396 Intelligence and Miscellaneous Articles. 



front the other metals of the platinum group. These two bodies, 

 by the maimer in which they behave in contact with oxygen, mani- 

 festly approach arsenic and antimony ; they might, like these, be 

 placed among the metalloids. It is not the same, we have said, 

 with rhodium, palladium, and iridium. These bodies once oxidized, 

 decomposing by heat, permit us to ascertain the laws of their dis- 

 sociation and the tension it takes at different temperatures. "We 

 will only speak, in this Note, of the oxide of iridium, the only one 

 we have at present completely studied. 



The oxide is put into a porcelain tray, this into a small platinum 

 waggon, which is introduced into a porcelain tube closed at one end 

 by a glass plate kept in its place by mastic. The other extremity, 

 by means of a lead pipe and a glass tube, attached one to the other 

 by mastic and to the porcelain tube, is put into communication 

 with a Greissler mercury pump, and a manometric tube dipping in 

 the mercury. 



Before introducing the iridium oxide into the porcelain^ tube, 

 we assure ourselves that it maintains a vacuum at the ordinary 

 temperature and at a red heat, which does not always happen. In 

 fact, porcelain tubes which are air-tight when cold, at a red heat 

 often let in hydrogen and carbonic oxide from the hearth. 



The porcelain tube is introduced into a cylindrical muffle capable 

 of containing at the same time a porcelain thermometer furnished 

 with its tube and compensator, of the form employed and described 

 by M. Troost and one of us*. The muffle is placed in a furnace 

 heated with petroleum or with heavy coal-oil, which is introduced 

 into the furnace through a piston-cock divided into at least 200 

 parts ; so that the flow of the oil can be varied, and consequently 

 the temperature, with a perfection that could not have been ex- 

 pected. The oil-reservoir is furnished with a Mariotte tube, which 

 maintains it at a constant pressure. With this apparatus the 

 porcelain can be completely fused. 



"We commence by heating the muffle to the point at which the 

 tension of the liberated oxygen is from 30 to 40 centims. and 

 returus to the same when the apparatus has been several times ex- 

 hausted by means of the Greissler pump. We are then sure that 

 the composition of the undecomposed oxide of iridium no longer 

 varies. Then with the cock we lessen the flow of the coal-oil until 

 the pressure of the oxygen does not exceed a few millims. and 

 remains constant. This is noted, and the temperature determined. 



We then successively augment the flow of the oil in order to 

 obtain higher temperaiures and stronger tensions of dissociation ; 

 these are noted when they have become constant. Thus the fol- 

 lowing numbers were found : — 



* This apparatus has been simplified by employing a Sprengel pump, 

 which permits the thermometric material (nitrogen) contained in the re- 

 servoir to be taken out and measured as often as we wish, and the tempe- 

 rature to be calculated. 



