2 FERGUSON AND DOWSON, Studies in Capillarity 



It is clear, then, that a few measurements of the surface tension of an 

 unassociated liquid at fairly widely separated temperatures will enable us 

 to calculate b and n, and will thus furnish us with a practical method for 

 measuring its critical temperature. 



But these measurements demand an accurate knowledge of the tem- 

 perature of the meniscus — a quantity by no means easy to determine, de- 

 manding, as it does, fairly elaborate thermostatic arrangements. Further 

 than that, either expense must be incurred by closing the thermostat with 

 windows of optical glass, or time must be spent in testing such pieces of 

 glass as may be at hand. 1 



The importance of surface tension measurements in modern colloidal 

 work renders it imperative to develop a method which shall be rapid, ac- 

 curate, and shall not make too heavy a demand on the instrumental equip- 

 ment of a technical laboratory. It is, we think, the unanimous opinion 

 of workers in this branch of physics that the capillary-rise method is very 

 difficult in practice, and is the reverse of rapid. 



Practically all these troubles are swept away if, instead of measuring 

 the rise of the liquid in a narrow tube, we force the liquid down to the 

 lower end of a tube immersed vertically therein, and measure, on a convenient 

 manometer, the pressure required to effect this. 



Consider the advantages of such an arrangement : — 

 i. Calibration troubles are completely avoided. It is only necessary 

 to measure, once for all, the bore at the end of a capillary tube of circular 

 cross-section. This end is the position of reference for all liquids. 



2. The capillary portion of the tube may be quite short, and the tube 

 is consequently much more easily cleaned, and kept clean. 



3. The thermostatic arrangements are much simplified. The liquid 

 may be heated electrically, and temperatures taken by means of a fine 

 thermo-couple placed quite close to the end of the capillary. 



4. The use of the cathetometer is greatly facilitated. It is far easier 

 to measure the difference of level of the surfaces of a liquid in the limbs 

 of a pressure gauge than to measure the rise in a capillary tube. More- 

 over, any convenient manometer may be used — a point of importance in 

 a laboratory where appliances are restricted. As is easily seen, if the 

 lower end of the capillary be just touching the liquid under observation, 

 and the same liquid be used in the manometer, the difference of level 

 observed will be equal to the height to which the liquid will rise in the 

 capillary tube in the ordinary capillary-rise experiment. Clearly, a gain 

 in sensitiveness is at once obtained by using a light liquid in the mano- 

 meter. But as said above, any convenient pressure gauge may be used — 

 the micro-manometer devised by Threlfall, the Chattock gauge, the differ- 

 ential liquid manometer, 2 a small receptacle closed by a thin metal disk 

 whose motion may be suitably magnified, or in the absence of these a 

 sloping tube attached to a wide vessel and read by a millimetre scale will 

 give satisfactory results. 



1 Harkins and Brown, I.e., p. 504. 



2 For a description of this simple and interesting instrument, see Barton, " Intro- 

 duction to the Mechanics of Fluids" (Longmans), p. 193. 



