PARAFFIN SECTIONS 91 



placed in the melted paraffin for from six to twenty hours. 

 It is then embedded by pouring a little of the melted 

 paraffin into a watch-glass, or into a small box formed 

 of folded paper or lead-foil, or by bringing together 

 two L- shaped pieces of brass on a glass plate so that a 

 rectangular cavity is produced. The pieces of tissue are 

 then taken out with a small warmed forceps or needle, 

 adjusted to the position they are required to occupy, 

 and more melted paraffin is poured in, so as to cover them. 

 When a film of solid paraffin has formed, the whole is 

 immersed in cold water so as to cool it rapidly. 



A new paraffin is frequently crystalline in structure, 

 and acts much better after it has been kept melted for 

 some weeks, or is much improved by heating nearly to 

 its boiling-point for five or six days (P. T. Beale). The 

 xylol for clearing may be used several times and the 

 paraffin repeatedly, the remains of old tissues being 

 removed. The time which the tissues require to remain 

 in the alcohol, xylol, and paraffin depends upon their 

 size ; very small pieces may be treated in a few hours, 

 large ones may require two or three days. 



Other clearing agents, such as chloroform, turpentine, 

 and cedar oil, may be used instead of xylol. The paraffin 

 method is usually straightforward, but small pieces of 

 tissue must not be left too long either in absolute alcohol 

 or in the paraffin bath, for they are liable to become too 

 hard to cut. Thyroid tissue and skin are also rather 

 troublesome ; they become very hard unless the whole 

 process is carried out as rapidly as possible. If the pieces 

 of tissue be large, the capsule of melted paraffin containing 

 the tissue may be placed under the receiver of an air- 

 pump, which is then exhausted. This causes the paraffin 

 to penetrate better, and the process may be repeated two 

 or three times during the period of infiltration. A special 

 form of paraffin oven has been devised by Cheatle for 



