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REAGENTS. 
Fixing and Killing Agents are more especially em- 
ployed in the study of the organized cell -contents, 
where by their use the life-processes of the cell are 
brought to a sudden termination, the object being to 
fix the contents in a condition approaching as nearly 
as possible the normal living state. In order to carry 
out this operation successfully, the living specimen 
must be placed in the fixing or killing agent as soon 
as collected, and if the specimen is large it should be 
cut into small pieces. The following are some of the 
common fixing agents: Chromic acid in 0'5 to 1 per 
cent, aqueous solution ; osmic acid in 1 to 2 per cent, 
aqueous solution; Flemming’s Mixture, which is an 
aqueous solution of chromic acid (0'25 per cent.) con- 
taining 01 per cent, of osmic acid and OT per cent, of 
acetic acid; picric acid in concentrated aqueous or 
alcoholic solution; picric -sulphuric acid, a concen- 
trated aqueous solution of picric acid containing 2 per 
cent, by volume of sulphuric acid ; and mercuric 
chloride (corrosive sublimate) used in OT to 1 per cent, 
aqueous or alcoholic solution. 
Hardening or Dehydrating Agents are those substances 
which are employed for the purpose of hardening the 
specimen so as to facilitate sectioning and for removing 
the water, which would interfere with its examination. 
Alcohol is to be regarded as the principal hardening 
or dehydrating agent, and considerable care is neces- 
sary in its use; the specimen is treated successively 
with alcoholic solutions of gradually increasing 
strength, beginning with a 35 per cent, solution, in 
which the specimen is kept for twenty-four hours; 
then it is placed in 50 per cent, alcohol for from six to 
twenty-four hours, and then in 70 per cent, alcohol in 
which it may be kept until ready for use. In order to 
avoid shrinking of the material at this stage, it may 
