Lord Rayleigh : Investigations in Capillarity. 333 



removed with strips of paper. The operation should be 

 repeated two or three times with intermediate insertion of 

 the balanced strips until it is certain that no grease remains, 

 competent to affect the tension even when concentrated. 

 The weights now necessary to bring the pointer to zero give 

 the standard with which the contaminated surfaces are to be 

 compared. 



If it be desired to begin with small contaminations, it is 

 best to contract the area, say to about one-half the maximum, 

 and then to apply the grease under examination with a 

 previously ignited platinum wire until a small effect, such 

 as '02 gm., is observed at the balance. If the surface be 

 now extended to the maximum, the attenuated grease will 

 have lost its power, and the original reading for clean 

 surfaces will be recovered. The barrier may now be ad- 

 vanced, readings being taken at intervals as the grease is 

 concentrated. It is often more convenient to make the final 

 adjustment by moving the barrier rather than by correcting 

 the weights. 



An example will make manifest at once the character of 

 the results obtained. On May 15, the weight for the clean 

 surface being 1*65 gm., the water was greased with castor- 

 oil. With the barrier at 63 cm. this grease had no effect. 

 The corrected length is 66, and the reciprocal of this, viz, 

 1 52, represents (for this series of observations) the density of 

 the oil. With the barrier at 40, viz. at density 233, there 

 was no change of the order of '005 gm. At 36 cm., or 

 density 256, the oil had just begun to show itself distinctly, 

 the weight being then 1*64. At density 278 the weight 

 became 162. From this point onwards increase of density 

 tells rapidly. At 308 the weight was 1*55, and at 334 the 

 weight was 1*40. A plot of these results is given in fig. 3, 

 and brings out more vividly than any description the striking- 

 character of the law discovered by Miss Pockels. 



The effect of concentration beyond 571, giving '70 gm., 

 could not be examined in the same series. It was necessary 

 to add more oil, and then of course the reciprocals of the 

 corrected lengths represent the densities on a different scale 

 from before. Corresponding to 63 cm., of which the reciprocal 

 is 159, the weight was now 1*20 gm., falling to l'OO at 175, 

 •80 at 204, -70 at 233, '60 at 351, -55 at 488, and finally -52 

 at 625. These values are plotted in fig. 4, and they show- 

 that from a certain density onwards the tension falls very 

 slowly. This curve may be continued backwards by means 

 of the results of fig. 3, for of course the densities correspond- 

 ing to any particular weight, e. g. 1*20 gm., are really the 

 same in the two series. 



