PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 201 



specimen will be fit for mounting 'dry' after having been carefully 

 cleansed from any adhering particles of putty-powder. 



197. Decalcification. When it is desired to examine the structure of 

 the Organic matrix, in which the Calcareous salts are deposited that give 

 hardness to many Animal and to a few Vegetable structures (such as the 

 true Corallines), these salts must be dissolved away by the action of some 

 Mineral Acid, which may be either Nitric or Hydrochloric. This should 

 be employed in a very dilute state, in order that it may make as little 

 change as possible in the soft tissue it leaves behind. When the Lime is 

 in the state of Carbonate (as, for example, in the skeletons of Echino- 

 derms, Chap, xiv.), the body to be decalcified should be placed in a 

 glass jar or wide-mouthed bottle holding from 4 to 6 oz. of water, and 

 the acid should be added drop by drop, until the disengagement of air- 

 bubbles shows that it is taking effect; and the solvent process should be 

 allowed to take place very gradually, more acid being added as required. 

 When, on the other hand, much of the lime is in the state of Phosphate, 

 as in Bones and Teeth, the strength of the acid solvent must be increased; 

 and for the hardening of the softer parts of the organic matrix, it is 

 desirable that Chromic acid should be used. In the case of small bones, 

 or delicate portions of large (such as the cochlea of the ear), a half 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 muriatic acid are mixed 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 is not completed in a 

 month, jnore 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 

 lamellae. 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 Fora- 

 minifera)', or for getting rid of them entirely, so as to bring into com- 

 plete view any 'internal cast' which may have been formed by the 

 silicification of its originally soft contents (Figs. 332, 337). It has been 

 in this mode, even more than by the cutting of thin sections, that the 

 structure of Eozoon Canadense (Plate xvn. ) has been elucidated by Pro- 

 fessor 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 disengagement 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. 



198. Preparation of Vegetable Substances. Little preparation is 

 required, beyond steeping for a short time in distilled water to get rid of 

 saline or other impurities, for mounting in preservative media specimens 

 of the minuter forms of Vegetable life, or portions of the larger kinds of 

 Alga, Fungi, or other succulent Cryptogams. But the Woody struc- 

 tures of Phanerogams are often so consolidated by gummy, resinous, or 



