When dissolved in water or in solutions of neutral salts, it communicates to the fluid a yellowish-brown or reddish-brown colour, which is destroyed by sulphuretted hydrogen, because the iodine is converted into hydriodic acid. In the colourless fluid thus formed, if treated with chlorine,—or in the original brown fluid without chlorine,—a solution of starch, obtained by ebullition and subsequently cooled, produces a fine blue colour and precipitate; and these, if the solution be sufficiently diluted, disappear on boiling, reappear on sudden cooling, and are removed permanently by a stream of sulphuretted hydrogen. This is a very delicate and characteristic system of tests. The best mode of using chlorine for decomposing hydriodic acid is to let it descend in the gaseous form from the mouth of a bottle of nitro-hydrochloric acid upon the fluid to be examined; In this way an excess is easily avoided, which bleaches out the blue colour. Sulphuric acid, though often recommended for the purpose, does not act unless it contains nitrous acid,—from which however the sulphuric acid of commerce is seldom quite free. When mingled with organic substances, the discovery of it is a matter of some nicety; because many substances of this nature, especially in the living body, quickly convert it into hydriodic acid.[330] Hence few cases can occur in medico-legal practice, where iodine will be discoverable in its free state. The following method of analysis will meet all possible cases. _Process for Compound Mixtures._—Add water if necessary, and filter. If either the fluid or solid part is little or not at all coloured, test it with cold solution of starch, assisting the action of the test on the solid part by trituration in a mortar. If a blue colour be struck, which disappears under ebullition, and reappears under refrigeration alone, or on subsequently allowing chlorine gas to descend on the surface of the fluid, there can be no doubt of the existence of iodine.—If the colour of the suspected mixture after filtration is so deep that the action of the starch cannot be expected to yield characteristic appearances, then both the solid and fluid parts should be agitated with a third of their volume of ether; and after the ethereal solution has arisen to the surface, it is to be removed and tested with solution of starch. The blue colour will be now perhaps struck, because the ether, in carrying off the iodine from the mixture, leaves many coloured organic principles behind. Should free iodine not be thus detected, strong presumptive evidence may still be procured of its actual presence, or of its having been at one time present, by continuing the examination with the view to detect hydriodic acid. This is described in p. 159.