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 * 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. 
1.768 
1.740 
1.658 
1.639 
1.621 
1.606 
1.605 
1.588 
1.561 
1.554 
1.544 
Liquid. 
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 light with 
Rock-Forming Minerals in Thin Sections, pp. 16-18 (1908). 
