OF MICROSCOPIC OBJECTS. ' 103 
laid upon it should be warmed and its surface covered with 
a thin layer of balsam, and then lowered gradually upon 
it, when no bubbles should be imprisoned. - A narrow 
piece of card-board at each end of the object, for the 
cover to rest upon, prevents any danger of crushing the 
specimen. 
This mode of mounting polypidoms, &c., seems to give 
almost the complete beauty of the fresh specimens. They 
are very beautiful objects when viewed with common light, 
but much more so when the polarizer is used (in the manner 
described a little farther on). 
To the above instructions there can be little to add; but 
I may here mention that some young students may not be 
possessed of the air-pump, and on this account put aside all 
search for those, specimens which need little looking for at 
the seaside. Many of these, however, though they lose some 
- beauty by the ordinary mode of drying, will, by steeping for 
some time in turpentine, not only be freed from the air- 
bubbles, but suffer so little contraction that they are a 
worthy addition to the cabinet. 
Another class of objects is the spicula met with in 
sponges, &c. These are often glass-like in appearance and 
of various shapes; many are found resembling needles 
(whence their name); some from the synapia are anchor- 
like, whilst others are star-like and of complex and almost 
indescribable combinations. As some of these are composed 
of silex and are consequently not injured by the use of nitric 
acid, the animal substance may be removed by boiling them 
in it. Those, however, which are calcareous must be treated 
with a strong solution of potash instead; but whichever 
way is used, of course they must afterwards be freed from 
every trace of residue by careful washing. 
These spicules may be often found amongst the sand 
which generally accumulates at the bottom of the jars in 
which sponges are kept by those who deal in them, and 
must be picked out with a camel-hair pencil. The speci- 
mens obtained by this means will seldom if ever require 
