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 orer 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 

 in 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 Equisetacese (i.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 



