WITH THE MICROSCOPE. 75 



without difficulty. Another advantage is, that by the addition of a 

 little water all the original characters of the tissues are restored. 



Further observations upon rendering tissues which are more or 

 less opaque, transparent, will be found in part III. 



141. Of Examining and Preserving Specimens in the Dry Way. 

 Any specimen examined, or preserved permanently, as a dry object 

 in air, must be protected from dust by being covered with thin glass, 

 and the pressure of the latter upon the specimen must be prevented 

 by the interposition of small pieces of cardboard at the edges of the 

 thin glass, slightly thicker than the specimen itself. Objects may be 

 mounted in the dry way in many of the cells described in 114 131 ; 

 but a simple cell made of wood or cardboard is sufficient for all prac- 

 tical purposes. The round vulcanised India-rubber rings cemented 

 to the glass slides make capital cells for mounting such preparations. 



The thin glass cover must be attached by a little very thick gum 

 or by a paste made of gum and flour or chalk. 



Among unorganised substances, there are many objects which 

 may be mounted or preserved with advantage in air. Many crystal- 

 line bodies found native, and some crystals derived from the organic 

 and inorganic kingdoms artificially prepared, may be examined or 

 preserved permanently in air. Many of these present very beautiful, 

 appearances. Arsenious acid, common salt, benzoic acid, uric acid, 

 crystals of the vegetable alkaloids, such as salicine and many crystal- 

 line salts, bone, teeth, hair, horn, the scales of butterflies and other 

 insects, are examples of structures which may be examined in 

 air and mounted dry. The general structure of many vegetable 

 preparations may be shown in this simple manner. The petals of 

 many flowers, different forms of vegetable cellular and vascular 

 tissue, the epidermis, hairs, and other parts of plants, the seeds and 

 seed vessels, spiral fibres, the stones of fruits, sections of wood, of 

 the pith from the stem of various plants, pollen, the spores of ferns, 

 mosses, and fungi, are examples of vegetable preparations which may 

 be examined and preserved in air. 



Thick objects preserved in the dry way are examined by reflected 

 light only, but very thin dry tissues, like the epidermis from different 

 parts of plants, may be examined by reflected or by transmitted light. 



142. Examination of Substances in Fluids. In choosing a fluid in 

 which the specimen is to be immersed, its chemical composition, its 

 transparency and its refractive power must be considered. The dif- 

 ferent preservative solutions described in pp. 53 59, may be used for 

 the preservation of a variety of objects in fluid. If we wish for a fluid 

 closely resembling water, but possessing the property of preserving the 

 specimen, we may use the solution of naphtha and creosote, 102, or 



