1890.] On the Superficial Viscosity of Water. 135 



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 observations upon 

 motes ; and it appeared that the last degrees of purification told 

 more upon the behaviour of the motes than upon the time of describ- 

 ing 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 clean 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 r 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 pro- 

 gressive diminution of the tension of well-protected water surfaces 

 observed by Quincke* 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 

 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 

 * Poggendorff's ' Annalen,' vol. 160, 1877, p. 580. 



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