MICROSCOPIC OBJECTS. 91 



Agate. — This form of silica is often found imperfectly 

 crystallized, and thin plates (3^0 th of an inch thick), when 

 viewed by polarized light, exhibit a rich motley colour- 

 ing. In some specimens a tessellated structure is seen. 

 The wavy lines which give the pattern are very fine and 

 close together. 



bismuth crystallized — requires a very low power to 

 show the regular rectangular form of its crystals. The 

 colour of the surface when oxidated increases the interest. 

 — Power 20 to 30 diameters. 



Borax crystallized in Glacial Phosphoric Acid. — These 

 crystals arrange themselves in circular masses or dodeca- 

 hedrons composed of needle-shaped crystals radiating 

 from the centre. These clusters exhibit the black cross 

 and coloured rings when submitted to polarized hght. — 

 See Microscopic Illustrations, 3rd edition, page 227. 



Carbonate of Lime. — Small spherules of this substance 

 are sometimes found in the urinary deposits of the horse. 

 These deposits are often composed of concentric layers ; 

 at other times the fibres are radial. Their diameter is 

 about -0^0^^ °^ ^^ inch. There are several varieties, which 

 should be numbered 1, 2, 3, &c. Illuminated by polar- 

 ized light and a power of 100 diameters, they are splen- 

 did objects. 



Crystallization of Salts as transjmi'ent objects by common 

 light. — Independently of the beautiful geometrical forms 

 assumed by different salts during their crystallization, a 

 vast variety of elegant configurations may be obtained 

 by mixing small quantities of the different solutions in a 



K 



