86 30 0.9957 0.992
95 35 0.9940 0.988
104 40 0.9923 0.983
-------------------------------------------------------------------
[Illustration: __Fig._ 8.--Aniline drops falling through cold water and
ascending through hot water._]
*Other Examples of Equi-Density.*--There are many other liquids which,
like orthotoluidine, may be heavier or lighter than water, according to
temperature, and I now wish to bring to your notice the remarkable
liquid _aniline_, which falls under this head. Aniline is an oily
liquid, which, unless specially purified, has a deep red colour. It
forms the basis of the beautiful and varied colouring materials known as
the aniline dyes, which we owe to the skill of the chemist. The
equi-density temperature of water and aniline is 147 deg. F. or 64 deg.
C.; that is, aniline will sink in water if both be colder than 147 deg.
F., and rise to the surface if this temperature be exceeded. We may
illustrate this fact by a simple but striking experiment. Here are two
tall beakers side by side, and above them a cistern containing aniline
(Fig. 8). The stem of the cistern communicates with the two branches of
a horizontal tube, the termination of one branch being near the top of
one of the beakers, whilst the other branch is prolonged to the bottom
of the second beaker, and is curved upwards at the end. Both branches
are provided with taps to regulate the flow of liquid, and to commence
with are full of aniline. Cold water is poured into the beaker
containing the shorter branch until the end is submerged; and water
nearly boiling is placed in the second beaker to an equal height. I now
open the taps, so that the aniline may flow gradually into each beaker;
and you notice that the drops of aniline sink through the cold water and
rise through the hot. We have thus the same liquid descending and
ascending simultaneously in water, the only difference being that the
water is cold on the one side and hot on the other. Prolonging the
delivery-tube to the bottom of the beaker containing the hot water
enables the rising drops to be observed throughout the length of the
column of water; and in addition enables the cold aniline from the
cistern to be warmed up on its way to the outlet, so that by the time it
escapes its temperature is practically the same as that of the water. If
this temperature exceed 147 deg. F., the drops will rise. We might, in
this experiment, have used orthotoluidine instead of aniline; or,
indeed, any other liquid equal in density to water at some temperature
intermediate between those of the hot and cold water--always provided
that the liquid chosen did not mix with water. Amongst such other
liquids may be mentioned _anisol_; _butyl benzoate_; and _aceto-acctic
ether_; but none of these possess the fine colour of aniline or its
chemical relative orthotoluidine, and in addition are more costly
liquids. Besides these are a number of other liquids rarer still,
practically only known to the chemist, which behave in the same way.
These liquids are all carbon compounds, and more or less oily in
character. There is a simple rule which may be used to predict whether
any organic liquid will be both lighter and heavier than water,
according to temperature. Here it is: If the density of the liquid at 32
deg. F. or 0 deg. C. be not greater than 1.12, the liquid will become
less dense than water below 212 deg. F. or 100 deg. C., at which
temperature water boils. This rule is derived from a knowledge of the
extent to which the expansion of organic liquids in general exceeds that
of water. I have considered it necessary to enter at some length into
this subject of equi-density, as much that will follow involves a
knowledge of this physical relation between liquids.
*Aniline Films or Skins.*--We have previously concluded, largely from
circumstantial evidence, that a liquid drop is encased in a skin or what
is equivalent to a skin, and I propose now to show by experiments with
aniline how we can construct a drop, commencing with a skin of liquid.
Here is some aniline in a vessel, covered by water. I lower into the
aniline a circular frame of wire, which I then raise slowly into the
overlying water; and you observe that a film of aniline remains
stretched across the frame. By lifting the frame up and down in the
water the skin is stretched, forming a drop which is constricted near
the frame (Fig. 9). On lifting the wire more suddenly, the skin of
aniline closes in completely at the narrow part, and a sphere of water,
encased in an aniline skin, then falls through the water in the beaker,
and comes to rest on the aniline below--into which, however, it soon
merges. You were previously asked to regard a drop of liquid as being
similar to a filled soap-bubble; and this experiment realizes the terms
of the definition. And it requires only a little imagination to picture
a drop surrounded by its own skin instead of that of another liquid. It
is easy to make one of these enclosed water-drops by imitating the
blowing of a soap-bubble--using, however, water instead of air. In order
to do this I take a piece of glass tubing, open at both ends, and pass
it down the vessel, until it reaches the aniline. Water, in the
meantime, has entered the tube, to the same height as that at which it
stands in the vessel. On raising the tube gently, a skin of aniline
adheres to the end; and as we raise it still further, the water in the
tube, sinking so as to remain at the level in the vessel, expands the
skin into a sphere (Fig. 9)--the equivalent of a filled soap-bubble. On
withdrawing the tube gradually, the composite sphere is left hanging
from the surface of the water.
[Illustration: __Fig._ 9.--Aniline skins enveloping water._]
*Surface Tension.*--Before proceeding further, it will be advisable to
introduce and explain the term "surface tension." We frequently use it,
without attaching to it any numerical value, to express the fact that
the free surface of a liquid is subjected to stretching forces, or is in
a state of tension; and thus we say that certain phenomena are "due to
surface tension." But the physicist does not content himself with merely
observing occurrences; he tries also to measure, in definite units, the
quantities involved in the phenomena. And hence surface tension is
defined as the force tending to pull apart the two portions of the
surface on either side of a line 1 centimetre in length. That is, we
imagine a line 1 centimetre long on the surface of the liquid, dividing
the surface into two portions on opposite sides of the line, and we call
the force tending to pull these two portions away from each other the
surface tension. Experiments show that this force, in the case of cold
water, is equal to about 75 dynes, or nearly 8/100 of a gramme. If we
choose a line 1 inch long on the surface of water, the surface tension
is represented by about 3 1/6 grains. It is always necessary to specify
the length when assigning a value to the surface tension; and unless
otherwise stated a length of 1 centimetre is implied. The values for
different liquids vary considerably; and it is also necessary to note
that the figure for a given liquid depends upon the nature of the medium
by which it is bounded--whether, for example, the surface is in contact
with air or another liquid. The following table gives the values for
several liquids when the surfaces are in contact with air:--
------------------------------------------------------------------
Liquid. Tension at 15 deg. C. (59 deg.
F.), dynes per cm.
------------------------------------------------------------------
Water 75