1890.] Experiments on Vapour-density. 55 



Experiments (1) and (2) are not reliable, as the vibrating glass 

 tube gave a different note from that produced in the experiments on 

 air ; it gave one of two notes, according to the way in which it 

 was clamped. The interval between these notes was estimated by ear 

 as a fourth, but it could not be determined accurately owing to the 

 highness of the notes, which appeared also to vary slightly in pitch. 

 The mean of experiments (3), (4), and (5) is 126'9. In the calcula- 

 tion, &! was taken as T405 (Rontgen), and & 2 as 1'31 (Strecker). These 

 results show conclusively that saturated iodine vapour, and therefore 

 liquid iodine also, are composed of diatomic molecules, I 2 . This 

 result agrees with the late work of Ernst Beckmann (' Zeitschrift 

 fur Physikal. Chem.,' vol. 5, 1890, p. 76) on the molecular weight of 

 iodine in solution in ether and carbon bisulphide ; the numbers varied 

 from 235 to 261 for solutions in ether, and from 263 to 283 for solu- 

 tions in carbon bisulphide. 



A single experiment was performed with bromine in the same 

 manner ; the mean distance between the heaps of silica was 12'22 mm. 

 at a temperature of about 29. Compared with air at the same tem- 

 perature, this gives a vapour-density 85'05. This points to a normal 

 density, as would be expected. There was considerable uncertainty 

 as to the temperature, because no heaps were formed while the bro- 

 mine vapour was saturated, and the tube was therefore heated with a 

 Bunsen's burner. Iodine vapour refused to give heaps in the same 

 way when left long enough in contact with liquid iodine to become 

 saturated. It was predicted by Professor Fitzgerald, in a private 

 letter to Professor Ramsay, that sound would not be propagated 

 through saturated vapour, because the rate of propagation of a com- 

 pression would not be the same as that of an expansion, owing to the 

 difference in elasticity in the two cases. Either the expansion or the 

 compression of a sound-wave would produce condensation in any 

 saturated vapour ; in the case of saturated steam, expansion causes 

 condensation, except at high temperatures (above the " temperature 

 of inversion "). At present there are not sufficient data to calculate 

 the effect of a wave of compression or expansion upon saturated 

 iodine vapour ; but condensation would take place either during a 

 compression or an expansion, and the ratio dpjdv and therefore the 

 elasticity, v dp/dv would be quite different in the two cases. The 

 sound-waves would interfere with one another, and become confused, 

 so that no sound would be propagated through the vapour. 



Induction Spark through Iodine Vapour. An experiment was then 

 made to determine if the passage of the induction spark through 

 iodine vapour effected its dissociation. Professor J. J. Thomson 

 obtained vapour-densities 137 and 130 for unsparked iodine, and 110, 

 115, 84, and 86 for sparked iodine, the last vapour-density being 

 determined twenty-four hours after sparking. Professor Thomson 



