130 Lord Rayleigh. [June 5, 



the answer of this question Marangoni himself made an important 

 contribution in the earlier of the memoirs cited. He found (p. 245) 

 that the substitution for the elongated needle of Plateau of a 

 circular disc of thin brass turning upon its centre almost obliterated 

 the distinction between liquids of the two first categories. The 

 ratio of the superficial to the internal viscosity was now even greater 

 for ether than for water. From this we may infer that the special 

 superficial viscosity of water is not called into play by the motions of 

 the surface due to the rotation of the disc, which are obviously of the 

 nature of shearing. 



A varied form of this experiment is still more significant. I have 

 reduced the metal in contact with the water surface to a simple (2") 

 ring ACBD of thin brass wire (fig. 1). This is supported by a fine 



silk fibre, so that it may turn freely about its centre. To give a 

 definite set, and to facilitate forced displacements, a magnetised 

 sewing needle, NS, is attached with the aid of wax. In order to make 

 an experiment, the ring is adjusted to the surface of water contained 

 in a shallow vessel. When all is at rest, the surface is dusted over 

 with a little fine sulphur,* and the suspended system is suddenly set 

 into rotation by an external magnet. The result is very distinct, 

 and contrasts strongly with that observed by Plateau. Instead of the 

 surface enclosed by the ring being carried round with it in its 

 rotation, not the smallest movement can be perceived, except perhaps 

 in the immediate neighbourhood of the wire itself. It is clear that 

 an ordinary water surface does not appreciably resist shearing. 



* Sulphur seems to be on the whole the best material, although it certainly com- 

 municates some impurity to the surface. Freshly heated pumice or wood-ashes 

 sink immediately ; and probably all powders really Iree from grease would behave 

 in like manner. 



