MR. TALBOT ON THE OPTICAL PHENOMENA OF CRYSTALS. 
33 
first place to polarize the light, and tourmaline or calcareous spar is used to analyse 
it : so that the analogy or rather identity of effect with the tourmaline is complete. 
I will now mention some other crystals which possess the analysing property, but 
not in such a high degree. 
2. Boracic acid. — If dissolved in boiling water, it yields in cooling irregular cry- 
stals which have considerable analytic power. A crystal which in one position is so 
translucent as to be hardly distinguishable from the water in which it floats, is in the 
transverse position very strongly defined. It does not become dark all over, but 
only in its outline. 
If now we employ it to analyse the tints of sulphate of lime, its outline becomes 
beautifully coloured. Nothing can exceed the delicacy of colouring which a number 
of these crystals exhibit when viewed together : those which lie in one direction ap- 
pearing, for instance, green ; those in a transverse direction red. The appearance is 
very unlike any other optical phenomenon that I know of, in consequence of two 
colours being seen in strong contrast, and without any intermediate tints ; and also 
from the outline only of the objects being coloured, while their interior remains with- 
out colour. It is only when the crystals have a fibrous or striated structure that the 
tint extends over all their surface. 
The boracic acid has the same analytic property and precisely the same appearance 
when it crystallizes from a solution of borax in phosphoric acid. The plumose cry- 
stals of it (No. 4. supra) are very delicately coloured with the two opposite tints. 
I obtained a very beautiful result by placing a drop of phosphoric acid upon a 
group of circular crystals. This caused a fresh deposition of boracic acid upon them 
as nuclei, which assumed the form of very delicate cilia, spreading in all directions 
as from a centre. These fringed circles showed the analytic property in an admirable 
manner, exhibiting four quadrants coloured alternately with complementary colours 
of great vivacity. 
3. Another instance which is worthy of mention is the oxalate af potash and chro- 
mium, a salt whose optical properties have been investigated by Sir David Brewster*. 
If some gum arabic is added to a solution of this salt, and a drop of it put between 
two plates of glass, it abandons its usual mode of crystallization for another, which 
resembles a microscopic vegetation composed of minute prisms growing one out of 
another, and variously arranged in sprigs and branchlets ; while in other places it 
assumes an undulating capillary form, much resembling in miniature the tufts or 
locks of a species of conferva which is seen growing in pools of water or in the sea. 
Now these objects are possessed of a high analytic power, insomuch that when a 
plate of sulphate of lime is placed beneath them they assume a colour of great inten- 
sity and splendour, which is changed for the complementary tint when the polariza- 
tion of the incident ray is reversed. 
4. Nitre , — If nitre and gum arabic are dissolved together in hot water, a drop of 
* Philosophical Transactions for 1835, p. 91. 
MDCCCXXXVII. 
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