184 BACTERIOLOGICAL DIAGNOSIS. 
tissues hayden them at the same time; this is necessary, 
for fresh tissue would yield before the sharpest knife 
and could not be cut into thin sections. These processes 
are always carried out, no matter what method of 
section cutting is to be adopted. 
In cutting sections it is necessary that the material 
should be sufficiently fivm and homogeneous in consistency. 
The former is secured to some extent by the process of 
hardening, but a properly hardened block is rarely firm 
enough to permit of its being cut into sections without 
further preparation. Further, it almost invariably hap- 
pens that some parts of the material are firmer or 
harder than others; and if such a substance were cut 
the harder parts might be sufficiently firm whilst the 
softer parts would simply crumble before the knife. 
There are two methods of overcoming this difficulty— 
freezing and embedding. 
The freezing process is very simple, and it is one which 
can easily be carried out at home. The sections which 
it yields are usually quite sufficient for purposes of 
histological research (the diagnosis of tumours, &c.), 
but they are rarely sufficiently thin for a proper demons- 
tration of the bacteria which they may contain. The 
sections are cut more easily by the freezing than by the 
paraffin process, but they are decidedly more difficult 
to manipulate. 
In the freezing process the block or tissue after fixing 
and hardening is dipped, or better soaked for some 
hours, in a thick solution of gum arabic. It is then 
placed on the plate of a microtome and frozen until the 
tissue assumes the consistency of fairly hard cheese and 
can be cut into thin sections. 
The embedding process should be called the infiltration 
process; the tissue to be cut is infiltrated throughout 
