24 
which would, by the chromic acid, be rendered solid, and so give 
rise to a false appearance in the section. Next we must consider 
why the hardening should not be continued in spirit, since so the 
process would be easier and much more rapidly completed. The 
reasons are not numerous, perhaps, but they are powerful. Spirit 
will harden sufficiently and much more rapidly, but it interferes 
with the shape of the individual cells of the tissue, acting differently 
upon different cell contents ; while chromic acid simply permanently 
fixes whatever the tissue contains. Chromic acid, moreover, itself 
stains the tissue, so that very good specimens are obtained by at 
once mounting sections in glycerine, which cannot be the case with 
spirit, for the tissue having been in part hardened by abstraction 
of water, when placed in glycerine becomes in some parts at once 
unduly distended, especially the connectie tissue, and in others too 
transparent for observation ; so that chromic acid seems to have 
weighty advantages over spirit for hardening animal tissues. But 
when all the elements of the tissue have been permanently fixed by 
it, a return to spirit is very successful. The substance becomes 
harder, more easy to cut, more easy to manipulate, and stains more 
readily. So that for converting a soft animal tissue, of which in its 
fresh state it is impossible to cut fine sections, into a tough, hard 
block, from which the finest shavings can be cut, we have only to 
place it first for 24 hours in equal parts of spirit and water, then 
for about a fortnight in 1 per cent. solution of chromic acid, 
changed three or four times, and finally into methylated spirit for a 
few days. And this is the method I should certainly recommend 
in all ordinary cases. There is, however, another method fre- 
quently of great service, especially for cutting sections of carmine 
gelatine injections, and that is freezing. The method is very 
simple. It is only necessary to make an ordinary freezing mixture 
with ice and salt, and to throw the substance to be cut into it for 
a few minutes, when it becomes as hard as a board, and yet very 
easy to cut. These are the only methods of hardening that I use, 
except when it is not convenient to freeze carmine injections, when 
spirit must be used instead, for chromic acid destroys the colour ; 
therefore I will pass on to the consideration of the next stage, that  . 
of cutting the sections. This is to be done either with a section- 
eutting machine, or a razor. Some very good machines have lately 
been constructed in which sections may be cut of hardened tissues 
embedded in paraffin, or fresh ones frozen; but for ordinary pur- 
poses a good razor answers admirably. Some difference of opinion 
exists as to the form of razor to be used. One ground flat on its 
under-side is generally recommended, but I think this to be a 
mistake, for with a little practice you will be able to rely on the 
steadiness of your hand for cutting an even section, when a razor 
ground hollow on both sides is infinitely superior on account of the 
beautiful edge which can always be so easily produced upon it. 
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