Intelligence and Miscellaneous Articles. 397 



Temperatures, 

 o 



Dissociation-tensions, 

 millim. 



822-8 

 1003-3 

 1112-0 

 1139-0 



5 



203-27 

 710-69 

 745-00 



If, after a determined temperature and tension have been attained, 

 oxygen is withdrawn by means of the G-eissler pump, the mercury 

 returns to the initial tension, provided, of course, undecomposed 

 oxide of iridium remains. The dissociation-tension of this oxide 

 depends, then, solely on the temperature. 



If the temperature be raised above 1139°, the dissociation-tension 

 soon exceeding that of the atmosphere, oxygen is rapidly liberated 

 through the mercury ; when this has quite ceased, we exhaust ; 

 and on taking out the tray containing the oxide from the cooled 

 tube, w T e find in it some metallic iridium, consequently reduced by 

 the mere action of heat. 



The tension of oxygen in air being about 152 millim s., it follows 

 from the above-cited numbers that the oxide of iridium decomposes 

 in the open air at a temperature below 1003°-3, and consequently 

 that at that temperature, or at any higher one, iridium is absolutely 

 non-oxidizable in the air. 



"When we break the porcelain tube in which the oxide was heated, 

 we observe that it is covered, in the parts which w r ere but little 

 heated, with a very thin layer of blue oxide of iridium — which de- 

 monstrates a slight volatility of this oxide at the relatively low 

 temperatures at which it can exist. Above 1000° volatilization 

 becomes impossible in our atmosphere, since the oxide of iridium 

 ceases to exist there and the metal is at least as fixed as platinum. 

 We have likewise proved this feeble volatility of the oxide of iridium 

 in other experiments, made with M, Stas. — Comptes Rendus de 

 VAcademie des Sciences, Sept. 23, 1878, tome lxxxvii. pp. 441-445. 



ON A UNIVERSAL LAW RESPECTING THE DILATATION OF BODIES. 

 BY M. LEVY. 



Between the specific volume of a body, its temperature, and the 

 pressure (supposed normal) which it supports, there exists, as is 

 known, a relation which permits one of these three quantities to be 

 expressed as a function of the other two — for example, the pressure 

 as a function of the volume and temperature. 



Hitherto (to my knowledge at least) theory has supplied no in- 

 dication upon the nature of this relation, and nothing permits us to 

 affirm with certainty that it may not change in some way when we 

 pass from one substance to another. For every substance the phy- 

 sicist is condemned to seek it in every particular by experiment, 

 which, in a manner, necessitates oo 2 number of observations. 



