28 THE MICROSCOPIC PREPARATION. 



can not be passed directly into paraffin, as alcohol dissolves only a small 

 percentage of paraffin, and, consequently, the preparation would not be 

 infiltrated with the imbedding mass. The pieces of tissue are therefore 

 first placed in some fluid which mixes with absolute alcohol and at the 

 same time dissolves the paraffin. There are many reagents which have 

 this property, such as xylol, toluol, chloroform, and a number of oils (oil 

 of turpentine, oil of cedar, oil of origanum, etc.). Of these xylol may 

 be recommended for general use. In the xylol the tissues remain for 

 from two to twelve hours, the time depending somewhat on the size of 

 the pieces and on the density of the tissue. When thoroughly permeated 

 by the xylol, they are transparent. From the xylol (toluol, chloroform, 

 or oils) the tissues are placed in melted paraffin. Two kinds of paraffin 

 are generally used, one having a melting point of 38 to 40 C. soft 

 paraffin and another with a melting point of 50 to 58 C. so-called 

 hard paraffin. The paraffin should always be filtered before using. This 

 is best done by using a hot-water filter. It is essential that melted 

 paraffin have a constant temperature while the tissues are being infiltrated. 

 This is attained by placing the receptacle containing the paraffin in a 

 paraffin oven regulated by means of a thermostat to a temperature about 

 two degrees above the melting point of the hard paraffin. 



Filtered hard and soft paraffin may be kept in suitable glass beakers 

 in respective compartments in the paraffin oven. After the tissues are 

 thoroughly permeated with the xylol, this is poured off and melted soft 



paraffin added, and the dish replaced 

 in the paraffin oven. In the soft 

 paraffin the tissues remain from one to 

 four hours, at the end of which time 

 the soft paraffin is poured off and 

 hard paraffin added, and the dish 

 again placed in the oven. In the 

 hard paraffin the tissues remain from 

 Fig. 3,-Box for imbedding tissues. two to twelve hours, depending on the 



size of the pieces. They are now 



ready to be imbedded. Two metallic L's are placed together on a 

 glass or metal plate in such a way as to make a rectangular box. 

 (Fig. 3.) This is filled with melted hard paraffin taken from the 

 oven. Before the paraffin cools, the piece of tissue to be imbedded 

 is taken from the hard paraffin in the oven and placed with one 

 of its flat surfaces against one end of the box. If several pieces of 

 tissue are to be imbedded, a piece may thus be placed in each end of 

 the box. While transferring the tissues from the hard paraffin to the 

 imbedding box they should be handled with forceps, the blades of 

 which have been warmed in a flame. As soon as the paraffin in which 

 the tissues are imbedded has cooled sufficiently to allow the formation of 

 a film over the melted paraffin, the imbedding box is placed in a dish of 

 cold water. This cools the paraffin quickly and prevents its becoming 

 brittle. A stay of from five to ten minutes in the cold water hardens the 

 paraffin so that the L's may be removed, and the paraffin block containing 

 the imbedded tissue may be taken from the plate. It is well to place the 

 paraffin block thus obtained back into the cold water for a short time, so 

 that it may become hard all the way through. As the paraffin often 

 adheres closely to the glass or metal plate and the L's, it is advisable to 

 cover these parts with a very thin layer of glycerin before imbedding. 

 There is then no difficulty in separating them from the paraffin block. 



