SECTION CUTTING. 187 
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 water. It yields very good results, and is 
perhaps the fluid which can be most warmly recom- 
mended 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 not a very good fixing fluid, as it tends to 
cause a good deal of shrinkage. When it is used the 
blocks should be cut small and placed at once in undi- 
luted methylated spirit. 
SECTION CUTTING BY THE FREEZING 
METHOD. 
Sections which are prepared by the freezing method 
are rarely as thin as those prepared by one or other of 
* Formalin should not be used for tissues which are to be searched 
for the tubercle bacillus, as it prevents the decolorising action of the 
acid. 
