262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



I. THE LIMIT OF THE EXPANSION OF OIL OVER WATER. 



We will first look at an experiment of elementary simplicity yet 

 fundamental. Let us pour some water into a pliotogi-aphic tray 

 and then remove all the impurities from the surface by placing 

 upon it just a sheet of thin paper. Then I scatter on the surface 

 a little talc powder and place upon it a trace of oil by means of a 

 very fine capillary tube. The oil spreads out rapidly from the talc in 

 a circle, since the normal surface tension of the water is considerably 

 lowered. But if there is very little oil, such as the capillary will 

 take up by just touching the stopple of the bottle, the expansion stops 

 suddenly, so that we have a circle of oil surrounded by free water. 



Yet is the water really free ? Perhaps there are traces of impuri- 

 ties which stop the extension of the oil. This is not the case. 



Let us touch with a trace of oil another point distant from the first 

 one touched ; a new circle forms and extends outward from the talc, 

 but the first one is in no way affected. No equilibrating impurity 

 exists outside of the first circle, otherwise its surface would have 

 been deformed and diminished. There is therefore a real limit to 

 the extension of oil upon water. And when that limit is reached 

 the surface tension is both that of pure water and of oiled water. 



Let us throw upon this water some grains of camphor dust. AX 

 once we see the grains in lively motion, but everywhere with appar- 

 ently the same speed whether within or without the oiled region. 



We may proceed differently. First spread over the water a sheet 

 of oil, powder it, and then try to enlarge a little portion of the oil 

 film by means of a strip of paper placed across it and over the edge 

 of this dish. At once the whole surface is covered, since the layer 

 of oil was somewhat thick. But there always comes a time when the 

 extension stops; the oiled region marked by the talc remains behind, 

 although there is a surface of water free from both talc and oil. 

 The limit is extremely clean cut and we have side by side two sur- 

 faces with the same surface tension — one of free water, the other of 

 oiled water at its maximum extension.^ 



If at this moment a little camphor dust is scattered on the surface, 

 the grains will be seen in active motion. In getting out of the way 

 of the talc they act like little tadpoles. If the surface is reduced to 

 one-half all motion stops suddenly and the talc gathers around each 

 particle of camphor. We may put upon the water a little tin boat 



1 In 1891, Mile. Pockel pointed out to Lord Rayleigh in a letter published in Nature 

 (English) on the 12th of March, p. 4?>7, some experiments relating to these facts. In 

 enlarging progressively a surface of oil upon water or of water soiled with any other 

 impurity, the tension of that surface varies continuously (abnormal condition) ; it in- 

 creases slowly at first, then very rapidly, and reaches a maximum. Any further extension 

 from the maximum point leaves the tension invariable (normal condition). If Mile. 

 Pockel had scattered an inert powder upon that surface to render it visible, she would 

 have realized that, as soon as the maximum is reached, the oil would extend no further. 



