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 fluid 

 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 10 H 4 O 6 + ICl = C 10 H 3 IO 6 + HC1. 



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 percbloride 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 cbromate 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 



