PREPARING SECTIONS OF SHELLS, ETC. 333 



stones that are tough, such as the cherry and plum, can be 

 easily made thin ; others that are more brittle will demand 

 some care in their preparation, whilst some few, as, for in- 

 stance, the ivory nut, are so hard as even to require the aid of 

 the cutting-machine or the lapidary's wheel, for their reduction 

 to a proper degree of thinness. The method generally em- 

 ployed to make sections of these hard tissues for the micro- 

 scope is very similar to that of bone before described, viz., to 

 cut as thin a slice as possible with the saw, then to reduce 

 this nearly to the requisite thinness by the file, and finish it 

 with the hones ; as all these tissues are more or less of a dark 

 colour, they will be best displayed in balsam, therefore the 

 process of poUshing on the buff leather with putty powder 

 may be dispensed with. The development of some of the hard 

 tissues may be very well seen in the scales of the cone of firs ; 

 these may be readily cut in the machine employed to make 

 sections of wood presently to be described, and may be 

 mounted in balsam in the usual manner. Another form of 

 hard tissue may be procured by maceration from the pear 

 tribe ; this is known to botanists as gritty tissue, and should be 

 mounted in fluid, as the balsam makes it too transparent. 



To prepare Siliceous Skeletons of Vegetables. — In all plants 

 known as grasses, silica or flint is more or less abundant ; its 

 presence may be recognised in many ways, but heat and nitric 

 acid are the agents generally .employed to separate it from 

 the other less durable substances with which it is intimately 

 connected. SiHca forms a coating to the stems of grasses ; it 

 is even found in small masses or concretions in the joints of 

 the bamboo, and is then known by the name of tahasheer. 

 The attention of microscopists was first directed to the siliceous 

 skeletons of certain parts of grasses by the Rev. J. B. Keade, 

 in the year 1835 ; the specimens first examined by him con- 

 sisted of the husks and parts of the stem of the wheat and 

 oat ; they were prepared by subjecting these parts to a very 

 high temperature in a platinum crucible, whereby all the 

 carbonaceous matter was burnt ofi^, and an ash of silica was 

 left ; this was removed and mounted in Canada balsam, when 

 a perfect cast even of the most minute vegetable structure in 



