SECTION CUTTING. 125 
corrosive sublimate, but the foregoing answer all practical 
purposes. 
When there are bone or lime salts in the tissues, chromic acid 
is the most useful reagent. It serves at the same time to 
harden the soft portions and to decalcify and thus soften the 
hard. ‘The object must be placed in a large quantity of the 
fluid of a greater strength (one to two per cent.) and the 
acid should be frequently changed until all lime salts have 
disappeared. Of course, with this increase in strength of acid 
and the length of time of immersion, one runs a risk of the 
other portion becoming brittle, but this cannot well be 
avoided. In some cases it is necessary to use dilute hydro- 
chloric acid in place of the chromic. ‘This should rarely be 
used of greater strength than one percent. After the sub- 
stance is thoroughly decalcified it is transferred to alcohol. 
Frequently specimens contain such a large amount of pig- 
ment matter as to render the thinnest section opaque and to 
utterly obscure all cell limits. In such cases it is necessary 
to immerse the tissues in 25 per cent. nitric acid, and to watch 
closely until the color disappears. This usually takes some 
hours, and the sections cut from such material are not very 
satisfactory but are the best that can be obtained. 
The process of embedding comes next in order. The sub- 
stances used are many, the most common being pure par- 
affine, a mixture of paraffine and oil or tallow, wax and tallow, 
transparent soap, gum arabic, and glycerine jelly. With most 
substances paraffine, without the admixture of anything, gives 
the best results, though many advise the addition of a fourth 
to a half of paraffine oil, or lard, or tallow, to render it softer. 
