120 PREPARATION AND MOUNTING 
portion as possible of the balsam diluted with turpentine, 
as before mentioned, be applied, they will cling to the glass 
and allow the pure balsam to flow readily over them with- 
out being so liable to imprison air-bubbles when the thin 
glass is put upon them. 
The raphides, which were fully described in Chapter III., 
when required for use with polarized light, must be mounted 
in balsam, and many are found which give-beautiful colours. 
They require no peculiar treatment, but must be washed 
quite clean before putting up. But in order to understand ° 
anything of the natural arrangement of raphides, it is 
necessary to mount certain parts of plants with these objects 
im situ. The most common is the coating of the onion, 
which must be soaked some time in turpentine or benzole, 
in order to render it transparent, and must then be mounted 
in balsam, as before said. We shall then be able to obtain 
such colours by the aid of polarized light, that the raphides 
are shown in wonderful distinctness, and somewhat of their 
nature will be perceived. 
There is one class of objects for the polariscope which 
differs in preparation from any we have yet considered, and 
affords very beautiful specimens. Some of the plants, in- 
cluding many of the grasses and the Equisetacez (7.e. horse- 
tails), contain so large a quantity of silica, that when the 
vegetable and other perishable parts are removed, a skeleton 
of wonderful perfection remains. This skeleton must be 
mounted in balsam, the method of performing which will 
now be considered. 
Sometimes the cuticle of the equisetum is removed from 
the plant, others dry the stem under pressure, whilst the 
grasses, of course, require no such preparation. They should 
then be immersed in strong nitric acid and boiled for a 
short time; an effervescence will go on as the organic matter 
is decomposed, and when this has ceased, more acid should 
be added. At this point the modes of treatment differ; 
some remove the object from the acid and wash, and having 
