IODINE. 93 



its volume is reduced to one half, consisting of 

 hydrogen gas. Here it is clear that hydriodic 

 acid is a compound of equal volumes of iodine 

 vapour and hydrogen gas, united together with- 

 out any change of volume. A volume of iodine 

 vapour, then, bears the same relation to an atom, 

 that a volume of hydrogen gas does to an atom. 

 But the specific gravity of hydrogen gas is ob- 

 tained by multiplying its atomic weight by half 

 the specific gravity of oxygen gas, or by 0-5555. 

 For 0-125 + 0-5555 = 0-0694. It will be shown 

 in the 7th chapter of this work that this law is 

 general. Consequently, to obtain the specific 

 gravity of iodine vapour, we have only to multi- 

 ply 15-5 by 0-5555. The product is 8-6111, 

 which is the true specific gravity of iodine va- 

 pour. 



The specific gravity of hydriodic acid is a 

 mean between that of iodine vapour and of hy- 

 drogen gas, or 4-34027. 



The atomic weight of hydriodic acid is 

 15-625. 



Iodine combines both with oxygen and with lodic and 

 chlorine, and forms with the former iodic acid, 

 and with the latter chloriodic acid. I have not 

 myself made any experiments to determine the 

 atomic weight of these acids ; but from Gay- 

 Lussac's experiments it is evident, that iodic 

 acid is a compound of one atom iodine, and five 

 atoms oxygen. It must therefore weigh 20-5. 



