88 MICROSCOPIC METHODS. 



solution is used. For small pieces of tissue J inch in thickness, 

 twelve hours' immersion is sufficient. If the pieces are larger, 

 twenty-four hours is necessary. It is very important for the 

 success of the subsequent procedures that the corrosive 

 sublimate should be now thoroughly washed out of the 

 tissues. They should be tied up in a piece of gauze, and 

 this placed in a stream of running water for from twelve to 

 twenty-four hours, according to the size of the pieces. They 

 must then be placed for twenty-four hours in each of the 

 following strengths of methylated spirit (free from naphtha l ) : 

 30 per cent, 60 per cent, and 90 per cent. Finally they 

 are placed in absolute alcohol for twenty-four hours and 

 are then ready to be prepared for cutting. If the tissue 

 is very small, as in the case of minute pieces removed for 

 diagnosis, the stages may be all compressed into twenty-four 

 hours. In fact after fixation in corrosive the tissue may be 

 transferred directly to absolute alcohol, the perchloride of 

 mercury being removed after the sections are cut as will be 

 afterwards described. 



Methylated Spirit. Small pieces of tissue may be placed 

 in methylated spirit, which is to be changed after the first day. 

 In six to seven days they will be hardened. If the pieces 

 are large, a longer time is necessary. 



The Cutting of Sections i. By Means of the Freezing 

 Microtome. Pieces of tissue hardened by any of the above 

 methods must have all the alcohol removed from them by 

 washing in running water for twenty-four hours. They are 

 then placed for from twelve to twenty-four hours (according 

 to the size) in a thick syrupy solution containing two parts 

 of gum arabic and one part of sugar. They are then cut 

 on a freezing microtome (of which Cathcart's is a good ex- 

 ample) and placed for a few hours in a bowl of water so that 



1 Ordinary commercial methylated spirit has wood naphtha added to it 

 to discourage its being used as a beverage. The naphtha being insoluble 

 in water a milky fluid results from the dilution of the spirit. By law 

 chemists can only sell 8 ounces of pure spirit at a time. Most pathological 

 laboratories are, however, licensed by the Excise to buy pure spirit in large 

 quantities. 



