Method for determining Surface-Tensions, 635 



shown that the contact-angle of ether with glass is about 16°, 

 that of turpentine with glass about 17°. Now as cos 16° is- 

 about -96, it follows that the surface-tension of these liquids, 

 as determined by capillary rise, would differ by about 

 4 per cent, from those determined by a method independent 

 of the contact-angle — a difference which is quite outside the 

 limits of experimental error. And for these two liquids, 

 other experiments (though, as Mr. Morgan has pointed out, 

 they are very few in number) go far to indicate that these 

 values are of the right order of magnitude. 



For instance, for turpentine at 18° using Jaeger's method, 

 I obtained the value 27*98 dyues per cm. for the surface- 

 tension. Its value at the same temperature as given by 

 capillary-rise experiments is about 26*7. If this latter value 

 be really T cos#, these figures give 17°'5 as the value of 6 in 

 good agreement with Magie's result. Similarly in the case 

 of ether, whose surface-tension at 20° is about 16*5 dynes 

 per cm. from capillary-rise experiments*, 17'3 dynes per cm. 

 from experiments carried out by Jaeger's method. This 

 gives 17° as the value of the contact-angle, again in fair 

 agreement with Magie's result. When it is remembered 

 that these values of the contact-augle are obtained by using 

 quite different functions of 6 — for the ratio of the two values 

 of the surface-tension as given by the capillary-rise and by 

 Jaeger's method is approximately cos 0, while the ratio of 

 the values obtained from measurements of the total and 



partial depths of large bubbles is cos 2 v — it becomes fairly 



probable that these values are fairly exact measures of the 

 contact-angles of these two liquids. 



Now, of the hundred liquids whose surface-tensions have 

 been studied by Mr. Morgan, seventy give values by the 

 drop-weight method which are in good agreement with the 

 values obtained from capillary-rise experiments: for the 

 other thirty the agreement is not so good. 



is benzene, for which a 1 is given as *05678 sq. cm., leading- to a value of 

 19*45 dynes per cm. for T at 20° C. The density given is -698, whilst the 

 density of benzene at 20° is '879. But even assuming that the density 

 has been misprinted and employing' the figure '879, we have 24/49 dynes 

 per cm. for the value of the surface-tension of benzene at 20° C. This 

 value seems to be quite out of the question, as practically all investigators 

 give values of about 28-S at '20° C. — certainly no results with which I 

 have met g-o so low as even 27. 



* This value is for ether in the presence of its own vapour — the 

 surface-tension for an ether-air surface would probably be very slightly 

 less. 



