CHAPTER I. 5 



5. Removal of Alcohol; Clearing. The water having been 

 sufficiently removed, as described in 3, the alcohol is in its turn 

 removed from the tissues, and its place taken by some anhydrous 

 substance, generally an essential oil, which is miscible with the 

 material used for imbedding or mounting. This operation is 

 generally known as Clearing. It is very important that the passage 

 from the last alcohol to the clearing agent be a gradual one. This is 

 effected by placing the clearing medium under the alcohol. A 

 sufficient quantity of alcohol is placed in a tube (a watch-glass will 

 do, but tubes are generally better), and then with a pipette a suffi- 

 cient quantity of clearing medium is introduced at the bottom of the 

 alcohol. Or you may first put the clearing medium into the tube, 

 and then carefully pour the alcohol on to the top of it. The two 

 fluids mingle but slowly. The objects to be cleared, being now 

 quietly put into the supernatant alcohol, float at the surface of 

 separation of the two fluids, the exchange of fluids takes place 

 gradually, and the objects slowly sink down into the lower layer. 

 When they have sunk to the bottom, the alcohol may be drawn 

 off with a pipette, and after some further lapse of time the 

 objects will be found to be completely penetrated by the clearing 

 medium. 



This method of making the passage from one fluid to another 

 applies to all cases in which objects have to be transferred from a 

 lighter to a denser fluid for instance, from alcohol, or from water, 

 to glycerin. 



This is a convenient stage for carrying out minute dissections, if 

 any such have to be done, a drop of clearing agent being a most 

 helpful medium for carrying out such dissections (see 9). 



At this point the course of treatment follows one of two different 

 roads, according as the object is to be mounted direct in balsam 

 ( 8), or is first to be sectioned ( 6). 



6. Imbedding, and Treatment of Sections. The objects are now 

 imbedded. They are removed from the clearing medium, and soaked 

 until thoroughly saturated in the imbedding medium. This is, for 

 small objects, generally paraffin, liquefied by heat, and for large 

 objects either paraffin or a solution of collodion or " celloidin " (in 

 this last case the clearing may be omitted and the tissues be 

 imbedded direct from the alcohol). The imbedding medium con- 

 taining the object is then made to solidify, and sections are made 

 with a microtome through the imbedding mass and the included 

 objects. The sections are then mounted on a slide by one of the 



