94 MICROSCOPIC METHODS. 



makers supply paraffins which, for ordinary purposes, are quite 

 as good, and much cheaper. The successive steps in the pro- 

 cess of paraffin embedding are as follows : — 



1. Pieces of tissue, however hardened, are placed in fresh absolute alcohol 

 for twenty-four hours in order to complete their dehydration. 



2. Transfer now to a mixture of equal parts of absolute alcohol and 

 chloroform for twenty-four hours. 



3. Transfer to pure chloroform for twenty-four hours. At the end of 

 this time the tissues should sink, or float heavily. 



4. Transfer now to a mixture of equal parts of chloroform and paraffin, 

 and place on the top of the oven for from twelve to twenty-four hours. If 

 the temperature there is not sufficient to keep the mixture melted, then they 

 must be put inside. 



5. Place in pure melted paraffin in the oven for twenty-four hours. For 

 holding the paraffin containing the tissues, small tin dishes such as are used 

 by pastry-cooks will be found very suitable. There must be a considerable 

 excess of paraffin over the bulk of tissue present, otherwise sufficient chloro- 

 form will be present to vitiate the final result and not give the perfectly hard 

 block obtained with pure paraffin. With experience, the persistence of the 

 slightest trace of chloroform can be recognised by smell. 



In the case of very small pieces of tissue the time given for each stage 

 may be much shortened, and where haste is desirable Nos. 2 and 4 may be 

 omitted. Otherwise it is better to carry out the process as described. 



6. Cast the tissues in blocks of paraffin as follows : Pairs of L-shaped 

 pieces of metal made for the purpose by instrument makers must be at hand. 

 By laying two of these together on a glass plate, a rectangular trough is formed. 

 This is filled with melted paraffin taken from a stock in a separate dish. In 

 it is immersed the piece of tissue, which is lifted out of its pure paraffin bath 

 with heated forceps. The direction in which it is to be cut must be noted 

 before the paraffin becomes opaque. When the paraffin has begun to set, 

 the glass plate and trough have cold water run over them. When the block 

 is cold, the metal L's are broken off, and, its edges having.. bgen. pared, it is 

 stored in a pill-box. 



The Cutting of Paraffin Sections. — Sections must be cut as 

 thin as possible, the Cambridge rocking microtome -being; on 

 the whole, most suitable. They should not exceed 8 w in thick- 

 ness, and ought, if possible, to be about 4 ix,. For their manipu- 

 lation it is best to have . 

 two needles on handles, 

 two camel's-hair brushes 



Fig. 51. — Needle with square of paper on end for 1. ji j ji 



manipulating paraffin sections. ^n handles, and a needle 



with a rectangle of stiff 

 writing paper fixed on it as in the diagram (Fig. 51). When cut, 

 sections are floated on the surface of a beaker of water kept at a 



