1881.] 



r On Pendent Drops. 



373 



The explanation is, I think, to be found in the fact that the liquid 

 film which wets the surface, is, by reason of the contact, in a modified 

 condition, viscous and unable to transmit pressure like the rest of the 

 liquid.* The existence of such modified covering films seems to have 

 been well established by Professor Quincke.*} - 



I have endeavoured to lessen the labour of calculating the volume 

 of liquid supported by the tension at any section, by cutting out of a 

 uniform sheet of card the area by whose revolution round the axis this 

 volume is generated. The magnitude of this area was found by 

 weighing, and the position of its centre of gravity by balancing on a 

 knife-edge, from which, by applying the well known " Property of 

 Guldinus," the volume of the solid was at once obtained. Owing, 

 however, chiefly to the unequal thickness of even the most uniform 

 material, whether card or metal foil, that I could procure, the results 

 obtained in this way were liable to differ from those obtained by the 

 previously described method of integration by 1 or 2 per cent., so that 

 the method cannot be recommended. I have, however, retained in 

 the following table the measure of T obtained in this way for water in 

 a particular case, and have also inserted for comparison the values thus 

 obtained at different sections of a chloroform drop. The table con- 

 tains the record of results obtained in the way described. 



In most cases a tube was used, the diameter of which is recorded, 

 instead of a plane surface of indefinite extent, since it was found 

 difficult to prevent the drop from gliding about and getting out of 

 focus, when pendent from the latter. 



The temperature (when recorded) is that indicated by a thermo- 

 meter placed close to the drop. 



The surface tension (T) is expressed in grams per centim. 



Olive Oil (density '92129). 



Indefinite plane. Temperature about 18° C. 



Calculated. Observed. 



T r = -04935 .... -04940 



T 2 = -05121 .... -05181 



T 3 = -05777 .... -05688 



T 4 = -06506 .... -06538 



c=- -09943 

 T= -03373 



* It sometimes seemed to me that the rest of the liqirid started out of this film, 

 at a small but definite angle, of which I obtained the following measures in the case 

 of turpentine hanging from a flat iron terminal, which it wetted to an indefinite 

 distance :— 18°, 20°, 21°-5, 20°'5, 19°-2 mean 19° 84. 



f " Phil. Mag.," June, 1878. 



