Sept. 28, 1888.] 



SCIENTIFIC NEWS. 



337 



SURFACE TENSION. 



( Continued from p. 314.^ 



T T ITHERTO we have operated only on masses of liquid 

 relatively considerable, and having only one free 

 surface. We may now mention new instances of tension 

 furnished by small masses having two surfaces in contact 

 with the air and capable of producing effects of a much 

 more striking character. 



Fifth Experiment. — We take a litre of water in which 

 have been dissolved twenty-five grammes of curd-soap 

 and the same weight of sugar-candy. We plunge into 

 it a wire frame, and then withdraw it. We find it occu- 

 pied by a flat plate which seems to be without weight, 

 so little is it bent. As it becomes attenuated, the con- 

 t-actile force of the two surfaces predominates more and 

 more. Let us place upon this plate a closed outline of 

 cotton or silk thread. It may take any form, because 

 there is a liquid film both within and without. But as 

 soon as we burst the interior film the thread becomes 

 perfectly circular (fig. 5). This is the effect of the 

 combined tensions of the two surfaces of the remaining 

 film. The thread takes the form for which the surface 

 which it bounds is as great as the length of the thread 

 permits, that is, a circle. 



Sixlh Experiment. — My assistant, Professor Schoentjes, 

 has endeavoured to modify this experiment, which I 

 published in 1866. He takes not a simple thread, but a 

 system composed ot solid rectilinear portions, and por- 

 tions of arbitrary form ; according to Steiner's theorem, 

 the maximum surface bounded by such an outline is 

 found when all the portions of arbitrary forms are arcs 

 of one and the same circumference of which the solid 

 rectilinear portions constitute so many chords. 



Prof. Schoentjes has realised such systems by means 

 of cocoon threads passed into the small free channels of 

 portions of the stems of fine graminaceous plants. Each 

 time Steiner's theorem has been perfectly verified when 

 one of these systems has been placed upon a plane 

 liquid film, and the inner portion has then been burst. 



(fig- S-) 



The foregoing experiments have been made each with 

 a single liquid. But it may be foreseen that if we bring 

 in contact two liquids of different tensions there will be 

 produced centripetal or centrifugal movements, according 

 to the value of these tensions. 



If the two liquids mix like water and alcohol, the ten- 

 sion of the surface of contact is null. Hence if we place 

 upon water a drop of alcohol (2-5) or of ether (i'S8), we 

 observe a violent movement of extension, because the 

 tension of water (7 '5) greatly exceeds that of alcohol 

 (2-5), or that of ether ( 1 -88). After this spreading out 

 the liquid retreats towards the point where it has been 

 deposited. It is because the evaporation of the alcohol 

 or the ether has cooled the subjacent liquid, which has 

 thus a stronger tension than that of the more distant 

 parts. 



When the two liquids brought into contact do not mix, 

 as, for instance, oil and water, the surface common to 

 these liquids has also a tension which cannot be neg- 

 lected. 



If we place a drop of oil of turpentine (2-90), ot oil of 

 lavender (3'0°), etc., upon pure water contained in a 

 large vessel, we observe at once a rapid diffusion of the 

 drop, which is transformed into a very thin film, often 

 presenting the most beautiful colours. This diffusion is 

 due to the fact that the tension of pure water (7 -5) is 



greater than the sum of the tensions of the oil and of the 

 surface common to the two liquids. 



As this rapid extension is accompanied by a fall of 

 the temperature of the original drop, the film does not 

 retain its form, but is subdivided into an infinity of very 

 minute portions, forming sometimes small curvilinear 

 polygons, and recalling the aspect of fine lace ; this is 

 what Mr. Tomlinson has called a "cohesion-figure." 

 With oil of lavender the figure is soon broken up into a 

 multitude of spots scarcely visible. 



The application of one drop or of two drops of oil upon 

 water is generally sufficient to render the extension of a 

 new drop impossible. This is because the surface ten- 

 sion has become equal or inferior to the sum of the 

 tensions of the oil and of the surface common to the two 

 liquids. 



Since 1869 I studied this kind of phenomena very 

 carefully, though Signor Marangoni, of Florence, had, 



Fig. 5. 



unknown to me, already given the explanation. If, upon 

 the surface of pure water, we deposit a small fragment 

 of a solid body which is more or less soluble, the equili- 

 brium of the surface of the water is destroyed, and if the 

 act of solution takes place unequally on different sides of 

 the fragment the latter presents abrupt movements of 

 translation and of rotation. 



Let us place, for instance, upon distilled water a 

 flexible, closed outline (of thread), 25 or 30 centimetres 

 in length, so that it touches the water on all its lower 

 side without being submerged at any point. Let us 

 then drop upon the surface of the water within the en- 

 closure one or more small fragments of camphor. Im- 

 mediately the fragments begin to rotate briskly, and to 

 change their place, and the thread will arrange itself in 

 a perfect circle. The interior liquid sometimes passes 

 over the flexible outline at one or more points, when the 

 entire circular film moves on the surface of the liquid. 

 All these phenomena are due to the unequal distribution 

 of the camphorated water, the tension of which is 4' 5 

 milligrammes. 



As regards the diffusion of oil upon water, I may be 

 permitted here to mention an important application 

 which I pointed out in 1882. Let us remember that per 



