On Calculating Surface- Tensions. 51 



eye-piece and stage-micrometer. Then the weight of mercury 

 (w), corresponding to an increase of temperature from 0° to 

 100° C., is given by 



iv = alxl3% (1) 



where, a is the area of the tube and / the length in centimetres 

 of 100° (t°) on the scale of the thermometer. 

 The weight (W) of mercury in the bulb is then 



w =^ & 



where a is the coefficient of apparent expansion of mercury 

 in glass. For lead-glass a=0 , 000155, and for soda-glass 

 0*0001586. Having thus determined the quantity of mercury, 

 the quantity of glass may be found as in the first method. 

 With a thermometer containing a known weight of mercury, 

 this method gave a w T ater- value for the immersed part of the 

 instrument which was 3*2 per cent, in error. I place much 

 less reliance upon this than upon the former method, as the 

 apparent coefficient of expansion of glass varies more than its 

 specific gravity. The first method is quick and simple, and 

 the results so excellent, that this second method may be almost 

 regarded as unnecessary; but should any case arise in which it 

 is found to possess advantages over the first, it might be possible 

 to determine the coefficient by observing the increase in length 

 of the thermometer when heated in steam. 

 University College, Liverpool. 



VIII. On the Error involved in Professor Quincke's Method 

 of Calculating Swf ace-Tensions from the Dimensions of Flat 

 Drops and Bubbles. By A.M. Worthington, M.A., 

 Clifton, Bristol*. 



IN one of a series of well-known papers (Pogg. Annal. 

 vol. cxxxix. part 1 ; and Phil. Mag. April 1871) Prof. 

 Quincke has recorded a large number of measures of flat 

 drops and bubbles, from which he has deduced the value of 

 the tensions, not only at the free surface of liquids, but also 

 at the common surface of two liquids in contact. 



The numerical results obtained exceed very appreciably 

 the values of the surface-tensions deduced from observations 

 with capillary tubes, and Prof. Quincke attributes the diffe- 

 rence partly to the exposure of the surface of the meniscus 

 in capillary tubes to atmospheric impurities, but chiefly to 

 the fact that with capillary tubes the edge-angle is not zero, 

 and that the quantity measured is not the surface-tension of 



* Communicated by the Physical Society : read June 13th ? 1885. 



E 2 



