204 Mr. J. F. Brown on a Gerio'al Method of Substituting 



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 yel- 

 lowish-white precipitate with nitrate of silver, soluble in am- 

 monia, 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 decomposed, 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 mix- 

 ture of oxide of copper and litharge. This would have been of 

 little moment in determining the constitution of a substance 

 such as iodopyromeconic acid, where the mode of its formation 

 sufficiently indicates its composition, 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 deter- 

 mination 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 introduced into a long combus- 

 tion 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 3 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 

 upwards, and by slight tapping a passage opened throughout 

 the whole length of the tube, while the pieces of lead projecting 

 above the surface melted on the application of heat, and by ex- 

 posing a metallic surface during the time combustion was going 

 on, served to retain all the iodine. 



The results of analysis are as follows: — The hydrogen of No. 1 

 was not weighed, as, from its combustion being effected in the 

 ordinary way without lead, the small end of the chloride of 



