402 On Iodine. [June, 



binations of iodine with the metals that decompose water possess 

 the same property. 



On the contrary, the metals that do not decompose water have 

 given me precipitates with tlie hydriodate of soda. The precipitate 

 of copper is whitish-gr6y ; that of lead, a fine orange-yellow ; that 

 of protoxide of mercury, greenish-yellow ; that of peroxide of 

 mercury, orange-red ; that of silver, white ; that of bismuth, 

 chesnut-brown. 



I consider all these precipitates as metallic iodurets, and with so 

 much the greater reason, that the hydriodates of the very oxidable 

 metals are changed into iodurets merely by drying them by a gentle 

 heat. Now the force which has detennined the insolubility of all 

 these precipitates ought to be considered as much more energetic 

 than a small change of temperature which is sufficient to convert a 

 hydriodate into an ioduiet. 



It will not be useless, in order to settle our opinions respecting 

 the nature of the combinations of the metals with sulphur, iodine, 

 and chlorine, when in contact with water, to show the analogy 

 which they have to one another. 



Among the sulphurets, those only of the metals, which have a 

 much greater affinity for oxygen than hydrogen has, are soluble in 

 water, and may be considered, with some probability, as hydro- 

 sulphates. Such are those of potassium, sodium, barium, &c. 



Though iron and zinc decompose water, they have not so superior 

 an affinity for oxygen above hydrogen, \\xAt the united affinities of 

 the metal for oxygen, and the sulphur for hydrogen, are stronger 

 than that of the oxygen for hydrogen, and tlae metal for-sulphur. 1 

 neglect here the affinity of the oxide for hydro-sulphuric acid, be- 

 cause it must be very weak relatively to the others. Tlie metal? 

 which readily yield their oxygen to hydrogen will form a fortiori 

 sulplwrets, which will not decompose water, and which will be 

 insoluble in that liquid. 



When we compare the iodurets with the sulphurets, we nvust 

 attend to this circumstance, that iodine has a stronger affinity than 

 sulphur for hydrogen, and that from this there ought to result an 

 augmentation of intensity in tlte tbices which tend to decompose 

 water.* VVe see, in fact, that all the metals which give soluble 

 compounds with sulphur form equally soluble ones with iodine; and 

 further, lliat the iodurets of the metals which decompose water 

 possess the same property. As to the iodurets of the metals, which 

 have less affinity for oxygen than hydrogen, they are insoluble, a« 

 >\-ell as their sulphurets. 



Pursuing tlie same comparison witb the chlorurets, we ought, 



• It nifiy becbjectefJ, thai'* ilie fmicea which teul tn dcroinposje water have 

 incicagod, becaine iodi;ie has more af5mt_v ihansu'.pt.ur lor h>dro5en, ilmfe which 

 lend tu prevent Us decomposition have also increased, beniuse iodine has Bior« 

 nffinity than sivlphur for polasfium and tiie other metallic bodipj. But we may 

 •appose, with 6uffioieotpr«bal>iliiL>., that the fir<t have increased in a greiitcr rati* 

 (hail the iVcoiid. 



