OF MICROSCOPIC OBJECTS. 79 



There is one class of objects for the polariscope which differs 

 in preparation from liny we have yet considered, and affords very 

 beautiful specimens. Some of the plants, including many of the 

 grasses and the Equisetaceae (i.e. horsetails), 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 perform- 

 ing 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 preparation. The vegetable should be immersed 

 in strong nitric acid and boiled for a short time ; an effervescence 

 will go on as the alkalies are being removed, 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 dried, burn it upon thin glass until all appears white, 

 when it must be carefully mounted in balsam. I think, however, 

 it is better to leave it in strong acid until all the substance, except 

 the required portion, is removed ; but this will take a length of 

 time, varying according to the mass, &c., of the plant. Of course, 

 when this latter method is used, the skeleton must be washed 

 from the acid, &c., before being mounted in balsam. 



These siliceous cuticles are readily found. The straws of most 

 of the cereals, wheat, oat, &c. ; the husks, also, of some of these ; 

 many canes ; the equisetum, as before described ; and some of 

 the grasses. Many of these are everywhere procurable, so that 

 the student can never want material for a splendid object for the 

 polariscope. 



In Chapter II. the scales (or hairs) which are often found upon 

 the leaves of plants were mentioned as beautiful objects when 

 mounted dry ; but some of these when detached from the leaf 

 which is easily done by gently scraping it, when dried, with a 

 knife present brilliant starlike and other forms, if mounted in 

 balsam and used with the polariscope. There is a little danger, 

 when placing the thin glass upon the balsam, of forcing out the 

 scales in the wave of matter which is always ejected ; this may be 

 overcome by applying to the slide, previously to placing the ob- 

 jects upon it, an extremely thin covering of the balsam diluted 



