142 PREPARATION AND MOUNTING 



acid is diluted with twenty times its volume of water, and 

 in this the shell is immersed. After a period, differing ac- 

 cording to the thickness of the shell, the carbonate of lime 

 will be dissolved away, and a peculiar membrane left, show- 

 ing the structure of the shell very perfectly. This may be 

 mounted dry, in balsam, or sometimes in liquid, according 

 to the appearance of the object ; but no rule can be given. 

 The discretion of the student, however, will enable him to 

 choose the most suitable method. 



From some shells it is easy to divide thin plates, or 

 laminae, which require nothing but mounting in Canada 

 balsam to show the texture very well. In working, however, 

 with those which are pearly, it will be found that expe- 

 rience and patience are needed, as they are very brittle and 

 peculiarly hard ; but a little practice will overcome these 

 difficulties. 



Amongst the Echinodermata, which include the star- 

 fishes, sea-hedgehogs, &c., there are many whose outer sur- 

 face is covered with spines, or thin projections. Some 

 of these are sharp and thorn-like, others blunt, longer or 

 shorter, and, indeed, of endless variety. In many of these, 

 when a section is made, rings are seen which have a common 

 centre, with radiating supports, resembling sections of some 

 of the woods. These are very beautiful objects, and methods 

 of procuring them may now be considered. It is the best 

 to cut as thin a section as can safely be got with the watch- 

 spring saw first, when the smooth sharpening stone may be 

 used to polish one side, which is easily accomplished with 

 water only. When this is effected, it must be washed clean, 

 and thoroughly dried, and then may be united to the slide 

 in the same manner as before recommended for the Orbito- 

 lite, &c. If it is ever necessary to displace it on account 

 of inequalities, bubbles, or other remediable fault, this may 

 be done by warming the slide ; though too much heat must 

 be avoided, otherwise fresh bubbles will certainly be pro- 

 duced. The covering with thin glass, balsam, &c., will 

 present no difficulty to the student; but he must remember 



