1815.] On lodhie. 107 



Concentrated sulphuric acid, nitric acid, and chlorine, imme- 

 diately decompose hydriodic acid. They seize upon its hydrogen, 

 and the iodine precipitates, or exhales in purple vapours. Chlorine 

 is one of tlie most (Iclicatc re-agents to detect small quantities of 

 hydriodic acid ; but it must be added cautiously : for when an ex- 

 cess is employed, it dis?)lves the iodine before it has time to pre- 

 cipitate, or at least colour the liquid. Like hydrosulphuric acid, 

 hydriodic acid is decomposed by solutions of peroxide of iron. 

 \Vhen healed by the oxides which give chlorine with hydrochloric 

 acid, iodiiic is evolved, together with a hydriodate or an ioduret. 

 If for example, it be heated with the black oxide of manganese, 

 we obtain iodine and hydriodate of manganese ; but with tlie red 

 oxide of lead, we obtain iodine and an ioduret. Hydriodic acid 

 forms compounds v/itli the different bases, which have a great deal 

 of resemblance to the hydrosulphates and the hydrochlorates. 



Let us recapitulate the principal characters of hydriodic acid. 

 In the gaseous state it is speedily decomposed by mercury, which 

 \i converted into an ioduret of a greenish yellow colotu'. With 

 Ghlorine it imtnediattly produces a fine purple vapour of irreat in- 

 tensity. In the liquid state it is speedily decomposed, and' coloured 

 vvlien exposed to tlic air. Concentrated sulphuric acid, nitric acid 

 and chloiine, separate iodine from it. Sulphureted hydrogen does 

 not alter it in the least. When poured into a solution of lead it 

 forms a fine orange precipitate. In the solution of peroxide of 

 jiicrcury it forms a red precipitate, and with silver a white preci- 

 j)itate insoluble in ammonia. 1 thought proper to give the pro- 

 jxTties of hydriodic acid in this place, because this will render 

 more intelligible the account which I am going to give of the com- 

 binations of iodine with other bodies. 



Iodine forms with sulphur a weak compound of a greyish black 

 colour, radiated like sulphuret of antimony. Iodine is separated 

 from it when it is distilled with water. 



Hydrogen, whether dry or moist, did not seem to me to have 

 any action on iodine at the ordinary temperature ; but if, as was 

 done by M. Clement, in an experiment in which I was present, we 

 expose a mixture of hydrogen and iodine to a red heat in a tube 

 they unite together, and hydriodic acPd is produced, which gives jt 

 reddibh brown colour to water. We found that 100 parts of k'/tline 

 abi.orbr.53 of hydrogen, in order to be converted into an a^'id. 

 liuf tiiib proportion is a gicat deal too great, as 1 found aftci-wards 

 tbai hydriodic acid Is composed of 100 iodine and 0-.S1.4 hydrotren. 

 Charcoal has no action upon iodine, either at a high or low teni- 

 |)eriiU;re. Several metals un the contrary, a« zinc, iron, tin, nier- 

 cury, and iwtassiutn attack it with facility, even at a low tem- 

 peiatiue, provided they be in u divided state. 'i'liough tlicso 

 ciiin!>inalions take place readily, they pioducc hut little hear 

 and but rarely any light. The compound of iodine and ziiu-' 

 which 1 call lodiiial <>/' zi/ic, is while. It melts readily, and il 

 »uniiincd in tht; Btute of fine ucicular four-iided prlsins. it is very 



