134 
METHODS IN PLANT HISTOLOGY 
4. With the saw, make another cut, as close to the glass as possible, 
so as to leave a thin section firmly fastened to the glass. 
5. Grind and polish until the section is as thin as possible, or as thin as 
you want it. 
6. Wash all polishing powder off with water. 
7. Dry completely and, either with or without moistening in xylol, mount 
in balsam. 
A word of suggestion in regard to these various points may not 
be amiss. 
1. Most rock-sections are cut with a rather expensive and quite 
complicated instrument, called a “petrotome.” The saw is of the 
circular type, is made of tin or other soft metal, has no teeth, but has 
diamond dust driven into the margin. A rigid clamp holds the 
object, and the saw, constantly cooled by a stream of water, gradually 
cuts through the specimen. If the piece to be cut is more than 5 
or 6 cm. in diameter, a band saw is better; and if the piece is 10 or 
20 cm: in diameter, the band saw is necessary. 
2. The cut surface is most easily polished on a revolving brass 
plate, kept wet and liberally powdered with fine carborundum. 
When the surface has become even and smooth, the specimen is 
ready for the next step. 
3. Fasten the polished surface to the glass slide upon which the 
section is to be mounted. Plate glass 3 or 4 mm. thick is best for 
sections larger than 3 or 4 mm. square. Gradually heat the slide 
until it is quite hot. Melt upon the slide the thin flakes of white 
shellac used by painters; heat the object and press the polished 
surface very firmly into the melted shellac. Canada balsam, from 
which the xylol has been driven off by heating, can be used instead 
of shellac. Much of the Paleozoic material is in the form of coal 
balls. After the ball has been cut in two, it is often difficult to hold 
the hemispherical piece in a clamp, especially if the piece is small. 
In such cases, it is better to fasten the polished surface to a convenient 
piece of marble, about 2.5 cm. thick, and 5 or 6 cm. square. The 
marble is easily held in the clamp. As soon as the slide, or marble, 
and object are cool, the next cut can be made. 
4. Fasten the object in the clamp and saw as close to the glass, 
or marble, as possible, thus leaving a thin section cemented to the 
slide or marble. If marble has been used, the section is removed 
by heating or by dissolving it off with xylol. It can then be fastened 
