158 METHODS IN PLANT HISTOLOGY 



9. Wash the film in 5 per cent hydrochloric acid to remove any excess 

 mineral, then wash gently in water. 



10. Dry in blotting paper under pressure, otherwise the peel curls badly. 



11. The entire film, or only desirable portions of it, may be cleared in 

 xylol and mounted in balsam. 



After every peel, polish the surface before making another. - 



The following variations, used by various investigators, are worth 

 trying: After the peel has been taken off, cleaned, and washed (stage 9 

 of Dr. Reed's schedule), instead of drying and using blotting pnper, 

 put the peel into 50, 70, 85, and 95 per cent alcohol, about 15 minutes 

 in each. If the celloidin appears to soften in 95 per cent alcohol, short- 

 en the time. Transfer to Eycleshymer's clearing fluid (which does not 

 cause such curling as the xylol), and mount in balsam. 



Silicified material is treated in the same way, except that hydro- 

 fluoric acid (10 per cent in water) is used instead of hydrochloric and 

 is allowed to act longer, about 3 or 6 minutes. 



Gelatin peels. — In Nature, of March 15, 1930, John Walton de- 

 scribes peels of gelatin. The rock surface is prepared and etched with 

 acid, as in case of celloidin peels, and allowed to dry. Then a hot 

 solution of jelly, containing formalin and glycerin, is poured on the 

 surface. The amount of the mixture poured on will depend upon the 

 size of the object and the desired thickness of the peel. To cover a sur- 

 face 1 square decimeter in area, use about 2 g. of the pure gelatin used 

 in making bacteriological cultures, 50 c.c. water, 0.5 c.c. glycerin, and 

 0.5 c.c. formalin. The surface must be surrounded before the etching 

 process with a rim of plasticene, or some other substance, and should 

 be leveled with a spirit level. The water and glycerin are mixed, heat- 

 ed, and the jelly stirred until dissolved. Heat to 60°-80° C, stir in 

 the formalin quickly, and immediately pour the mixture over the sur- 

 face of the petrifaction. Let the jelly set and remove to a warm well- 

 aired place to dry. Avoid dust. When the jelly is dry, peel it off. It 

 may be lizard to start the peel but, once started, it comes off rather 

 easily. Clear in xylol and mount in balsam. 



It used to be claimed that in petrifactions all organic matter has 

 been replaced by the mineral. The fact that a peel can be made proves 

 that the old claim is incorrect. The mineral matter is etched down by 

 the acid, the organic matter remaining and coming off in the peel. 



