SURFACE ACTION 



49 



at the bottom, so as to form a lower stratum of slightly higher specific gravity, ortho-toluidine 

 can be run in at the junction of the two liquids by means of a tap-funnel, and spheres of 

 o to 8 centimetres in diameter can be made. 



It is interesting to note that the phenomena shown by such suspended spheres of liquid 

 were chiefly investigated by Plateau, the physicist of Ghent, after he became blind owing 

 ioJ; aZ1 T g at midday sun for experiments on vision. His researches were published in 



1873. In his work he was assisted by his son-in-law, van der Mensbrugghe, whose name 

 we have already met with. 



A method of measurement of surface tension is by the use of Searle's apparatus, 

 made by Pye, of Cambridge (Fig. 31). The pull of the tension of a liquid film 

 is made to twist a wire of phosphor-bronze by a known amount, which is 

 compared with that effected by a known weight. "A rectangular glass microscope 

 slide is clipped to one end of the lever, which also carries a scale pan. The 

 counterpoise is then adjusted so that the lever is horizontal when the lower edge 

 of the slide is just immersed in the liquid. The reading on the scale is noted 

 and the liquid removed. The lever will rise considerably. After drying the 

 glass slide, the lever is 

 brought down to its pre- 

 vious position on the scale 

 by adding weights to the 

 scale pan ; in other words, 

 a force is applied to twist 

 the wire to the same ex- 

 tent as the surface tension 

 of the liquid did. If A 

 is the length of the slide 

 in centimetres and T its 

 thickness, the total length 

 of the film is 2(A + T), 

 since both sides of the slide 

 are active. Then M being 

 the mass in grammes added 

 to the scale pan, its weight 

 is 98 1M in dynes, and the 

 surface tension in dynes 

 per centimetre is 



981 xM 



2(~A + T)' 



If it be wished to obtain a measurement of the absolute surface tension at a water-air inter- 

 face, it is best to use tap water, since this is less likely than distilled water is to contain greasy 

 matter, which has a powerful effect in lowering surface tension, as we shall see later. With 

 Searle's apparatus I have found no difficulty in a lecture experiment in obtaining readings of 

 71 '6 dynes, or 98 per cent, of the correct value, 73. The weight needed in an actual 

 experiment to produce the same torsion of the wire as the pull of the water did was I'll grams, 

 a sufficiently obvious weight. 



The effect of surface tension in regulating the size of drops falling from an 

 orifice is also used as a method of measuring the surface tension of liquids. It 

 is sometimes called the " stalagmometer " method, and is due to Quincke. The 

 size of a drop will increase until its weight balances the tension of its surface 

 film, which is holding it up against gravity. As soon as this size is exceeded 

 the drop will fall. In practice, the number of drops in a known volume of the 

 liquid is counted, and this number is, obviously, inversely proportional to the 

 size of the drops, and this again is proportional to the surface tension the larger 

 the drop the greater the surface tension. Account must be taken of the weight 

 of the drop, that is, the specific gravity of the liquid must be known. The 

 formula is : 



FIG. 31. SEARLE'S TORSION BALANCE FOR MEASURING 

 SURFACE TENSION OF LIQUIDS. 



(As made by Pye & Co., Cambridge.) 



d g 



Number of drops of water x density of liquid 

 Number of drops of liquid 



Another method is founded "on the rise or fall of the level of a liquid in a 



