j6z CLINIcXl bacteriology ANft HiE'MATOLOGY 



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 proportion of 

 7 grammes to a litre (about 3| grains to the ounce), and saturating 

 this solution whilst hot with perchloride 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 penetration 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 subsequently), 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 (o'8 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 

 blocks should be cut small and placed at once in methylated 

 spirit diluted with an equal quantity of water. 



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

 tubercle bacillus, as it prevents the decolorizing action of the acid. 



