OF MICROSCOPIC OBJECTS. 155 
into the pith, and when this is dry thin sections could be 
cut and the gum washed out again, and these could be 
mounted in balsam. 
The plan adopted by most practical histologists for cut- 
ting sections of soft tissues is as follows :—The tissue to be 
cut is first hardened by immersion in a chromic acid solu- 
tion varying in strength from 0°25 to 2 per cent., or by im- 
mersion in alcohol. The substance to be cut may then be 
embedded in melted wax and spermaceti, in proportions 
suitable to the nature of the substance to be cut; when 
this is cold the section may be cut with a razor ground 
flat on one side, and may then be floated off in spirits of 
wine. 
These sections mount very well in Canada balsam, if 
after being removed from the spirit they are immersed in 
oil of cloves till they become clear, then put into turpentine 
before the balsam. The thinness of the section will depend 
very much on the dexterity of the operator, but section- 
cutting instruments for soft tissues can now be obtained at 
most scientific instrument shops. 
When sections of softer substances are required, no instru- 
ment can be compared with “ Valentin’s knife,” which con- 
‘sists of two steel blades lying parallel with each other and 
attached at the lower end. The distance of separation may 
be regulated at will by a small screw near the handle. 
When, therefore, a section is wanted, the substance must 
be cut through, and betwixt the blades a thin strip will be 
found, which may be made of any thickness, according to 
the distance of their separation. By loosening the screw 
the blades may be extended, and the section may be floated 
out in water if the damp will not injure it. The knife cuts 
much better if dipped in water or glycerine immediately be- 
fore use, and also when the substance to be operated upon 
is wet, or even under water altogether; but care must be 
taken, after use, to clean the blades thoroughly and oil them 
before laying by, if the place is at all damp. This instru- 
ment is most useful in such subjects as anatomical prepara- 
