INTRODUCTORY HISTOLOGICAL STUDY 
23 
This, like any efficient killing and fixing reagent, preserves the 
cells without either shrinking or swelling the parts, and holds the 
various cell structures in as nearly as possible the same relation¬ 
ships that they possessed in life. From this fixing reagent the 
pieces must be transferred without undue handling to 50% alcohol 1 
which should be changed after an hour or two to 70% alcohol. 
Several changes of 70% alcohol are desirable before the material 
is finally left in this or a somewhat higher grade of alcohol (80%) 
until further use is to be made of it. 
Decalcifying. —In case the material contains any calcarious 
deposit (as, for example, would be the case with bone) it is neces¬ 
sary that this calcarious matter be removed. A reagent com¬ 
monly used for this purpose is a saturate aqueous solution of 
picric acid to which a sufficient amount of nitric acid is added to 
make about 1%. In order to transfer the material to this solu¬ 
tion it is necessary to gradually replace the 70% alcohol in which 
it is preserved, by water, and for this purpose the material is 
transferred successively to weaker grades of alcohol (55%, 40%, 
20%) and finally to water. 
As a general rule to insure the desired replacement of one fluid 
by another, the.material should be left in each new fluid for from 
a half hour to an hour for each millimeter of the shortest dimen¬ 
sion of the object.. It should always be kept in mind that water 
and grades of alcohol below 70% tend to injure tissues by soften¬ 
ing them and thus prolonged exposure to these fluids should 
always be avoided. 
From water the material is placed in the decalcifying fluid 
where it should remain for 24 hours or longer. 
It must then have the excess of picric acid removed by wash¬ 
ing in several changes of water, after which it is passed through 
the successive grades of alcohol until 70% alcohol is reached. 
1 To mix any grade of alcohol from any higher grade: Take as many units of the 
higher grade as are equal to the percentage of the desired lower grade, and add to it 
enough water to make of the mixture as many units as are equal to the percentage 
of the higher grade. For example, to mix 70% alcohol from 95%, take 70 c.c. of 
the 95% alcohol and add 25 c.c. of water. 95% alcohol (i.e., commercial alcohol) 
is the grade from which the lower grades are usually obtained; 100% alcohol 
is very expensive and hence must always be used sparingly , and never used to mix 
with water for the lower grades of alcohol. 
