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IODINE FOR HYDROGEN IN ORGANIC COMPOUNDS, &c. 51 
conic acid. The solution is instantly decolorized, and the new acid is deposited 
in the form of fine delicate plates, which are so abundant as to render the fiuid 
almost semisolid. The only precautions necessary are to avoid the use of a hot 
solution of the acid, and an excess of the chloride, as the product, under such 
circumstances, becomes coloured, owing to the occurrence of a further decompo- 
sition, to be afterwards referred to. The change which first takes place is re- 
presented by the equation :— 
C,H, 0, + ICl = C,, H, 10, + HCL. 
The crystals of iodopyromeconic thus precipitated are in a short time filtered off 
and washed with cold water; they are then finally dissolved in boiling alcohol, 
from which they again deposit themselves, upon cooling, in perfectly colourless 
plates, having a high degree of lustre. The acid is sparingly soluble in cold 
water; but at a boiling heat it dissolves more readily, and crystallizes again from 
the solution in long, slender needles, possessing a slightly acid reaction. Acids and 
alkalies increase its solubility in water, but it is easily decomposed, if boiled with 
strong caustic potash. It is also decomposed by concentrated nitric acid, with 
the separation of free iodine. It gives a yellowish-white precipitate with nitrate 
of silver, soluble in ammonia, and with perchloride of iron it produces a deep 
purple colour, but no precipitate. It suffers no loss of weight at 212°, but heated 
to a higher temperature, it first fuses to a black fluid, and is then suddenly decom- 
posed, with the evolution of a large quantity of iodine. 
The combustion of iodopyromeconic acid was attended with some difficulty, 
for it was found that not only the acid itself, but even its lead salt permitted 
the iodine to escape in the free state, when burned either with chromate of lead, 
or with a mixture of oxide of copper and litharge. This would have been of 
little moment in determining the constitution of a substance such as iodopyro- 
meconic acid, where the mode of its formation sufficiently indicates its com- 
position, and the determination of the carbon and iodine would have been quite 
sufficient to fix its formula ; but having observed the same peculiarity in another 
substance afterwards to be described, in which the exact determination of the 
hydrogen was essential to the establishment of its formula, I was compelled to 
devise some method by which the iodine might be retained, and the following is 
that which I found most successful. 
The substance to be analysed was mixed with chromate of lead, and a small 
quantity of fused litharge reduced to a fine powder; the mixture was then intro- 
duced into a long combustion tube, held with its point downwards, and at the same 
time there were dropped into it small pieces of metallic lead, which remained at 
the under-side of the tube, and so arranged as to be about three inches apart. 
After the whole of the mixture had been introduced, and the remaining space 
in the interior of the tube filled up with chromate of lead, the point was turned 
