MOLECULAR WEIGHT OF IODINE 233 



be left out of account. There remains only the effect of the 

 air left in the apparatus by the air-pump. It [808] is obvious 

 that equilibrium being once established, and the tempera- 

 ture in all parts remaining the same, the pressure of the air 

 in the one half will always counterbalance that in the other. 

 In fact, partial exhaustion was only resorted to as a means of 

 preventing too great an outward pressure during the course 

 of the experiment, since it was difficult to prevent leakage 

 where there was any outward pressure upon the stoppers. 

 Partial exhaustion, besides obviating this difficulty, proved 

 directly advantageous by promoting a more rapid diffusion 

 of the vapors, and thereby shortening the duration of the 

 observations. 



The apparatus having been made ready, communication 

 between the two halves was temporarily interrupted, and the 

 tubes containing the liquids broken by shaking the two 

 bottles simultaneously. After ten to fifteen minutes communi- 

 cation was restored, and now the level of the water in the two 

 manometer tubes, equal before, was seen to differ consider- 

 ably, indicating a higher pressure in the bottle containing the 

 pure solvent. Readings being made from time to time, this 

 difference of level sometimes appeared virtually constant for 

 hours, whilst in other cases it would exhibit considerable 

 variations, which I ascribe to slight inequalities of tempera- 

 ture and to the unequal concentration of the solution in dif- 

 ferent parts of its bottle. After standing twenty-four hours, 

 the aqueous vapor from the manometer tubes generally began 

 to diffuse into the bottles, and by moistening the ether or 

 carbon bisulphide rendered further observations useless. 



The readings give the difference between the vapor tension 

 of the pure solvent and that of the solution; that is, the de- 

 pression of tension which corresponds with the proportion 

 of iodine to solvent in the solution. To calculate the con- 



