OX IMPACT WITH A LIQUID SURFACE. 191 



polished metal spheres were dusted nearly to dimness that the splash was invariably 

 altered, but when we let such a visibly dusty sphere drop through a flame and then 

 caught it in a conical wire cage, the dust was not found to be burned away or 

 appreciably altered, nor in this case ircw the splash altered by passage through the 

 flame. This shows that there is a kind of dust which cannot be removed by a 

 flame, and it is only a conjecture, however probable, that there is a kind which can. 

 We have sought to bring the matter to a crucial test by dropping the sphere 

 through filtered and, presumably, dust-free air contained in a long wide iron pipe 

 whose lower end was just submerged in water ; but though the result of many 

 such trials seemed to show that the splash near the critical height was more 

 often " smooth " in the dust-free air than in ordinary air, there were too many 

 exceptions for the matter to be put beyond doubt. 



In further confirmation of our view that the leading clue to the explanation of 

 the motion is the struggle between the adhesion of the rigid sphere and the 

 tangential momentum of the liquid, we may cite the following points : 



A liquid sphere (see Series L, II., III. of Paper I.) makes a "rough" splash, and 

 the photographs obtained show that the lower part of the in-falling drop is swept 

 away by the tangential flow, while the upper part is still undistorted. 



Here we have cohesion but no rigidity. And we have found that the " rough " 

 splash is obtained by any process which gives a non-rigid surface to the sphere. 

 Thus the splash made by a marble freshly roughened by sand-papering, or by 

 grinding between two files and let fall from the very small height of 7 '5 

 centims., can be practically controlled by attending to the condition of the surface. 

 If the surface is quite dry and still covered with the fine powder resulting from 

 the process of roughening, the splash is " rough," and a great bubble of air is 

 taken down. But if this coat of powder, which has neither cohesion nor shearing 

 strength, be removed by rubbing, the splash (under this low velocity) is " smooth.'' 

 Again, a marble freshly sand-papered and covered with the resulting powder, if 

 let fall from 12 or 15 centims., gives a rough splash. The same marble picked 

 out of the liquid and very quickly dropped in again from the same height, will 

 give again a rough splash. Here the liquid film is thick and "shearable." But 

 if the same sphere be allowed to drain or be lightly wiped, the splash will be 

 smooth. Here we may conjecture that enough fluid is left to fill up the inter- 

 stices, but that the coat is not thick enough to shear easily. If, however, the 

 sphere be thoroughly dried the splash becomes " rough " again. This gives us the 

 explanation of the facts already recorded in respect of the splash of a wet sphere 

 (Series VII. of Paper L). This splash was always irregular ; the liquid drifted to one 

 side where it would shear, while it disappeared from the other or became there too 

 thin to shear, though sufficient to fill up crevices. 



