July 17, 1890^ 



NA TURE 



285 



A very large number of consistent observations have been 

 recorded. The return of the needle, after deflection to 90°, is 

 limed 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. As an example, I may quote some observations 

 made on April 11. 



The apparatus wa> 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 opera- 

 tion to 8. The assistance of convection currents was then 

 appealed to, and the time fell to 6^, 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 con- 

 ditions 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 sam-; 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 1\. The alcohol thus 

 presents, as Plateau found, a great contrast with the unprepared 

 water ; but 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 behaves in the manner described 

 by Plateau ; that is, it takes part in rotation of the needle, almost 

 from the first moment. Under the action of wind a progressive 

 change is observed. After a time the motes do not begin their 

 movement until the needle has described a considerable arc. At 

 the last stages of purification, a mote, situated upon a radius 

 distant 30° or 40° from the initial direction of the needle, retains 

 its position almost until struck ; behaving, in fact, exactly as 

 Plateau describes for the case of alcohol. I fancied, however, 

 that I could detect a slight difference between alcohol and water 

 even in the best condition, in favour of the former. With a 

 little experience it was easy to predict the "time" from obser- 

 vations upon motes ; and it appeared that the last degrees of 

 purification told more upon the behaviour of the motes than upon 

 the time of describing the arc of 60°. It is possible, however. 



that a different range from that adopted might have proved more 

 favourable in this respect. 



The special difficulties under which Plateau experimented are 

 well known, and appealed strongly to the sympathies of his 

 fellow workers ; but it is not necessary to refer to them in order 

 to explain the fact that the water surfaces that he employed were 

 invariably contaminated. Guided by a knowledge of the facts, 

 I have several times endeavoured to obtain a clear surface without 

 the aid of wind, but have never seen the time less than 10. More 

 often it is 12, 13, or 14. It is difficult to decide upon the source 

 of the contamination. If we suppose that the greasy matter is 

 dissolved, or, at any rate, suspended in the body of the liquid in 

 a fine state of subdivision, it is rather difficult to understand the 

 comparative permanence of the cleansed surfaces. In the case 

 of distilled water, the condition will usually remain without 

 material change for several minutes. On the other hand, with 

 tap water (from an open cistern), which I have often used, 

 although there is no difficulty in getting a clean surface, there is 

 usually a more rapid deterioration on standing. The progres- 

 sive diminution of the tension of well-protected water surfaces 

 observed by Quincke {Poggendtrff' s Annalett, vol. clx., 1877, 

 p. 580) is most readily explained by the gradual formation of a 

 greasy layer composed of matter supplied from the interior, and 

 present only in minute quantity ; although this view did not 

 apparently commend itself to Quincke himself. If we reject the 



NO. ic8l, VOL. 42] 



supposition that the greasy layer is evolved from the interior of 

 the liquid, we must admit that the originally clean free surface, 

 formed as the liquid issues from a tap, is practically certain to 

 receive contamination from the solid bodies with which it 

 comes into contact. The view, put forward hypothetically 

 by Oberbeck, that contamination is almost instantly received 

 from the atmosphere is inconsistent with the facts already 

 mentioned. 



Some further observations, made in the hope of elucidating 

 this question, may here be recorded. First, as to the effect of 

 soap, or rather oleate of soda, A surface of distilled water was 

 prepared by wind and heat until the time was 5^, indicating a 

 high degree of purity. The door being closed, so as to isolate 

 the two parts of the surface, and the wind being maintained all 

 the while, a few drops of solution of oleate were added to the 

 water in the tail-piece. With the aid of gentle stirring, the 

 oleate found its way, in a few minutes, under the door, and 

 reached the surface of the water in the well. The time gradually 

 rose to 13, 14, 15 ; and no subsequent treatment with wind and 

 heat would reduce it again below 12. In this case there can be 

 no doubt that the contamination comes from the interior, and is 

 quickly renewed if necessary ; not, however, so quickly that the 

 tension isjconstant in spite of extension, or the surface would be 

 free from superficial viscosity. 



In like manner, the time upon the surface of camphorated dis- 



