IDENTIFICATION OF FERTILIZER MATERIALS. 3 



The accessories axe a quartz wedge, a quarter undulation mica 

 plate, a gypsum plate, a Bertrand lens, a Bertrand ocular, an eyepiece 

 micrometer, mainly, with others, a description of which, with their 

 uses, will be found in the standard texts. 



In examining substances with a view to obtaining their optical 

 constants a series of oils of definite refractive indices are used as 

 embedding mediums. Johannsen l gives the following series, which 

 contains oils suitable for practically all purposes: 



Refractive indices of various liquids used as embedding mediums. 



Liquid. 



Carbon bisulphide 



Iodo methylene 



a — Monobrom naphthalene 

 a — Monochlor naphthalene 



Mono-iodo benzene 



Cassia oil at 21° 



Cinnamon oil 



Bromoform 



Monobrom benzene 



Nitrobenzene 



Clove oil 



Index. 



Liquid. 



1.768 

 1.740 

 1.658 

 1.639 

 1.621 

 1.606 

 1.605 

 1. 58S 

 1.561 

 1. 554 

 1.544 



Ethylene bromide. . . 

 Monochlor benzene.. 



Cedar oil 



Benzene 



Xylene 



Beechnut oil 



Almond oil 



Carbon tetrachloride 



Glycerine 



Amyl alcohol 



Index. 



1.536 

 1.527 

 1.516 

 1.501 

 1.495 

 1.477 

 1.469 

 1.466 

 1.460 

 1. 4075 



Various mixtures of almond oil and cassia oil can be made with 

 refractive indices running from 1.474 to 1.562. 



The indices of these liquids vary somewhat on standing, and they 

 should be checked up to at least the second decimal place at occasional 

 periods. Any good refractometer will serve the purpose, or mineral 

 grains of known refractive indices may be used as will be presently 

 explained. 



The substance to be examined is mounted on a slide in one of the 

 oils, preferably one of medium index, and covered with a cover glass. 

 It is then ready for observation. If the indices of the liquid and the 

 substance are fairly different the outlines of the particles show up 

 distinctly, and the crystal habit, cleavage angles, color, and so forth, 

 should be noted. Crystallographic and cleavage angles may be 

 approximately measured by aid of the cross hairs and the rotating 

 stage. Efforts should be made to determine whether the color of 

 colored grains is that of the substance itself or of inclusions. High- 

 power lenses will usually accomplish this. However, the majority 

 of the salts are colorless, and consequently color is only rarely of 

 importance. These preliminary observations made, the optic proper- 

 ties of the substance are determined as outlined below. 



ISOTROPIC SUBSTANCES. 



All substances are either amorphous or crystalline ; and all crystal- 

 line substances are either isotropic or anisotropic, i. e., they transmit 

 light with equal velocity in all directions or they transmit fight with 



i Rock-Forming Minerals in Thin Sections, pp. 16-18 (1908). 



