on the volatile and fixed Oils, 211 
their chemical agencies *. As oil of turpentine separates iodine 
from its solution in water, and in cases when acids anda num- 
ber of metallic salts are present, it may, in many instances, af- 
ford a useful test to detect the presence of iodine, or be employed 
as a means of separating it in a fluid form from other substances 
with which it may exist in solution. The nitrates of silver and 
mercury seem to offer the best means of detecting and separating 
iodine from its solution in turpentine ; the iode of silver is of a 
paler and duller yellow colour than that of mercury. Polished 
silver, which Sir H. Davy found to be one of the best tests of 
the presence of iodine in compounds dissolved in water t, does 
not furnish satisfactory indications of its presence in turpentine, 
especially when it exists only in minute quantity. Except in cases 
when the fixed alkalies and ammonia are present in excess, starch 
seems in general to be a very delicate and unexceptionable test 
of the presence of iodine ; but when added to a solution of iodine 
in turpentine, it merely acquires a yellow tint. The addition of 
starch to a solution of iodine in water, alcohol, &c. occasions, as 
is well known, the immediate formation of the purple compound 
of starch and iodine. But if starch in its common state of dry- 
ness be pulverized and mixed with iodine in sinall’ proportion, 
a very peculiar effect will take place, which I have not seen any 
where noticed. The mixture, at first, is of a grayish colour; but 
in a little time it acquires a faint purple tint, which gradually be- 
comes deeper and deeper, till it appears almost black. These 
changes are probably connected with the absorption of moisture 
from the atmosphere ; for if water be added to the above mixture, 
the purple compound will be directly produced. The agency of 
water or moisture seems to be necessary to the formation of the 
purple compound of iodine and starch, as may, I think, be de- 
duced from the following experiments : I put some iodine into a 
small tube, and nearly filled it with starch in powder, which had 
been well dried: no apparent effect took place; the tube was 
gently heated so as to raise the iodine in vapour, and the starch 
was agitated. The same process was again repeated, but the 
starch merely assumed a light-brown colour. On exposing it to 
the atmosphere it slowly acquired a purple tint, and when moist- 
ened with water, or placed on wet paper, it immediately became 
of a bright purple colour. 
Royal Cork Institution, March 11, 1822. 
* Phil. Trans. 1814, 
T Abide { 
Dd2 XLVII. On 
