6o 
POISONS : THEIR EFFECTS AND DETECTION. 
TABLE I.— continued . 
Index of 
Refraction. 
Name. 
Approximate 
Boibng -point. 
Approximate 
Density. 
147 
Olive oil . 
0-91 
148 
Castor oil. 
• . 
0-96 
149 
Xylene .... 
136° 
0-86 
149 
Benzene .... 
80° 
0-88 
1-50 
Clove oil . 
• • 
105 
1-51 
Cedarwood oil . 
• • 
0-98 
1-52 
Mono chlorbenzene . 
132° 
104 
1-55 
Nitrobenzene . 
209° 
1-20 
1-56 
Monobromobenzene . 
155° 
149 
1-57 
Orthotoluidine . 
197° 
100 
1-58 
Monobromo phenol . 
195° 
• • 
1-58 
Bromoform 
149° 
2-83 
1-61 
Quinaldin 
240° 
105 
1-62 
Monoiodo benzene 
187° 
1-83 
1-625 
Carbon bisulphide 
46° 
1-29 
1-63 
Alpha-monochlornaphtha- 
lene. 
255° 
L50 
1-65 
Alpha-mono bro mnaptha- 
lene. 
277° 
1-50 
1-76 
Methylene iodide 
180° 
3-34 
1-95 
Phenyl sulphide 
272° 
{ 
142 
In order to apply this method to crystals, the crystal is immersed 
in one or other of the liquids given in Table I. ; it is easily seen whether 
the crystal has a higher or lower refraction than the liquid chosen, 
and it is equally easy to make a mixture by trial approximating to 
the true refraction. Obviously, the crystal must not be soluble in the 
liquid ; this limits the choice with organic substances somewhat; but 
if a suitable liquid is found and a particular alkaloid known or sus¬ 
pected, which alkaloid is more or less soluble in the liquid, a saturated 
solution or solutions of the alkaloid can be made, its refraction care¬ 
fully determined, and then used. 
Crystals are divided into isotropic and anisotropic. In isotropic 
crystals, such as the alums, common salt, nitrates of barium and stron¬ 
tium, and silver chloride, the light rays, no matter in what direction the 
rays are sent, are equal in velocity ; hence the refractive index found is 
the true index. On the other hand, anisotropic substances have either 
two or three indices of refraction ; the alkaloids, e.g., have at least two 
different values according to the position in which the crystals lie on 
the stage of the microscope. 
The uniaxial crystals belonging to the tetragonal and sexagonal 
systems have two refractive indices. Biaxial crystals belonging to the 
orthorhombic, monoclinic, and triclinic systems have three indices. 
In uniaxial crystals, the index of refraction ascertained by light 
vibrations transmitted transversly to the optical (vertical) axis is 
