APPENDIX II 185 



is essential, to cut thin sections transversely across and longi- 

 tudinally through the axis of the specimen and to grind them 

 down till they are so transparent that they can be studied 

 through the microscope. The cutting can be done on a lapi- 

 dary's wheel, where a revolving metal disc set with 'diamond 

 powder acts as a knife. The comparatively thin slice thus 

 obtained is fastened on to glass by means of hard Canada 

 balsam, and rubbed down with carborundum powder till it is 

 thin enough. 



The process, however, is very slow, and an amateur cannot 

 get good results without spending a large amount of time and 

 patience over the work which would be better spent over the 

 study of the plant structures themselves. Therefore it is usually 

 more economical to send specimens to be cut by a professional, 

 if they are good enough to be worth cutting at all, though it 

 is often advisable to cut through an unpromising block to see 

 whether its preservation is such as would justify the expense. 



In the case of true " coal balls " much can be seen on the cut 

 surface of a block, particularly if it be washed for a minute in 

 dilute hydrochloric acid and then in water, and then dried 

 thoroughly. The acid acts on the carbonates of which the 

 stone is largely composed, and the treatment accentuates the 

 black-and-white contrast in the petrified tissues (see fig. 10). 

 After lying about for a few months the sharpness of the surface 

 gets rubbed off, as the acid eats it into very delicate irregularities 

 which break and form a smearing powder; but in such a case 

 all that is needed to bring back the original perfection of 

 definition is a quick wash of dilute acid and water. If the 

 specimens are not rubbed at all the surface is practically per- 

 manent. Blocks so treated reveal a remarkable amount of 

 detail when examined with a strong hand lens, and form very 

 valuable museum specimens. 



The microscope slides should be covered with glass slips (as 

 they would naturally be if purchased), and studied under the 

 microscope as sections of living plants would be. 



Microscopic slides of fossils make excellent museum speci- 

 mens when mounted as transparencies against a window or 

 strong light, when a magnifying glass will reveal all but the 

 last minutiae of their structure. 



4. Labelling and numbering of specimens is very important, 

 even if the collection be but a small one. As well as the paper 

 label giving full details, there should be a reference number on 

 every specimen itself. On the microscope slides this can be cut 

 with a diamond pencil, and on the stones sealing wax dissolved 



