1890.] On the Superficial Viscosity of Water. 137 



question whether the ether may be regarded as originally free from 

 dissolved oil of an involatile character. In practice, both methods 

 have been used. 



The results were not quite so regular as had been hoped, the 

 difficulty appearing to be that the oil left by evaporation upon 

 platinum was not completely transferred to the water surface when 

 the platinum was immersed, even although the operation was per- 

 formed slowly, and repeated two or three times. On the other 

 hand, there was no difficulty in cleansing a large surface of platinum 

 by ignition in the flame of a spirit lamp, so that it was absolutely 

 without perceptible effect upon the movement of the needle over a 

 purified water surface. 



The first solution that was used contained 7 milligrams of oil in 

 50 c.c. of ether. The quantities of solution employed were reckoned 

 in drops, taken under conditions favourable to uniformity, and of 

 such dimensions that 100 drops measured 0'6 c.c. The following is 

 an example of the results obtained : On April 25, the apparatus was 

 rinsed out and recharged with distilled water. Time = 13. After- 

 purification of surface by wind and heat, 5^, rising, after a consider- 

 able interval, to 6. After insertion of a large plate of platinum, 

 recently heated to redness, time unchanged. A narrow strip of 

 platinum, upon which, after a previous ignition, three drops of the 

 ethereal solution had been evaporated, was then immersed, with the 

 result that the time was at once increased to 8. In subsequent trials 

 two drops never failed to produce a distinct effect. Special experi- 

 ments, in which the standard ether was tested after evaporation upon 

 platinum, showed that nearly the whole of the effect was due to the 

 oil purposely dissolved. 



The determination of the number of drops necessary to check the 

 movements of camphor upon the same surface seemed to be subject to 

 a greater irregularity. In some trials 20 drops sufficed ; while in 

 others 40 or 50 drops were barely enough. There seems to be no 

 doubt that the oil is left in a rather unfavourable condition,* 

 veiy different from that of the compact drop upon the small 

 platinum surface of former experiments ; and the appearance of the 

 platinum on withdrawal from the water often indicates that it is still 

 greasy. Under these circumstances it is clearly the smaller number 

 that should be adopted ; but we are safe in saying that T ' T of the oil 

 required to check camphor produces a perceptible effect upon the 

 time in Plateau's experiment, and still more upon the behaviour of 

 the surface before the advancing needle, as tested by observation of 

 motes. At this rate the thickness at which superficial viscosity 



* It should be stated that the evaporation of the ether, and of the dew which 

 was often risible, was facilitated by the application of a gentle warmth. 



L 2 



