106 Oil Iodine. [Fkh. 



with hydrogen and ammonia. We must not then reject the in- 

 fluence of a very small quantity of matter in a compound, unless it 

 be demonstrated that the density of its vapour bears a considerable 

 ratio to that of the other elements of the compound, or, which 

 comes to the same thing, unless it be demonstrated that its capacity 

 of saturation is small. 



Hydriodic gas is partly decomposed at a red heat. The decom- 

 position is complete if it be mixed with oxygen. Water is i-ormed 

 and iodine separated. 1 find on the other hand, that when iodine 

 and vapour of water are passed through a red-hot porcelain tube, 

 no decomposition takes place. This constitutes the great diiferenc^j 

 between iodine and chlorine ; for this last substance takes hydrogen 

 from oxygen. But the circumstance shows a nearer relation 

 between iodine and sulphur, as 0X3'gcn takes hydrogen from boih. 



Hydriodic gas is very soluble in water, it gives it a great den- 

 sity, and renders it smoking when it is dissolved in greiit quantity. 

 The most convenient way of obtaining liquid hydriodic acid is to 

 dissolve ioduret of phosphorus in water. It is scf arated from 

 phosphorous acid by heat. Hydriodic acid is prepared with stiH 

 greater facility, by putting iodine in water and making a current of 

 hydrosulpiiuric gas pass through the liquid. This gas gives out it« 

 hydrogen to the iodine, and allows its sulphur to precipiiate. The 

 liquid is heated to drive off the excess of hydrosulphuric acid. It 

 is then filtered, or the liquid is decanted oif the pretijiitaied sul- 

 phur. By this method a colourless hydriodic acv.' is detained. 

 'J'he acid prepared by these means is not very coiK-entrated, unless 

 care has been taken, in distilling that prepared by phosphorus, not 

 to mix the last distilled portions with tlie first, which consist of 

 little else than pure water. This acid, like sulj)liurie, may be 

 concentrated by evaporation. While its temperature is lower tiian 

 257^, the liquid that distils over may be thrown away, because it 

 contains very little acid. Above this point the acid itself Iv-gins to 

 distil over, and the temperature remains constant at 26j-^°. The 

 specific gravity of the acid is then l*/, and it does not sensibly 

 vary. This property of hydriodic acid to boil only at 2624-°, ren- 

 ders it a powerful acid, and prevents it from being disengaged from 

 its combinations by the volatile acids. 



Hydriodic acid always becomes more or less coloured when dis- 

 tilled. It even becomes coloured at the ordinary temperature, 

 provided it be in contact of the air. Oxygen is absorbed, water 

 formed, and the iodine, instead of precipitating, dissolves in tiie 

 acid, and gives it a brownish red colour, the deepness of which is 

 proportional to the quantity of iodine lield in solution. 1 have 

 attempted, hut in vain, to destroy this colour by boiling the acid ; 

 hence 1 conclude, that iodine has a great affinity for hydriodic 

 acid : for ir" it? volatility were not diminished by the combination, 

 it ought to be disengaged with the vapour of the water. However 

 I do not think that we ought to consider this coloured acid as a 

 peculiar acid. 



