THE VARIATION OF ANGLES OBSERVED IN CRYSTALS. 
515 
(3.) Sodium Nitrate. 
The preceding observations relate to isotropic substances; it appeared advisable to 
apply the same method to some substance not belonging to the cubic system and 
possessing strong double refraction. Sodium nitrate 
lends itself admirably to this purpose and both 
cleaves and crystallises readily in the form of 
rhombohedra of 73° 30'. The refractive index of a 
saturated solution of sodium nitrate was deter¬ 
mined above to be about 1 •38905 at 19° (mean of 
1-38850, 1-38913, 1-38927, 1-38930). With a prism 
of 73° 30' this would give for a cleavage rhomb of 
sodium nitrate in contact with a saturated solution 
an angle of total reflection of 28° 57' and an angle of 
emergence of 
14° 
15f', taking o) = 1-58739, and 
T1 lis leads 
using only the ordinary index (see fig. 20). 
A cleavage crystal having an angle of 73° 35' 
between two very good faces was immersed in 
saturated solution and gave an angle of emergence of d = — 14° 11'. 
to the value 1-39085 for the refractive index of the licpiid. 
Another cleavage fragment gave 
a = 73° 34', 6> = - 14° 1,71-', /r = 1-38954 at 17^°. 
A third cleavage fragment immersed in a saturated solution kept at about the 
same temperature by surrounding the trough with beakers of ice gave 
a = 73° 33', d = — 14° 151', p = 1-38908. 
A fourth, for which the solution was cooled down to 10° by the same method, gave 
a = 73° 35', d=-14°12', p= 1-39067, ^ = 16°. 
A fifth gave 
o 
a 
= 73° 35', e = 
14° 131', 
p = 1-39042, 
t = 17^. 
a 
Finally, then, we have for the refractive index of the liquid in contact with 
growing crystal of sodium nitrate (mean of 1-38954 and 1-39042), p=: 1-38998 
at 17^°. 
Interpreting these results by the curve of Diagram IV. we arrive at the conclusion 
that at 17|-° C ;— 
(1) A saturated solution of .sodium nitrate contains about 47-65 grammes of the 
sub.stance in 100 grammes of the .solution ; 
(2) The lici[uid in contact with a growing crystal contains about 48-8 grammes 
of the substance in 100 grammes of the solution. 
3 u 2 
