OF MICKOSCOPIC OBJECTS. 11 



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 Strieker 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. 



