58 PREPARATION AND MOUNTING 



must be watched carefully. Often, however, this is an advantage, 

 as where the structure alone is wanted, the removal of the colour- 

 ing matter renders it more transparent. There are objects, how- 

 ever, which retain the air with such tenacity that soaking alone 

 will not remove it. If these will bear heat without being injured, 

 they may be boiled in turpentine, or even in balsam, when the air 

 will be partly or totally expelled. But where heat is objection- 

 able, they must be immersed in the turpentine, and so submitted 

 to the action of the air-pump. Even with this aid, sometimes 

 days are required to accomplish it perfectly, during which time 

 the air should be exhausted at intervals of five or six hours, if con- 

 venient, and the objects turned over now and then. 



Sometimes the objects are so minute that it is impossible to sub- 

 mit them to any soaking, and in this case they must be laid upon 

 the slide at once, and the turpentine applied to them there. But 

 it must not be forgotten that there are some few which are much 

 better mounted in such a way that the balsam may thoroughly 

 surround, and yet not penetrate, the substance more than necessary. 

 Sections of teeth are amongst these, which will be noticed in 

 another place, and some insects (see Dr. Carpenter) when required 

 to show the " ramifications of the trachea?." 



Having freed the object, then, from these two enemies damp- 

 ness and air we must proceed to mount it. 



The slide must first be cleaned ; then on the centre a quantity 

 of balsam must be placed with a bluntly-pointed glass rod, accord- 

 ing to the size of the object about to be mounted. To this a 

 slight heat must be applied, which will cause any bubbles to rise 

 from the surface of the slide, so that they may be readily removed 

 with a needle. The object should be freed from all air by steep- 

 ing in turpentine, as before described, and then from superfluous 

 liquid by a short drainage, and carefully laid upon, or where it is 

 practicable thrust into, the balsam, prepared on the slide as above. 

 In the former case, or where the balsam has not totally covered 

 the object, a small quantity must be taken, warmed, and dropped 

 upon it, and any bubbles removed by the needle as before. To 

 cover this, the thin glass must be warmed, and beginning at one 

 side, allowed to fall upon the balsam, driving a small " wave" be- 

 fore it, and thus expelling any bubbles which may remain. This 



