298 
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 
1 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 0 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 of these remarks, that is 
to say, the cause and nature of 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, 4 in 
which he seeks to prove that the viscidity upon the outer 
portion of the liquid is identical with that which is 
beneath the surface. According to him, in such liquids 
as water, for instance, which does not produce bubbles, 
1 A leune sperienze considerazioni intorno all ’ adesione tra solidi e 
liquidi. Turin, 1870. 
u Sul principio della viscosita superjiciale dei liquidi 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 1 . 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 tne 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 of 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 of 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 itself tends to re- 
move the dirt 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 of 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. 
3 Rdponse aux objections de M. Marangoni contre le principe de la 
viscosity superjiciale des liquides. 
4 Di/esa della teorica del? elasticita superficiale dei liquidi. 1878. 
