FIXING MATERIAL FOR CUTTING 215 



the hardening fluid, it is sufficient to say that they should 

 never exceed J inch in thickness, and if perchloride of mer- 

 cury is used should be even thinner. The other dimensions 

 of the block are of less importance, 



The bulk of the fluid in which the block is placed should 

 be at least twenty times that of the block, and it is not 

 advisable to place two blocks in the same vessel. 



The fluids which we shall recommend for this purpose are : 



1. Perchloride of mercury in normal saline solution. This 

 is prepared by dissolving* common salt in water in the pro- 

 portion of 9 grammes to a litre (about 4 grains to the 

 ounce), and saturating this solution whilst hot with per- 

 chloride of mercury. The solution must be allowed to cool 

 completely; as it does so, crystals of the mercury salt will 

 separate out. 



This fluid fixes completely in twenty-four hours, or less, 

 and gives most excellent results. Its powers of penetra- 

 tion are not very great, so that slices of tissue which are to 

 be fixed in it should be thin. 



The after-treatment of the blocks fixed in this fluid must 

 be described briefly. They are allowed to remain in the 

 solution for twenty-four hours and no longer, and are then 

 washed for twenty-four hours in running water to remove 

 the perchloride of mercury. They are then passed through 

 the various strengths of spirit (as will be described subse- 

 quently), a little tincture of iodine being added to each to 

 remove any mercury which may still remain. The other 

 steps are the same as those which are used if other methods 

 of fixation have been adopted. 



2. Formalin. This should be used in a 5 per cent, solution 

 in normal saline solution (0*9 per cent.). It yields very good 

 results, and is perhaps the fluid which can be most warmly 

 recommended to a practitioner who is going- to send his 

 material to a public laboratory.* The fluid has very great 

 powers of penetration, and the slices may be much thicker 

 than we have recommended. The one objection to the fluid 

 is that it interferes somewhat with the way in which the 

 sections stain. 



3. Alcohol is a very good fixing fluid. When it is used the 



* Formalin should not be used for tissues which are to be searched for the 

 tubercle bacillus, as it interferes with the decolorizing action of the acid. 



