Special Methods 127 



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, driven at a great speed and 

 constantly cooled by a stream of water, gradually cuts through 

 the specimen. 



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 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 brown or white shellac used by 

 painters; heat the object and press the polished surface very firmly 

 into the melted shellac. As soon as the slide and object are cool, the 

 next cut can be made. 



4. Anyone who can handle tools should soon be able to cut a 

 section 1 mm. thick. A skilled technician can cut sections as thin 

 as 0.5 mm. 



5. The second grinding must be very careful and accurate. Do 

 the polishing on the revolving disk. The glass slide allows one to 

 note how the process is progressing. 



6. When the section becomes thin enough, or even before if it 

 begins to crack, wash off the powder. If the slide has been damaged 

 and the section is holding together, the shellac may be dissolved 

 with absolute alcohol, thus freeing the section, which may now be 

 mounted on another slide. 



