512 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



actuates a vertical spindle, /, which rotates a horizontal cast-iron 

 plate at m for polishing. 



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 struct vires (such 

 as the true corallines), these salts must be dissolved away by the 

 action of some acid, such as 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 jthe 



FIG. 412. Treadle machine for cutting hard sections. 



lime is in the state of carbonate (as, for example, in the skeletons of 

 echinodenns), 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 



