some time they will be 

 found to have sunk to the bottom of the clearing medium. 

 They may then be removed by means of a pipette, or the 

 supernatant alcohol drawn off and the preparations allowed to 

 remain until wanted. 



The chief clearing agents are essential oils. A classifica- 

 tion of these is given below (No. 343, Stieda and Schieffer- 

 decker). 



The penetration of all clearing media may be hastened by 

 using them warm. Directions for clearing are given when 

 necessary under the heads of the different organs and tissues. 

 It will suffice here to advise the beginner to keep on his table 

 the following: Oil of cedar, for general use; clove oil, for- 

 making minute dissections in cases in which it is desirable to 

 take advantage of the property of that essence of forming very 

 convex drops on the slide, and of imparting a remarkable 

 brittleness to soft tissues ; carbolic acid, for rapidly clearing 

 imperfectly dehydrated objects. 



It frequently happens that the essential oil with which ob- 

 jects are being treated in a watch-glass or on a slide becomes 

 cloudy after a short time, and fails to clear the tissues. This 

 is owing to a combination between the essential oil and mois- 

 ture, derived, I think, rather from the air than from the ob- 

 jects themselves. The cloudiness can usually be removed by 

 warming (as pointed out by HATCHETT JACKSON, Zool. Anzeig., 

 1889, p. 630), but this remedy is not always successful, for in 

 certain states of the atmosphere the cloudiness will persist, 

 notwithstanding continued warming. It is for this reason 

 that I advise that clearing be done, whenever possible, in 

 shallow well-corked tubes, under which conditions the pheno- 

 menon rarely occurs. 



343. Classification of Clearing Agents (STIEDA). Stieda's 

 experiments with essential oils led him to establish the follow- 

 ing classification : 



A. The turpentine group, capable of clearing in a short time 

 perfectly dehydrated sections, but clearing watery sections 

 only after many hours or not at all. 



01. Terebinthinas. 

 01. Absinthii. 



01. Balsam. Copaivse. 

 01. Cor tic. Aurantiorum. 



