COLLECTING AND PREPARING. 817 



on one side are also to be cemented to the glass for grinding the 

 other side. The glass should be about an inch square and perhaps 

 a quarter of an inch in thickness. For cementing the specimens 

 the following mixture has been found most satisfactory : Slowly 

 melt together in an evaporating dish or casserole 16 parts by weight 

 of viscous Canada balsam and 50 parts of common (orange) 

 shellac. Keep the mixture heated for some time, and allow to 

 cool slowly. Before it is quite cool, draw the viscous mass out into 

 strings, and roll between the hands into rods half an inch in 

 diameter and of convenient length. This will keep indefinitely. 



Before cementing the specimen to the glass slide heat it on an 

 iron or brass stand over a bunsen burner or an alcohol lamp, in 

 order to drive off all the moisture. Heat the glass plate at the 

 same time, and melt a small amount of the cement by holding the 

 stick upon the hot glass ; or better melt a small piece of the cement 

 in a separate dish, and spread a thin coat of the molten cement over 

 the gently heated glass plate. Then lay the specimen with the 

 polished side on the cement, and heat gently, taking care that no 

 bubbles are forming. Allow to cool under pressure. When thor- 

 oughly cool and hard, grind down the other side, on a grindstone 

 or emery wheel first, and later on a glass plate with fine emery and 

 finally pumice powder. Care must be exercised that the section 

 becomes of uniform thickness, and that the rubbing is stopped 

 when the section shows signs of breaking. 



When the section is complete, gently warm an ordinary object 

 glass, preferably of the shorter type used by petrographers. This 

 should first be thoroughly cleaned. Put a drop of Canada balsam 

 on the warm glass and allow it to spread, but not to boil. At the 

 same time heat the old glass until the cement is thoroughly softened, 

 and the thin section floats in it ; then slide, or transfer the section 

 with a mounted needle, to the new slide put another drop of balsam 

 on the section, heat gently and press a clean warm cover glass upon 

 it. Allow to cool under slight pressure. When cool remove the 

 superfluous balsam with a warm knife and alcohol, or with benzine. 

 Label the slide at once. 



In making sections of stromatoporoids, care must be taken that 

 the longitudinal sections are parallel to the vertical or radial 

 elements (see Vol. I, p. 35), and the transverse sections parallel as 

 52 



