206 Mr. J. F. Brown on a General Method of Substituting 



ammonia. As thus prepared, it is apt to carry down an excess 

 of oxide of lead, which is easily removed by warm acetic acid. 

 It is of sparing solubility in alcohol or water, and becomes 

 highly electrical when rubbed. This salt, contrary to expecta- 

 tion, evolves free iodine, like the acid itself, when burned with 

 chromate of lead ; but the phsenomenon is probably owing to the 

 decomposition of the salt taking place at a temperature much 

 lower than that sufficient to enable the iodine to combine with 

 lead. For this reason I contented myself with a determination 

 of its oxide of lead, which was effected by igniting the salt, after 

 the addition of a few drops of concentrated sulphuric acid : — 

 6*15 grains substance, dried in the air, gave 2-76 grains sul- 

 phate of lead, corresponding to the formula PbO, C^^ H* 10^ 

 as shown by the subjoined calculation : — 



I have already mentioned, that when the quantity of chloride 

 of iodine employed, is larger than is requisite for the production 

 of iodopyromeconic acid, the fluid acquires a yellow colour, due 

 to the presence of another compound, of very remarkable cha- 

 racters, produced by a further decomposition of pyromeconic 

 acid, and to which I give the name of iodomecone. When 

 potash is gradually added to the fluid, after separation of the 

 iodopyromeconic acid, a blackish precipitate immediately falls, 

 which rapidly dissolves on agitation of the fluid, while a peculiar 

 odour is evolved. After the addition of the potash has been 

 continued for some time, a point is reached at which the pre- 

 cipitate assumes a lighter colour, is no longer dissolved, and is 

 not increased by further addition of the alkali. The precipitate 

 is then filtered from the alkaline fluid, washed with cold water, 

 and purified by repeated crystallization from boiling alcohol. 

 By subsequent experiments, I ascertained that it was easy to 

 convert pyromeconic acid entirely into this compound, by adding 

 a large quantity of chloride of iodine, when the crystals of iodo- 

 pyromeconic acid at first formed rapidly disappeared, and car- 

 bonic acid was evolved. Exactly similar effects are produced by 

 bromide of iodine. 



Iodomecone is obtained in large hexagonal plates of a bright 



