10 BULLETIN 97, U. S. DEPARTMENT OF AGRICULTURE. 
In the triclinic system the arrangement of colors in the inter- 
ference figure is unsymmetrical. 
In actual determinative work it will seldom be found necessary to 
determine all of the foregoing optical properties of crystals. But in 
doubtful cases, the more data available the better; and even in seem- 
ingly clear cut cases it is sometimes advisable to make as thorough 
a study of the substance as possible. The methods described herein 
are, in general, those of the simplest and most accessible nature, but 
with their aid and a little practice anyone should soon be able to 
determine many of the materials ordinarily used in the manufacture 
of fertilizers. However, it should be remarked that problems contin- 
ually arise for the solution of which v/ide experience and a compre- 
hensive knowledge of all theoretical questions involved are none too 
much. 
OPTICAL CONSTANTS OF FERTILIZER MATERIALS. 
The diagnostic optical constants of the substances mentioned on 
page 2 were taken mainly from Dana and Groth 1 and are as follows: 
Potassium chloride. Sylvite. KCl. — Isometric. Habit cubic. 
Cleavage cubic perfect. Refractive index, 1.4903. Sometimes ex- 
hibits anomalous double refraction. 
Potassium sulphate. Arcanite. K 2 S0 4 . — Orthorhombic. 
Twinning yields pseudohexagonal forms. 
Optically positive ( + ). Ax. pi. || a. Bxl c. 
Dispersion feeble, p>\ oil, p< v in air. 
Refractive indices, a= 1.4920, 0=1.4935, j -1.4970. 
2V = 66°30', 2E = 109° 57'. 
Ammonium sulphate. Mascagnite. (NH i ) 2 S0 4: . — Orthorhombic. 
Pseudohexagonal. Optically positive ( + ). Ax. pi. || b. Bx JL a. 
Dispersion weak, ,o< v. 2E = S7° 44', 84° 6'. 2V = 52° 12'. Refrac- 
tive indices, a =1.5209, £=1.5230, y =1.5330. 
Kainite. MgSO±.KCl+3H 2 0. — Monoclinic. Optically negative 
( — ). Ax. pi. || b. Dispersion inclined, very distinct. 2V = 84° 33'. 
2E = 141° approx. 
Sodium nitrate. — Rhombohedral. Uniaxial. Optically negative 
( — ). Double refraction strong. Refractive indices for yellow, w = 
1.587, £=1.336. 
Calcium nitrate. Tetraliydrate. Ca{N0 2 ) 2 .J { .E 2 0. Monoclinic. Op- 
tically negative ( — ) . Optic angle small. Double refraction high. 
Apatite. (Ca(F,Cl)) Ca 4 (P0 4 ) 3 . — Hexagonal. Uniaxial. Opti- 
cally negative ( — ). Double refraction weak. Refractive indices 
very close to 1.63. 
J Dana, System of Mineralogy. Groth, Chemische Krystallographie. 
