DESILICIFICATION 5 1 3 



used. In the case of small bones, or delicate portions of large 

 (such as the cochlea of the ear), a ^ per cent, solution of chromic 

 acid will itself serve as the solvent ; but larger masses require either 

 nitric or hydrochloric acid in addition, to the extent of 2 per cent, 

 of the former or 5 per cent, of the latter. By some the chromic and 

 the nitric or hydrochloric acid are mixed-in in the first instance, 

 while by others it is recommended that the bone should lie first in 

 the chromic acid solution for a week or ten days, and that the 

 second acid should be then added. If the softening be not com- 

 pleted in a month, more acid must be added. When thoroughly 

 decalcified, the bone should be transferred to rectified spirit ; and 

 it may then be either sliced in the microtome' or torn into shreds 

 for the demonstration of its lamelbe. Acid solvents may also be 

 employed in removing the outer parts of calcareous skeletons, for the 

 display of their internal cavities (a plan which the Author has often 

 found very useful in the study of Foraminifera), or for getting rid of 

 them entirely, so as to bring into complete view any ' internal cast ' 

 which may have been formed by the silicification of its originally 

 soft contents. It has been in this mode, even more than by the 

 cutting of thin sections, that the structure of Eozoon canadense has 

 been elucidated by Professor Dawson and the Author. For the first 

 of these purposes strong acid should be applied (under the dissecting 

 microscope) with a fine camel's-hair pencil ; and another such pencil 

 charged with water should be at hand, to enable the observer to stop 

 the solvent action whenever he thinks it has been carried far enough. 

 For the second it is better that the acid should only be strong enough 

 for the slow solution of the shelly substance, as the too rapid disen- 

 gagement of bubbles often produces displacement of delicate parts 

 of the substituted mineral ; whilst, if the acid be too strong, the 

 ' internal cast ' may be altogether dissolved away. 



Busch suggests nitric acid as the best of all agents for decalcifica- 

 tion, insomuch as it does not cause ' swelling up,' nor injuriously 

 attack the tissue elements. 



One volume of chemically pure nitric acid of specific gravity 1'25 

 diluted with ten volumes of water may be employed for large and 

 tough bones ; but it may be diluted to 1 per cent, for young bones. 



The method given is that fresh bones should be laid in alcohol 

 of 95 per cent, for three days ; they must then be placed in the 

 nitric acid, which must be changed daily for eight days. They must 

 not remain after the decalcification is complete, or they will become 

 yellow. On removal the bones must be washed for a couple of hours 

 in running water and placed again in 95 per cent, alcohol, and in a 

 few days placed again in fresh alcohol. 



Desilicification. It is desirable to be able to remove siliceous as 

 well as calcareous elements from objects. To do this a glass vessel 

 should be carefully coated with paraffin internally, to prevent the 

 action of the acid used taking place on the sides of the vessel. The 

 subject to be cleared of its silica is placed in alcohol in the coated 

 vessel, and hydrofluoric acid is added drop by drop. As the mucous 

 membranes are fiercely attacked by this acid, great care must be 

 exercised in its use ; but small sponges and other similar siliceous 



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