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SCIENCE. 



ON THE SUPERFICIAL VISCIDITY OF LIQUIDS. 



Translated for " Science " from the French of J. Plateau, 



By the Marchioness Clara Lanza. 



In the year 1638 Descartes affirmed that the surface 

 of water presented a resisting tendency as though it 

 were covered with a thin pelicle. Several other learned 

 men have asserted the same fact and sought to verify it 

 by various experiments. Some limited their researches 

 to water alone ; others maintained that the tests were 

 applicable to all liquids. In my own observations I have 

 described experiments and facts which, in my opinion, at 

 once remove all doubt as to the reality of a peculiar re- 

 sisting force manifested by the surface of water, 

 solutions, etc., and I have attributed this resistance 

 to a characteristic viscidity or glutinous matter pertain- 

 ing exclusively to the outer coating of these liquids. 

 Nevertheless, I have demonstrated that certain other 

 fluids are totally exempt from this peculiarity, and I hope 

 I have fully established the fact, heretofore, that in 

 several among them, such as alcohol, spirits of turpentine, 

 sulphuric ether, etc., the invisible particles of the outer 

 layer offer, on the contrary, less resistance in regard to 

 relative displacement than those within the mass. 



Permit me to recall the facts of which my principal ex- 

 periments consist. A magnetized needle is placed upon a 

 pivot in the centre of a glass cylinder ; the liquid to be 

 tested is then poured into the vessel — just enough to 

 come in contact with the needle; the latter is then turned 

 to a meridian of about 90°, then in a few moments is left 

 to itself, and the time which it takes to travel over a 

 determined angle must be correctly ascertained. In 

 my experiments the angle was 85 . More liquid must 

 then be introduced into the cylinder so that the needle is 

 completely covered, the liquid rising at least two-thirds 

 of an inch above it. The needle is then again turned to 

 a meridian of 90% and one must remark the time taken 

 to describe the preceding angle. Now, for example, 

 when the liquid is distilled water, my needle took pre- 

 cisely twice as long when upon the surface as when be- 

 neath it, notwithstanding that in the first case the under 

 surface of the needle alone came in contact with the 

 water, while in the latter it was completely immersed. 

 When the liquid employed was alcohol or turpentine the 

 time required by the needle, when upon the surface, was 

 less by half than when beneath it. 



I must add that in those liquids on whose surface the 

 magnet moves more slowly than when beneath it, the entire 

 outer coating moves also, although somewhat less rapidly. 



Two liquids, one a solution of albumen and the other 

 of a saponaceous consistency, have exhibited superficial 

 viscidity in an extremely forcible manner. After having 

 moved with the utmost slowness, the needle stopped at 

 an angle of 35" on the surface of the former. It did not 

 move at all upon the latter liquid. 



I omit purposely the various details relative to those 

 experiments, as well as other facts belonging to the sub- 

 ject in question. I shall mention them further on as the 

 reasons present themselves, and at this time merely con- 

 fine myself to the special object ol these remarks, that is 

 to say, the cause and nature ot these phenomena. 



In a notice published in 1870, M. Luvini expressed 

 doubt in regard to the superficial viscidity of liquid mat- 

 ter 1 . He presumes that the effects I myself have ob- 

 served are due to some alteration in the outer surface 

 caused by the contact of the liquid with the air, or else 

 by particles of dust floating about. 



In 1872 M. Marangoni published a paper,-' in 

 which he seeks to prove that the viscidity upon the outer 

 portion ol the liquid is identical with that which is 

 beneath the surface. According to him, in such liquids 

 as water, tor instance, which docs not produce bubbles, 



1 A hunt sperienr.e considerazioni intorno all' adtsiont tra solicit t 

 liguidi. Turin, 1870. 



1 Sul principle delta vitcotita suptr/iciale dti liguidi stabilito dal 

 Signor J Plateau. 



the resistance is increased by a capillary action exercised 

 by the glass upon the needle ; while, when liquids which 

 bubble easily are used, the resistance springs from a thin 

 cuticle in coating of a nature peculiar to the liquid itself. 



I replied to both these articles 3 ; but M. Marangoni 

 attacked me again last year 4 . In his second work he 

 substitutes, for some unknown reason, particles of dirt 

 for the word cuticle. When speaking of the sapona- 

 ceous solution, he states that the carbonic acid in the air 

 decomposes the soap and produces an alkaline carbonate 

 which removes the fatty acids and forms a kind of emul- 

 sion upon the surface. As to the solutions of albumen 

 he thinks probably that the coating of dust is produced 

 by the evaporation of the water. 



He does not positively deny that the surface of liquids 

 cannot possess a viscidity of its own apart from that 

 which is in the mass ; but he is persuaded that the influ- 

 ence of the viscidity peculiar to the surface is very small 

 indeed when compared with that which effects the final 

 results. The following is the substance of his theory : 



We all know that if we place upon any liquid a drop 

 of another possessing less external elasticity, the drop 

 will spread itself in a thin coating upon the surface of 

 the former. Consequently, when a liquid is covered with 

 a layer of dirt, we may reasonably admit that this layer 

 possesses an elastic force much inferior to that which 

 belongs to the pure, fresh surface of the underlying 

 liquid. Now proper experiments show us, first of all, 

 that the tension of this coating is effectively much less 

 forcible than that of the liquid beneath ; secondly, that if 

 the coating becomes sufficiently thick, the elasticity dis- 

 appears entiiely, or very nearly; thirdly, that in any 

 saponaceous solution the film can be accumulated upon 

 certain portions ot the surface and removed upon others. 



When a bubble is blown from one of these liquids the 

 layer of dirt extends in both sides of it and thus prevents 

 its breaking. Liquids such as alcohol, ether, turpentine, 

 etc., cannot, owing to the slight elastic force they pos- 

 sess, be covered with a coating of dirt, and for this rea- 

 son they are unable to produce bubbles. 



The retarded movement of the needle upon the surface 

 of the liquid does not arise from any viscidity of the outer 

 layer, for, in the saponaceous solution at least, this coating 

 is very movable, as the two following facts will show : 



In the first place, when a large soap bubble is blown, 

 reflecting various colors, the slightest breath of air will 

 cause it to whirl rapidly backwards and forwards. In the 

 second place, if a certain amount of soap-suds be put into 

 a horizontal brass tube sufficiently large for the purpose, 

 and a magnet be placed inside upon a pivot, directed toward 

 the magnetic meridian, and then left to oscillate at will, 

 you will perceive that the vibrations are very nearly as 

 rapid as when the magnet moves freely in the air, not- 

 withstanding the fact that it has to overcome the resistance 

 offered by the two outer coatings ot the liquid. 



When a coating of dirt. exists, the somewhat retarded 

 motion of the magnet upon the surface, together with the 

 rotating movement of the entire mass, can be explained in 

 the following manner : The magnet itselt tends to re- 

 move the din which is behind it and accumulate it all in 

 front ; this produces an excess of elasticity along the 

 posterior contour, directly opposed to the natural motion, 

 and at the same time a diminutive expansive force along 

 the anterior contour. Furthermore, behind each half of 

 the magnet, the superfluity ol expansive force on the fresh 

 surface draws together the edges of that portion which is 

 already freed from dirt as though to close the rent, and, 

 at the same time, as in each of the anterior parts, the por- 

 tions far removed from the edge of the magnet possess 

 a weaker expansive force than those nearer to it ; the 

 former attract the latter and thus determine the rotation 

 of the entire mass. 



a Rttponst aux objections de M. Marangoni contre It principt de la 

 viscosite 1 superjit tale ties tiguitles. 



* Di/tsa delta ttorica dell' elasticita suptrficialt del ligttitti. 1878. 



