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 
laming, 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 Hchinodermata, 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, &., will 
present no difficulty to the student; but he must remember 
