THE CUTTING OF SECTIONS. 99 



chloride solution. Dissolve the sublimate in the salt 

 solution by heat. The separation of crystals on cooling 

 shows that the solution is saturated. For small pieces of 

 tissue \ 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 are then placed 

 for twenty-four hours in each of the following strengths of 

 methylated spirit (free from naphtha 1 ): 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 after- 

 wards 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 their size) in a thick syrupy solution containing two parts 

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



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 patho- 

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

 in large quantities. 



