1820.] Specific Gravitt/ of Gases. 265 



The only person who took the specific gravity of this gas. 

 before our experiments were made was M. Gay-Lussac. He 

 fomid it 4-443. But I have not the least doubt that our experi- 

 ments, from the great care employed in collecting the gas pure,, 

 are much nearer the truth than his. 



It has been shown by Gay-Lussac that hydriodic acid gas is a 

 compound of equal volumes of hydrogen gas and vapour of 

 iodine united together without any diminution of volume. Let 

 X — specific gravity of vapour of iodine : then 



0-0094 + x . o^-^A 



= 4-37o66 



From this it is easy to deduce the specific gravity of vapour of 



iodine, or a: = 8'68188. 



It is obvious that hydriodic acid is a compound of one atom 

 of hydrogen and one atom of iodine. To find the weight of an 

 atom of iodine we have only to reduce its specific gravity to 

 what it would be on the supposition that the specific gravity of 

 oxygen is unity, and multiply that specific gravity by 2. The 

 product will be the weight of the atom. The specific gravity of 

 the vapour thus reduced becomes 7'8137, and 7-8137 x 2 = 

 15"6274; so that the weight of an atom of iodine deduced from 

 the specific gravity of hydriodic acid, as determined in my labo- 

 ratory, is 15-6274. 



We have seen that it is a fundamental law in chemistry that 

 all the atomic weights of bodies are multiples of the atom of 

 hydrogen, or of 0-125. It is obvious that 15-6274 is not a mul- 

 tiple of 0-125; but it requires only a very slight alteration to 

 become a multiple. We have only to change it into 15-625. I 

 conclude, therefore, that the true v/eight of an atom of iodine is 

 15-6-25. The half of this number is 7-8125; and 1 : Mill :: 

 7-8125 : 8-68053047 = specific gravity of vapour of iodine. 

 The true specific gravity of hydriodic acid gas will be obtained 

 by adding the specific gravity of hydrogen gas to this number, 

 and dividing the sum by 2. 



8-68053047 

 0-06944444 



2)8-74997492 

 4-37498746 



Thus tlie true specific gravity of hydriodic acid gas is 4-375, 

 Now the specific gravity which we found dificrs only^^-th part 

 from this calculated specific gravity. There cannot, tlierefore, 

 be the least doubt that the true specific gravity of hydriodic acid 

 is 4-375, and that tlie weight of an atom of iodine is 15-625. 

 This is anmitiple of the atoni of hydrogen ; for 0-125 x 125 = 

 15-62o. Iodine, therefore, agrees with the other bodies exa- 



