1890.] On the Superficial Viscosity of Water. 139 



multiple of that at which superficial viscosity tends to disappear. 

 At the same time, the estimate of the total quantity of oil which 

 must be placed upon a really pure surface in order to check the 

 movements of camphor must be somewhat raised, say, from 1'6 to 

 1'9 micro-millimetre. Jt must be remembered, however, that on 

 account of the want of definiteness in the effects, these estimates are 

 necessarily somewhat vague. By a modiOcation of Plateau's appa- 

 ratus, or even in the manner of taking the observations, such as 

 would increase the extent of surface from which the film might be 

 accumulated before the advancing edge of the needle, it would doubt- 

 less be possible to render evident still more minute contaminations 

 than that estimated above at one-tenth of a micro-millimetre. 



[Postscript, June 4. In order to interpret with safety the results 

 obtained by Plateau, I thought it necessary to follow closely his experi- 

 mental arrangements; but the leading features of the phenomenon 

 may be well illustrated without any special apparatus. For this pur- 

 pose, the needle of the former experiments may be mounted upon the 

 surface of water contained to a depth of 1 or 2 Cinches in a large flat 

 bath. Ordinary cleanliness being observed, the motes lying in the 

 area swept over by the needle are found to behave much as described 

 by Plateau. Moreover, the motion of the needle under the action of 

 the magnet used to displace it is decidedly sluggish. In order to 

 purify the surface, a hoop of thin sheet brass is placed in the bath, so 

 as to isolate a part including the needle. The width of the hoop must 

 of course exceed the depth of the water, and that to an extent suffi- 

 cient to allow of manipulation without contact of the fingers with the 

 water. If the hoop be deposited in its contracted state, and be then 

 opened out, the surface contamination is diminished in the ratio of the 

 areas. By this simple device there is no difficulty in obtaining a 

 highly purified surface, upon which motes lie quiescent, almost until 

 struck by the oscillating needle. In agreement with what has been 

 stated above, an expansion of three or four times usually sufficed to 

 convert the ordinary water surface into one upon which superficial 

 viscosity was tending to disappear. 



I propose to make determinations of the actual tensions of surfaces 

 contaminated to various degrees ; but in the meantime it is evident 

 that the higher degrees of purity do not imply much change of tension. 

 In the last experiment upon a tolerably pure surface, if we cause the 

 needle to oscillate rapidly backwards and forwards through a some- 

 what large angle, we can clear away the contamination from a certain 

 area. This contamination will of course tend to return, but observa- 

 tion of motes shows that the process is a rather slow one. 



The smallness of the forces at work must be the explanation of the 

 failure to clean the surface in Plateau's apparatus by mere expansion. 



