134 Lord Rayleigh. [June 5. 



may be revived, and the purification accelerated, by the application 

 of heat to the bottom of the well at the part furthest removed from 

 the tail-piece. It may, perhaps, be thoaght that convection currents 

 might be substituted altogether for wind ; but in my experience it is 

 not so. Until a high degree of purity is attained, the operation of 

 convection currents does not extend to the surface, being resisted by 

 the film according to Marangoni's principle. 



When the apparatus was designed, it was hoped that the door 

 could be made a sufficiently good fit to prevent the return of the 

 greasy film into the well ; but experience showed that this could not 

 be relied upon. It was thus necessary to maintain the wind during 

 the whole time of observation. The door was, however, useful in 

 intercepting mechanical disturbance. 



A very large number of consistent observations have been recorded. 

 The return of the needle, after deflection to 90, is timed over an arc 

 of 60, viz., from 90 to 30, and is assisted by a fixed steel magnet 

 acting in aid of the earth's magnetism. A metronome, beating three 

 times per second, facilitates the time measurement. An an example, 

 I may quote some observations made on April 11. 



The apparatus was rinsed and carefully filled with distilled water. 

 In this state the time was 12 (beats). After blowing for a while 

 there was a reduction to 10, and after another operation to 8. 

 The assistance of convection currents was then appealed to, and 

 the time fell to 6f , and after another operation to 6. This appeared 

 to be the limit. The door was then opened, and the wind stopped, 

 with the result that the time rose again to 12. More water was then 

 poured in until the needle was drowned to the depth of about half an 

 inch. Under these conditions the time was 6|. 



It will be seen, that while upon the unprepared surface the time 

 was nearly twice as great as in the interior, upon the purified surface 

 the time was somewhat less than in the interior. 



For the sake of comparison, precisely similar observations were 

 made upon the same day with substitution for water of methylated 

 alcohol. Before the operation of wind the time was 5 ; after wind, 

 5 ; on repetition, still 5. Nor with the aid of convection currents 

 could any reduction be effected. When the needle was drowned, the 

 time rose to 7J. The alcohol thus presents, as Plateau found, a 

 great contrast with the unprepared water ; bat comparatively little 

 with the water after treatment by wind and heat. 



An even more delicate test than the time of vibration is afforded 

 by the behaviour of the surface of the liquid towards the advancing 

 edge of the needle. In order to observe this, it is necessary to have 

 recourse to motes, but all superfluity should be avoided. In a good 

 light it is often possible to see a few motes without any special dusting 

 over. In my experience, an unprepared water surface always 



