OF MICROSCOPIC OBJECTS. 1t 
Bichloride of mercury acts, in hardening tissues (like 
most of the preceding, probably), by combining and forming 
an insoluble compound with their albuminoid elements. It 
is not much employed for this purpose, but is principally 
of use in certain preservative solutions mentioned elsewhere 
in these pages. 
Tannic acid forms insoluble compounds with a great 
variety of organic and especially animal substances, as solu- 
tions of starch and gelatine, solid muscular fibre and skin, 
&c., which then acquire the power of resisting putrefac- 
tion. It scarcely colours animal membrane. Dr. Beale 
says that its action upon red blood corpuscles is “ very 
peculiar.” The solution used is three grains to an ounce of 
water. Other uses of tannin (tannic acid) will be found 
elsewhere in this work, and the intelligent student will easily 
thence infer its action and properties. 
Drying may be effected either in a current of warm dry 
air, or under a bell-glass over sulphuric acid, or over a layer 
of parched oatmeal; or a cheap form of water bath may be 
employed, such as will be found described in this work. 
Another very speedy method is to soak the specimen in 
strong alcohol for a sufficient time, remove it, and expose to 
a current of warm dry air. 
Boiling.—Tissues may be hardened by boiling in a fluid 
consisting of 8 parts water, 1 part creosote, and 1 part 
vinegar, for two or three minutes. They may then be laid 
out to dry. After two or three days they acquire a firm- 
ness admirably adapted for section; but if they remain too 
long uncut they become of a consistence unfit for that pur- 
pose. On the whole, boiling is not to be recommended, 
though Stricker says that it has its occasional uses. 
Freezing may be employed for otherwise unmanageable 
structures, such as brain, spinal cord, &c. (though there 
seems to be an objection of a theoretical kind to this use of 
it, viz., that it may injure or alter the cells), or other tissues 
which will not admit the use of chromic acid, or which it may 
be.desired to view under other aspects. 
