METHODS OF IMPREGNATION. 293 



from tissues embedded in colloidin ; but sections of large area may 

 be obtained, which is often of greater importance. For very thin 

 sections it is better to use paraffin for the embedding-material, 

 although the resulting sections will have to be smaller. 



Paraffin is insoluble in alcohol of all strengths. It is therefore 

 necessary to remove the absolute alcohol from the tissues before 

 they can be impregnated with paraffin. This may be done by 

 immersing the tissues in some liquid that is a solvent for paraffin 

 and is also miscible with alcohol. For this purpose, xylol, chloro- 

 form, or oil of cedar-wood may be used. Xylol yields the most 

 rapid results, but its use is contraindicated when it is desired to 

 retain fatty substances that have been colored with osmic acid, as 

 the xylol extracts them. If their preservation within the tissues 

 is important, chloroform should be used ; but the sojourn even in 

 that liquid should be as short as possible. Oil of cedar-wood prob- 

 ably causes less change in tissues than chloroform, but the method 

 is more protracted, and, requiring longer treatment in the paraffin- 

 oven, probably has little ultimate advantage over chloroform for 

 general purposes. 



If xylol is used, the tissues are transferred from the absolute 

 alcohol to xylol, on which they at first float. Subsequently they 

 sink, and are gradually rendered transparent as the alcohol is 

 expelled by the xylol. When there are no opacities left the speci- 

 men is ready for the paraffin-oven. These changes take from two 

 to twenty-four hours. 



The treatment with chloroform is similar to that with xylol, but 

 after the tissues have been cleared in chloroform (six to twenty-four 

 hours) they are immersed in a saturated solution of paraffin in 

 chloroform for about the same length of time. They are then ready 

 for the paraffin-oven. 



When oil of cedar-wood is used the pieces should be soaked in 

 two successive portions of the oil, about twelve hours in each, to 

 insure removal of the alcohol. 



The foregoing steps are all preliminary to the actual impregnation 

 with paraffin. 



It is important that the paraffin used for impregnation and 

 embedding should have a wax-like, and not a crystalline, texture, 

 and that its melting-point should be such that its consistency will 

 be favorable for cutting at the average temperature of the labora- 

 tory. Griibler, of Leipzig, furnishes excellent qualities of paraffin. 



