720 PALEOBOTANY 



to the plane from which the section is desired. Immerse this 

 surface for a time in hydrochloric acid (coal balls and other 

 calcareous masses) to dissolve away the carbonates. Find the 

 concentration of the acid and the duration of the etching process 

 by trial. Wash with water and flood the etched surface with a 

 liquid {e.g. the trade preparation Durofix) which forms a tough 

 film on drying. When dry peel off the film, which will carry with 

 it a thin section of the fossil. Wash in acid and in water, dry 

 clear and mount in Canada balsam under a coverslijD. The 

 thickness of the section depends upon the time of action of and 

 the strength of the acid used in etching. Virtually serial sections 

 may be obtained. For silicified material, hydrofluoric acid replaces 

 the hydrochloric acid used for etching. 



Koopman's {Geol. Bur. Netherlands, Heerlen, cxxxi-cxxxii, 

 1929) uses a celluloid solution. He moistens the surface with 

 amyl acetate to avoid air bubbles. Barnes and Duerden {New 

 Phyt., xxix, 1930, p. 74) remove bubbles from the drying film by 

 pouring ether fumes on them and polish the lower surface of the 

 film with fine knife powder. The transfer is washed and well 

 smeared with Mayer's albumen. A slide is warmed and the 

 transfer is well pressed on to the slide ; flattening and adhesion 

 is aided by a few drops of absolute alcohol. The slide is then placed 

 nearly vertically in equal parts of absolute alcohol and ether to 

 remove the celluloid. 



Duerden {Ann. Bot., xlv, 1931, p. 376) adds a plasticiser 

 (castor oil, triacetin, or benzyl abietate) to the cellulose acetate 

 or cellulose nitrate (pyroxylin) solution, to avoid puckering of 

 the sections due to unequal drying. The following mixtures are 

 good : (1) Cellulose nitrate in equal parts of alcohol and ether, 

 plus 5 per cent, of castor oil ; (2) cellulose nitrate in 2 parts of 

 acetone and 1 part of amyl acetate, plus 2 per cent, triacetin ; 

 (3) cellulose acetate in 4 parts of acetone and 1 part of diacetone 

 alcohol, plus 1 per cent, each of benzyl abietate and triacetin. A 

 mixture of the solvents is poured on the surface of the rock before 

 pouring the film. The film mixtures should be of the consistency 

 of pure glycerin. 



Graham {Stain Tech., viii, 1933, p. 65) finds a most satisfactory 

 film is given by 20-second nitrocellulose 20 grm., butylacetate 

 200 C.C., tricresyl phosphate or methyl phthalate about 1 c.c, 

 toluene or xylol 10 to 20 c.c. It dries in four to six hours, but is 

 best left one to two days. 



See also Krick, Bot. Gaz., xciii, 1932, p. 153. 

 1417. Impressions and Incrustations. Walton's Canada 

 Balsam Transfer Method {Ann. Bot., xxxvii, 1923, p. 379). Clean 

 the visible surface of the material and set it on a glass slide with 

 hard to brittle balsam. Imbed the mount in paraffin, leaving only 

 the remnants of the excess of rock matrix exposed. Dissolve these 



