186 BACTERIOLOGICAL DIAGNOSIS. 
for themselves. It happens to every medical man to 
find it necessary to send tumours, &c. to a laboratory to 
obtain a pathological diagnosis; and in very many 
cases the materials are treated in a way which abso- 
lutely prevents good sections from being obtained. 
Many fixing fluids are in use, and any of them may 
be selected, but it is absolutely necessary that the 
material to be investigated should be cut into small 
pieces and put into a large bulk of the fluid at once. 
This is especially necessary in the case of material 
removed at a post-mortem examination, where the 
tissues and organs have already undergone alteration. 
As regards the size of the slices which are to be 
placed in the hardening fluid, it is sufficient to say that 
they should never exceed } inch in thickness, and if 
perchloride of mercury 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 
wis not advisable to place two blocks in the same vessel. 
The fluids which we shall recommend for this purpose 
are i— 
1. Perchloride of mercury in normal saline solution. 
This is prepared by dissolving common salt in water 
in the proportion of seven grammes to a litre (about 34 
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 
