1815.] On Iodine. 105 



with wTiter, there can remain no doubt about the nature of 

 hytlriodic gas. 



This gas is colourless. Its odour is similar to that of hydrochloric 

 ga<, {muriatic acid.) Its taste is very acid. It contains half its 

 bulk of hydrogen, and saturates its own liulk of ammoniacal gas. 

 Chlorine instantly deprives it of its hydrogen. A beautiful violet 

 vapour is produced, and hydrochloric gas is formed. 1 took its 

 specific gravity by weighing it in a glass vessel the capacity of 

 which 1 knew, weighing it in succession full of air and of gas.* 

 I found by this method in a first experiment, that it is 4-602 times 

 heavier than common air; and in a second experiment, more exact 

 than the first, that its specific gravity is 4'443. This specific gra- 

 vity ought to be a little too great, because notwithstanding the pre- 

 caution which I took to pass the gas through a glass tube cooled 

 down to — 4°, slight traces of moisture were seen in the weighing 

 bottle. These were more conspicuous in the first experiment than 

 in the second. When we compare hydriodicgas with hydrociiloric, 

 we obtain a density differing but little from my last experiment, and 

 which ouglit to be the true one. 



I must put the reader in mind in the first place, that from the 

 experiments of M. Thenard and myself, one volume of chlorine by 

 combining with one volume of hydrogen produces exactly two 

 volumes of hydrochloric gas ; hence it follows, that the density of 

 this last gas is the mean of tliat of chlorine and hydrogen. But 

 since a volume of chlorine takes one volume of hydrogen, its ratio 

 to oxygen in bulk will be that of two to one, and its ratio in weight 

 may be immediately deduced from this. We do not know tiie den- 

 sity of the vapour of iodine ; but from experiments to be stated 

 below, I have found that the ratio of oxygen to iodine is 1 to 

 15-621. Now the density of a demivolume of oxygen being 

 0-55179, 0-55179 X 15-621 = 8-6195 will represent tlie density 

 of iodine under the volume taken for unity. If to this density we 

 add that of hydrogen, 0-07321, and take half the sum, we have 

 4-428H for the density of hydriodic gas ; and it is composed by 

 weight of 100 iodine and 0849 hydrogen. The density of the 

 vapour of iodine is the greatest of all those with wliich we are ac- 

 quainted. It is 1 17*71 greater than that of hydrogen ; and as it is 

 according to the volume of their vapours that bodies combine, this 

 explains why iodine does not take the hundredth of its weight of 

 hydrogen to be converted into an acid. With a denser vapour, as 

 would be the case with that of mercury, the proportion of hydrogen 

 requisite would be still smaller, as in the combination of mercury 



• I,p( ]) he Itic weight of the vessel full <if air; P it-; weight full of water: for 

 H first a|i|)roxiiiiaiioii I* — p will (•xj)ress tlic voliiine of water uliicli it roiitaiiH. 

 ir ; represent the ratio hetueen tlie density of airami that of water under a (;iveii 

 jirrssure and trinj)rralMri-, I* - p -\- (V — ()) V will lie a second ex|iresiii<in of 

 ihe I aparity of the ve't-el more exnet llian lln- lir-^t, and whieh in urUinary cases i» 

 yjfliciinl. Greater ac'iuiucy inaj he attained by the formula : — 



P - J» + d' - p) » + (P - p) V H *<-. 



