MESSRS. A. M. WOIM'm.NCTo.N AND K. 8, COl.K 



than the cohesion, and, as later photographs will clearly show, is incompetent to 

 produce the effects observed. 



In order to test this general explanation further experiments were made. 



Experiments on the Influence of Dust. 



In the first place, to test the influence of dust, the experiment was made of 

 deliberately dusting the surface of the sphere. For this purpose highly polished 

 nickelled spheres, of the three sizes mentioned on p. 179, were held in a pair of 

 crucible tongs by an electrified person standing on an insulating stool, and by him 

 presented to any dusty object that stood or could be brought within reach. The 

 particles of dust soon settled on the electrified sphere, which was then carefully 

 placed on the dropping ring with the dusty side lowest. The liquid used was 

 Alexandra oil, and the height of fall was 317 centims., at which each of these 

 spheres when not dusted gave still a quite airless splash. When dusted an 

 enormous bubble of air was carried down by each. Although the spheres when 

 laid on the dropping ring must have completely lost their electrical charge, yet 

 it seemed worth while to go through the same electrifying process without dusting 

 them. The result showed that no change was produced. In order to see how 

 far the influence of dust would go, the height of fall was now reduced, and it 

 was found that with sphere (l) a fall of 17'1 centims. gave a perfectly rough 

 splash when the surface was visibly dimmed with fine dust, and with sphere (3) 

 a fall of 16 '7 centims. availed. If the surface was only slightly dusty, then at 

 these heights the splash remained " smooth." 



It then occurred to us to try the effect of partial or local dusting, for we had 

 already found by experimenting with a marked sphere that the method of dropping 

 did not impart any appreciable rotation to the sphere, which reached the liquid 

 in the attitude with which it started from the dropping ring. Accordingly, after 

 dusting the sphere in the manner already described, the dust was carefully rubbed 

 away from all but certain parts whose position was recorded. The experiments 

 were very successful, and the results are shown in Series XX. The liquid used was 

 water, and the sphere was of polished serpentine, 257 millims. in diameter, falling 

 14 centims. (cf. Series XIII., Plate 3). 



In fig. 1 of Series XX. (see Plate 3) the sphere was dusted on the right-hand 



atmospheric pressure on the outside, which would be about 1,033 grams per sq. centim. If the actual 

 centripetal pull per unit area is less than this, then the hydrostatic pressure, even at the inner side of the 

 film, will still be positive. If a greater pull than this is required, the hydrostatic pressure near the inner 

 side of the film must be negative, and the liquid there will be in a state of true tension. Experiments 

 which we have conducted in vacua, and which will be described later, show that when this true tension is 

 reached the liquid is Iwble to separate from the solid and to " cavitate," and the phenomenon of a smooth 

 splash then ceases with the guiding influence of the sphere. Thus the limiting value of the cohesion 

 which can 1 reached in practice is probably about 1,038 grams per sq. centim. 



