REAGENTS 37 
equal parts of the | and absolute, and for the T V, equal parts of the £ 
and absolute. The J can be guessed at with sufficient accuracy. 
Some prefer a closer series of xylols, using 2J, 5, 7|, 10, 15, 20, 
30, 40, 50, 75, and 100 per cent. It is claimed that infiltration with 
paraffin is more thorough after this series. About one hour in each 
grade is enough for filamentous algae and fungi, fern prothallia, and 
similar objects. An hour and a half is enough for onion root-tips. 
For quarter-inch cubes, try 3 hours. For larger pieces the times 
should be longer. In all cases, the pure xylol should be changed 
two or three times. While the pure xylol must not be used again for 
this purpose, it is still good for dissolving paraffin ribbons when 
staining on the slide. 
Xylol is the best agent for clearing sections just before mounting 
in balsam. Preparations cleared in xylol harden more rapidly, 
and this is such a decided advantage that even when sections have 
been cleared in cedar oil or clove oil it is worth while to give them a 
minute or two in xylol before mounting. Besides, clove oil is a solvent 
of many of the most frequently used stains and, consequently, 
preparations in such stains would fade, if transferred directly from 
clove oil to balsam. 
Xylol evaporates so rapidly that one must take care not to let 
sections become dry before applying the balsam. Thin sections 
perfectly dehydrated will clear in a few seconds; but, even with very 
thin sections, it is better to let the xylol act for at least a minute. 
Sections 20 /z in thickness should remain in the xylol 2 to 5 minutes 
before mounting in balsam. If there is much moisture in the air, 
or if the absolute alcohol is not above suspicion, clear sections in 
clove oil before transferring to xylol. 
Chloroform. — Some botanists use chloroform to precede the 
infiltration with paraffin. In the later stages of infiltration it is more 
easily removed than xylol. It seems to possess no other advantages, 
and for clearing sections just before mounting in balsam it is inferior 
to xylol or clove oil. Its value in hardening celloidin and as a fixing 
agent entitles it to a place in the histological laboratory. 
Cedar Oil.—It is not always easy to get good cedar oil. If the 
stuff offered for sale looks like turpentine and smells like it, it is 
worthless for histological purposes. Good cedar oil has a slightly 
amber tint, the color resembling a weak clove oil. It should have 
the pleasant odor of cedar wood. The very expensive cedar oil 
