406 Mr. A. Ferguson on the Surface-tensions of 



bubble every two or tbree seconds — a condition absolutely 

 necessary for the success of the experiment. 



To the "vertical" limb of the Y-tube was attached a 

 T-pioce, the other limbs of the T-piece leading to the mano- 

 meter and capillary respectively. The manometer was a 

 wide U-tube attached to a vertical board. It is very neces- 

 sary that the diameter of the manometer-tube should be 

 large in comparison with that of the capillary, otherwise the 

 change in surface-tension when the surface of the liquid in 

 the inner tube of the manometer comes into contact with 

 the various gases employed might have an appreciable effect 

 on the value of h^ 



To the vertical capillary tube was attached a needle-point,, 

 and the distance (7i') from this needle-point to the end of the 

 capillary was carefully measured. In all cases, the liquid to 

 be investigated was placed in a beaker standing on an ad- 

 justable table, and the table was raised until the needle-point 

 just touched the surface of the liquid — a setting that could 

 be made with considerable accuracy. (In the case of the 

 more volatile liquids used, this setting had to be made at 

 frequent intervals.) 



The tubes and beakers employed were cleansed between 

 each set of experiments by successive rinsings with acid y 

 water, alkali, and water, finally being dried in a current of 

 hot air. 



That part of the apparatus which lay between the lower 

 limb of the Y-tube and the capillary was kept as small in 

 volume as possible in order to replace one gas by another 

 without any undue delay. 



It is to be remembered that two factors contribute to the 

 total effect caused by the gas — that actually due to the nature 

 of the gas in contact with the liquid and that due to the 

 solubility of the gas in the liquid. These two factors seem 

 to be inseparable, although reasons may be advanced which 

 point to the fact that the first factor is the more important 

 of the two. One might be inclined to think that, so far as 

 the solubility-effect goes, the surface-pellicle, which deter- 

 mines the value of the surface-tension measured, is always 

 saturated with the gas with which it is in contact, whatever 

 may be the state of the body of the liquid. Were this the 

 case, it would be impossible to state with certainty that any 

 part of the effect observed was due to the nature of the gas, 

 as the whole effect might be due simply to the difference 

 between the surf ace- tensions of solutions saturated with air 

 and carbon dioxide respectively* 



But if it be the case that the surface-pellicle is always 



