OSMIUM. 477 



does not amount to quite so much as the fifth of an 

 atom of water. This explains the reason why 

 the iridium is not precipitated by alkalies, as is 

 usually the case with metallic oxides. 



When the solution of this salt in water is made 

 without heat, it has a fine green colour ; but if 

 we boil the green liquid, it gradually loses its 

 original colour and becomes red. This change 

 of colour has been generally ascribed to the ad- 

 dition of oxygen to the oxide of iridium. It has 

 been concluded from it that there are two oxides 

 of iridium, the protoxide is green and the per- Oxides of 

 oxide red. Whether this explanation be the true m 

 one I have not been able to determine, as all my 

 attempts to obtain the oxides of iridium in a 

 separate state have ended in disappointment. If 

 there be two oxides of iridium, the protoxide is 

 doubtless composed of 1 atom iridium and 1 

 atom oxygen, and its atomic weight must be 

 4'75. Peroxide of iridium must be a compound 

 of 1 atom iridium and 2 atoms oxygen, and its 

 atomic weight must be 5^5. 



SECT. VI. 



OF THE ATOMIC WEIGHT OF OSMIUM. 



I AM still unacquainted with the atomic weight 

 of osmium the quantity of that anomalous sub- 



