334 MANIPULATION. 



flint was found on microscopical examination. One of the 

 most beautiful specimens for exhibiting the arrangement of 

 silica in its stem is an Equisetum, sold in the oil and colour 

 shops under the name of the Dutch rush ; it is used by the 

 cabinet-makers as a substitute for sand or glass paper, for 

 rubbing down the inequalities in the surface of wood ; this is 

 best prepared by cutting the stems into short pieces, and 

 boiling them in strong nitric acid in a tall vessel; copious 

 fumes of gas wiU be given off as the carbon is being removed, 

 the vessel should then be laid aside for a time, and more acid 

 added when the effervescence has ceased ; if the specimen 

 be not immediately wanted, it may be kept in the acid 

 uutU the perfect removal of all the other constituents has 

 been effected. A portion of this plant, when well prepared, 

 should be perfectly free from all foreign matter, and after being 

 thoroughly washed, may be mounted either in fluid, in balsam, 

 or even dry. In balsam it forms a beautiful object for polarized 

 light, but in fluid, its true nature is best exhibited. The 

 palecB or bracts of a grass, known as the Festuca pratensis, 

 exhibit a beautiful arrangement of silica without any pre- 

 paration by acid ; they can be shown dry as opaque objects, 

 and for the purpose may be cemented to one of the discs 

 described at page 319; or, if prepared, may be mounted in 

 fluid, and then examined by transmitted light. But the 

 palesB of the wheat and oat, which are known as chaff, 

 from being more opaque and less abounding in silica, will 

 require either the aid of acid or of heat for its exhibition. 



