90 MICROSCOPIC METHODS. 



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

 paraffin and place on the top of the oven from twelve to twenty-four 

 hours. If the temperature there is not sufficient to keep the mixture 

 molten 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, other- 

 wise sufficient chloroform 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 fresh melted paraffin. 

 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 been pared, it is stored in a pill-box. 



Fixing of Sections on Slides. Sections must be cut as 

 thin as possible, the Cambridge rocking microtome being, 

 on the whole, most suitable. They should not exceed 8 //, 

 in thickness, and ought, if possible, to be about 4 p. For 

 their manipulation it is best to have two needles on handles, 



FIG. 35. Needle with square of paper on end for 

 manipulating paraffin sections. 



two camel's-hair brushes on handles, and a needle with a 

 rectangle of stiff writing paper fixed on it as in the diagram 

 (Fig. 35). When cut, sections are floated on the surface of 

 a beaker of water kept at a temperature about 10 C. below 

 the melting-point of the paraffin. On the surface of the 

 warm water they become perfectly flat. 



