Chapter I: On Water and Its Compounds (2)

+------------+---------+-------------+----------+
|Temperature | Tension | Temperature | Tension |
+------------+---------+-------------+----------+
| -20° | 0·9 | 70° | 233·3 |
| -10° | 2·1 | 90° | 525·4 |
| 0° | 4·6 | 100° | 760·0 |
| +10° | 9·1 | 105° | 906·4 |
| 15° | 12·7 | 110° | 1075·4 |
| 20° | 17·4 | 115° | 1269·4 |
| 25° | 23·5 | 120° | 1491·3 |
| 30° | 31·5 | 150° | 3581·0 |
| 50° | 92·0 | 200° | 11689·0 |
+------------+---------+-------------+----------+

The table shows the boiling points of water at different
pressures. Thus on the summit of Mont Blanc, where the average
pressure is about 424 mm., water boils at 84·4°. In a rarefied
atmosphere water boils even at the ordinary temperature, but in
evaporating it absorbs heat from the neighbouring parts, and
therefore it becomes cold and may even freeze if the pressure
does not exceed 4·6 mm., and especially if the vapour be rapidly
absorbed as it is formed. Oil of vitriol, which absorbs the
aqueous vapour, is used for this purpose. Thus ice may be
obtained artificially at the ordinary temperature with the aid
of an air-pump. This table of the tension of aqueous vapour also
shows the temperature of water contained in a closed boiler if
the pressure of the steam formed be known. Thus at a pressure
of five atmospheres (a pressure of five times the ordinary
atmospheric pressure--_i.e._ 5 × 760 = 3,800 mm.) the temperature
of the water would be 152°. The table also shows the pressure
produced on a given surface by steam on issuing from a boiler.
Thus steam having a temperature of 152° exerts a pressure of
517 kilos on a piston whose surface equals 100 sq. cm., for the
pressure of one atmosphere on one sq. cm. equals 1,033 kilos,
and steam at 152° has a pressure of five atmospheres. As a
column of mercury 1 mm. high exerts a pressure of 1·35959 grams
on a surface of 1 sq. cm., therefore the pressure of aqueous
vapour at 0° corresponds with a pressure of 6·25 grams per
square centimetre. The pressures for all temperatures may be
calculated in a similar way, and it will be found that at 100°
it is equal to 1,033·28 grams. This means that if a cylinder
be taken whose sectional area equals 1 sq. cm., and if water
be poured into it and it be closed by a piston weighing 1,033
grams, then on heating it in a vacuum to 100° no steam will be
formed, because the steam cannot overcome the pressure of the
piston; and if at 100° 534 units of heat be transmitted to each
unit of weight of water, then the whole of the water will be
converted into vapour having the same temperature; and so also
for every other temperature. The question now arises, to what
height does the piston rise under these circumstances? that is,
in other words, What is the volume occupied by the steam under
a known pressure? For this we must know the weight of a cubic
centimetre of steam at various temperatures. It has been shown by
experiment that the density of steam, which does not saturate a
space, varies very inconsiderably at all possible pressures, and
is nine times the density of hydrogen under similar conditions.
Steam which saturates a space varies in density at different
temperatures, but this difference is very small, and its
average density with reference to air is 0·64. We will employ
this number in our calculation, and will calculate what volume
the steam occupies at 100°. One cubic centimetre of air at 0°
and 760 mm. weighs 0·001293 gram, at 100° and under the same
pressure it will weigh 0·001293/1·368 or about 0·000946 gram,
and consequently one cubic centimetre of steam whose density
is 0·64 will weigh 0·000605 gram at 100°, and therefore one
gram of aqueous vapour will occupy a volume of about 1·653 c.c.
Consequently, the piston in the cylinder of 1 sq. cm. sectional
area, and in which the water occupied a height of 1 cm., will
be raised 1,653 cm. on the conversion of this water into steam.
This piston, as has been mentioned, weighs 1,033 grams, therefore
the _external work of the steam_--that is, that work which the
water does in its conversion into steam at 100°--is equal to
lifting a piston weighing 1,033 grams to a height of 1,653 cm.,
or 17·07 kilogram-metres of work--_i.e._ is capable of lifting
17 kilograms 1 metre, or 1 kilogram 17 metres. One gram of water
requires for its conversion into steam 534 gram units of heat or
0·534 kilogram unit of heat--_i.e._ the quantity of heat absorbed
in the evaporation of one gram of water is equal to the quantity
of heat which is capable of heating 1 kilogram of water 0·534°.
Each unit of heat, as has been shown by accurate experiment,
is capable of doing 424 kilogram-metres of work. Hence, in
evaporating, one gram of water expends 424 × 0·534 = (almost) 227
kilogram-metres of work. The external work was found to be only
17 kilogram-metres, therefore 210 kilogram-metres are expended in
overcoming the internal cohesion of the aqueous particles, and
consequently about 92 p.c. of the total heat or work is consumed
in overcoming the internal cohesion. The following figures are
thus calculated approximately:--

+------------+----------------+-----------------+--------------+
| | Total work of |External work of | |
|Temperature | evaporation in | vapour in | Internal |
| |kilogram-metres |kilogram-metres |work of vapour|
+------------+----------------+-----------------+--------------+
| 0° | 255 | 13 | 242 |
| 50° | 242 | 15 | 227 |
| 100° | 226 | 17 | 209 |
| 150° | 209 | 19 | 190 |
| 200° | 192 | 20 | 172 |
+------------+----------------+-----------------+--------------+

The work necessary for overcoming the internal cohesion of
water in its passage into vapour decreases with the rise in
temperature--that is, corresponds with the decrease of cohesion;
and, in fact, the variations which take place in this case are
very similar to those which are observed in the heights to which
water rises in capillary tubes at different temperatures. It is
evident, therefore, that the amount of external--or, as it is
termed, useful--work which water can supply by its evaporation
is very small compared with the amount which it expends in its
conversion into vapour.

In considering certain physico-mechanical properties of water, I
had in view not only their importance for theory and practice,
but also their purely chemical significance; for it is evident
from the above considerations that even in a physical change
of state the greatest part of the work done is employed in
overcoming cohesion, and that an enormous amount of internal
energy must be expended in overcoming chemical cohesion or
affinity.

[12] When it is necessary to heat a considerable mass of liquid in
different vessels, it would be very uneconomical to make use of
metallic vessels and to construct a separate furnace for each;
such cases are continually met with in practice. Steam from a
boiler is introduced into the liquid, or, in general, into the
vessel which it is required to heat. The steam, in condensing and
passing into a liquid state, parts with its latent heat, and as
this is very considerable a small quantity of steam will produce
a considerable heating effect. If it be required, for instance,
to heat 1,000 kilos of water from 20° to 50°, which requires
approximately 30,000 units of heat, steam at 100° is passed into
the water from a boiler. Each kilogram of water at 50° contains
about 50 units of heat, and each kilogram of steam at 100°
contains 637 units of heat; therefore, each kilogram of steam in
cooling to 50° gives up 587 units of heat, and consequently 52
kilos of steam are capable of heating 1,000 kilos of water from
20° to 50°. Water is very often applied for heating in chemical
practice. For this purpose metallic vessels or pans, called
'water-baths,' are made use of. They are closed by a cover formed
of concentric rings lying on each other. The vessels--such as
beakers, evaporating basins, retorts, &c.--containing liquids,
are placed on these rings, and the water in the bath is heated.
The steam given off heats the bottom of the vessels to be heated,
and thus effects the evaporation or distillation.

Water is mechanically attracted by many substances; it adheres to their surfaces just as dust adheres to objects, or one piece of polished glass adheres to another. Such attraction is termed 'moistening,' 'soaking,' or 'absorption of water.' Thus water moistens clean glass and adheres to its surface, is absorbed by the soil, sand, and clay, and does not flow away from them, but lodges itself between their particles. Similarly, water soaks into a sponge, cloth, hair, or paper, &c., but fat and greasy substances in general are not moistened. Attraction of this kind does not alter the physical or chemical properties of water. For instance, under these circumstances water, as is known from everyday experience, may be expelled from objects by drying. Water which is in any way held mechanically may be dislodged by mechanical means, by friction, pressure, centrifugal force, &c. Thus water is squeezed from wet cloth by pressure or centrifugal machines. But objects which in practice are called dry (because they do not feel wet) often still contain moisture, as may be proved by heating the object in a glass tube closed at one end. By placing a piece of paper, dry earth, or any similar object (especially porous substances) in such a glass tube, and heating that part of the tube where the object is situated, it will be remarked that water condenses on the cooler portions of the tube. The presence of such absorbed, or 'hygroscopic,' water is generally best detected in non-volatile substances by drying them at 100°, or under the receiver of an air-pump and over substances which attract water chemically. By weighing a substance before and after drying, it is easy to determine the amount of hygroscopic water from the loss in weight.[13] Only in this case the amount of water must be judged with care, because the loss in weight may sometimes proceed from the decomposition of the substance itself, with disengagement of gases or vapour. In making exact weighings the hygroscopic capacity of substances--that is, their capacity to absorb moisture--must be continually kept in view, as otherwise the weight will be untrue from the presence of moisture. The quantity of moisture absorbed depends on the degree of moisture of the atmosphere (that is, on the tension of the aqueous vapour in it) in which a substance is situated. In an entirely dry atmosphere, or in a vacuum, the hygroscopic water is expelled, being converted into vapour; therefore, substances containing hygroscopic water may be completely dried by placing them in a dry atmosphere or in a vacuum. The process is aided by heat, as it increases the tension of the aqueous vapour. Phosphoric anhydride (a white powder), liquid sulphuric acid, solid and porous calcium chloride, or the white powder of ignited copper sulphate, are most generally employed in drying gases. They absorb the moisture contained in air and all gases to a considerable, but not unlimited, extent. Phosphoric anhydride and calcium chloride deliquesce, become damp, sulphuric acid changes from an oily thick liquid into a more mobile liquid, and ignited copper sulphate becomes blue; after which changes these substances partly lose their capacity of holding water, and can, if it be in excess, even give up their water to the atmosphere. We may remark that the order in which these substances are placed above corresponds with the order in which they stand in respect to their capacity for absorbing moisture. Air dried by calcium chloride still contains a certain amount of moisture, which it can give up to sulphuric acid. The most complete desiccation takes place with phosphoric anhydride. Water is also removed from many substances by placing them in a dish over a vessel containing a substance absorbing water under a glass bell jar.[14] The bell jar, like the receiver of an air pump, should be hermetically closed. In this case desiccation takes place; because sulphuric acid, for instance, first dries the air in the bell jar by absorbing its moisture, the substance to be dried then parts with its moisture to the dry air, from which it is again absorbed by the sulphuric acid, &c. Desiccation proceeds still better under the receiver of an air pump, for then the aqueous vapour is formed more quickly than in a bell jar full of air.

[13]

In order to dry any substance at about 100°--that is, at the
boiling point of water (hygroscopic water passes off at this
temperature)--an apparatus called a 'drying-oven' is employed.
It consists of a double copper box; water is poured into the
space between the internal and external boxes, and the oven is
then heated over a stove or by any other means, or else steam
from a boiler is passed between the walls of the two boxes.
When the water boils, the temperature inside the inner box will
be approximately 100° C. The substance to be dried is placed
inside the oven, and the door is closed. Several holes are cut
in the door to allow the free passage of air, which carries
off the aqueous vapour by the chimney on the top of the oven.
Often, however, desiccation is carried on in copper ovens
heated directly over a lamp (fig. 13). In this case any desired
temperature may be obtained, which is determined by a thermometer
fixed in a special orifice. There are substances which only
part with their water at a much higher temperature than 100°,
and then such air baths are very useful. In order to determine
directly the amount of water in a substance which does not part
with anything except water at a red heat, the substance is placed
in a bulb tube. By first weighing the tube empty and then with
the substance to be dried in it, the weight of the substance
taken may be found. The tube is then connected on one side with
a gas-holder full of air, which, on opening a stop-cock, passes
first through a flask containing sulphuric acid, and then into a
vessel containing lumps of pumice stone moistened with sulphuric
acid. In passing through these vessels the air is thoroughly
dried, having given up all its moisture to the sulphuric acid.
Thus dry air will pass into the bulb tube, and as hygroscopic
water is entirely given up from a substance in dry air even at
the ordinary temperature, and still more rapidly on heating,
the moisture given up by the substance in the tube will be
carried off by the air passing through it. This damp air then
passes through a U-shaped tube full of pieces of pumice stone
moistened with sulphuric acid, which absorbs all the moisture
given off from the substance in the bulb tube. Thus all the water
expelled from the substance will collect in the [U] tube, and so,
if this be weighed before and after, the difference will show the
quantity of water expelled from the substance. If only water (and
not any gases) come over, the increase of the weight of the [U]
tube will be equal to the decrease in the weight of the bulb tube.

[14] Instead of under a glass bell jar, drying over sulphuric acid is
often carried on in a desiccator consisting of a shallow
wide-mouthed glass vessel, closed by a well-fitting ground-glass
cover. Sulphuric acid is poured over the bottom of the
desiccator, and the substance to be dried is placed on a glass
stand above the acid. A lateral glass tube with a stop-cock is
often fused into the desiccator in order to connect it with an
air pump, and so allow drying under a diminished pressure, when
the moisture evaporates more rapidly. The fact that in the usual
form of desiccator the desiccating substance (sulphuric acid) is
placed beneath the substance to be dried has the disadvantage
that the moist air being lighter than dry air distributes itself
in the upper portion of the desiccator and not below. Hempel,
in his desiccator (1891), avoids this by placing the absorbent
above the substance to be dried. The process of desiccation
can be further accelerated by cooling the upper portion of the
desiccator, and so inducing ascending and descending currents of
air within the apparatus.

From what has been said above, it is evident that the transference of moisture to gases and the absorption of hygroscopic moisture present great resemblance to, but still are not, chemical combinations with water. Water, when combined as hygroscopic water, does not lose its properties and does not form new substances.[15]

[15] Chappuis, however, determined that in wetting 1 gram of charcoal
with water 7 units of heat are evolved, and on pouring carbon
bisulphide over 1 gram of charcoal as much as 24 units of heat
are evolved. Alumina (1 gram), when moistened with water, evolves
2-1/2 calories. This indicates that in respect to evolution of
heat moistening already presents a transition towards exothermal
combinations (those evolving heat in their formation).

The attraction of water for substances which dissolve in it is of a different character. In the solution of substances in water there proceeds a peculiar kind of indefinite combination; a new homogeneous substance is formed from the two substances taken. But here also the bond connecting the substances is very unstable. Water containing different substances in solution boils at a temperature near to its usual boiling point. From the solution of substances which are lighter than water itself, there are obtained solutions of a less density than water--as, for example, in the solution of alcohol in water; whilst a heavier substance in dissolving in water gives it a higher specific gravity. Thus salt water is heavier than fresh.[16]

[16] Strong acetic acid (C_{2}H_{4}O_{2}), whose specific gravity at
15° is 1·055, does not become lighter on the addition of water
(a lighter substance, sp. gr. = 0·999), but heavier, so that a
solution of 80 parts of acetic acid and 20 parts of water has a
specific gravity of 1·074, and even a solution of equal parts of
acetic acid and water (50 p.c.) has a sp. gr. of 1·065, which is
still greater than that of acetic acid itself. This shows the
high degree of contraction which takes place on solution. In
fact, solutions--and, in general, liquids--on mixing with water,
decrease in volume.

We will consider _aqueous solutions_ somewhat fully, because, among other reasons, solutions are constantly being formed on the earth and in the waters of the earth, in plants and in animals, in chemical processes and in the arts, and these solutions play an important part in the chemical transformations which are everywhere taking place, not only because water is everywhere met with, but chiefly because a substance in solution presents the most favourable conditions for the process of chemical changes, which require a mobility of parts and a possible distension of parts. In dissolving, a solid substance acquires a mobility of parts, and a gas loses its elasticity, and therefore reactions often take place in solutions which do not proceed in the undissolved substances. Further, a substance, distributed in water, evidently breaks up--that is, becomes more like a gas and acquires a greater mobility of parts. All these considerations require that in describing the properties of substances, particular attention should be paid to their relation to water as a solvent.

It is well known that water dissolves many substances. Salt, sugar, alcohol, and a number of other substances, dissolve in water and form homogeneous liquids with it. To demonstrate the solubility of gases in water, a gas should be taken which has a high co-efficient of solubility--for instance, ammonia. This is introduced into a bell jar (or cylinder, as in fig. 14), which is previously filled with mercury and stands in a mercury bath. If water be then introduced into the cylinder, the mercury will rise, owing to the water dissolving the ammonia gas. If the column of mercury be less than the barometric column, and if there be sufficient water to dissolve the gas, all the ammonia will be absorbed by the water. The water is introduced into the cylinder by a glass pipette, with a bent end. The bent end is put into water, and the air is sucked out from the upper end. When full of water, its upper end is closed with the finger, and the bent end placed in the mercury bath under the orifice of the cylinder. On blowing into the pipette the water will rise to the surface of the mercury in the cylinder owing to its lightness. The solubility of a gas like ammonia may be demonstrated by taking a flask full of the gas, and closed by a cork with a tube passing through it. On placing the tube under water, the water will rise into the flask (this may be accelerated by previously warming the flask), and begin to play like a fountain inside it. Both the rising of the mercury and the fountain clearly show the considerable affinity of water for ammonia gas, and the force acting in this dissolution is rendered evident. A certain period of time is required both for the homogeneous intermixture of gases (diffusion) and the process of solution, which depends, not only on the surface of the participating substances, but also on their nature. This is seen from experiment. Solutions of different substances heavier than water, such as salt or sugar, are poured into tall jars. Pure water is then very carefully poured into these jars (through a funnel) on to the top of the solutions, so as not to disturb the lower stratum, and the jars are then left undisturbed. The line of demarcation between the solution and the pure water will be visible, owing to their different co-efficients of refraction. Notwithstanding that the solutions taken are heavier than water, after some time complete intermixture will ensue. Gay Lussac convinced himself of this fact by this particular experiment, which he conducted in the cellars under the Paris Astronomical Observatory. These cellars are well known as the locality where numerous interesting researches have been conducted, because, owing to their depth under ground, they have a uniform temperature during the whole year; the temperature does not change during the day, and this was indispensable for the experiments on the diffusion of solutions, in order that no doubt as to the results should arise from a daily change of temperature (the experiment lasted several months), which would set up currents in the liquids and intermix their strata. Notwithstanding the uniformity of the temperature, the substance in solution in time ascended into the water and distributed itself uniformly through it, proving that there exists between water and a substance dissolved in it a particular kind of attraction or striving for mutual interpenetration in opposition to the force of gravity. Further, this effort, or rate of diffusion, is different for salt or sugar or for various other substances.[16 bis] It follows therefore that a peculiar force acts in solution, as in actual chemical combinations, and solution is determined by a particular kind of motion (by the chemical energy of a substance) which is proper to the substance dissolved and to the solvent.

[16 bis] Graham, in the jelly formed by gelatine, and De Vries in
gelatinous silica (Chapter XVIII.) most frequently employed
coloured (tinted) substances, for instance, K_{2}Cr_{2}O_{7},
which showed the rate of diffusion with very great clearness.
Prof. Oumoff employed the method described in Chapter X., Note
17, for this purpose.

Graham made a series of experiments similar to those above described, and showed that the _rate of diffusion_[17] in water is very variable--that is, a uniform distribution of a substance in the water dissolving it is attained in different periods of time with different solutions. Graham compared diffusive capacity with volatility. There are substances which diffuse easily, and there are others which diffuse with difficulty, just as there are more or less volatile substances. Seven hundred cubic centimetres of water were poured into a jar, and by means of a syphon (or a pipette) 100 cub. centimetres of a solution containing 10 grams of a substance were cautiously poured in so as to occupy the lower portion of the jar. After a lapse of several days, successive layers of 50 cubic centimetres were taken from the top downwards, and the quantity of substance dissolved in the different layers determined. Thus, common table salt, after fourteen days, gave the following amounts (in milligrams) in the respective layers, beginning from the top: 104, 120, 126, 198, 267, 340, 429, 535, 654, 766, 881, 991, 1,090, 1,187, and 2,266 in the remainder; whilst albumin in the same time gave, in the first seven layers, a very small amount, and beginning from the eighth layer, 10, 15, 47, 113, 343, 855, 1,892, and in the remainder 6,725 milligrams. Thus, the diffusive power of a solution depends on time and on the nature of the substance dissolved, which fact may serve, not only for explaining the process of solution, but also for distinguishing one substance from another. Graham showed that substances which rapidly diffuse through liquids are able to rapidly pass through membranes and crystallise, whilst substances which diffuse slowly and do not crystallise are _colloids_, that is, resemble glue, and penetrate through a membrane slowly, and form jellies; that is, occur in insoluble forms,[18] as will be explained in speaking of silica.

[17] The researches of Graham, Fick, Nernst, and others showed that
the quantity of a dissolved substance which is transmitted
(rises) from one stratum of liquid to another in a vertical
cylindrical vessel is not only proportional to the time and to
the sectional area of the cylinder, but also to the amount and
nature of the substance dissolved in a stratum of liquid, so that
each substance has its corresponding co-efficient of diffusion.
The cause of the diffusion of solutions must be considered
as essentially the same as the cause of the diffusion of
gases--that is, as dependent on motions which are proper to their
molecules; but here most probably those purely chemical, although
feebly-developed, forces, which incline the substances dissolved
to the formation of definite compounds, also play their part.

[18]

The rate of diffusion--like the rate of transmission--through
membranes, or _dialysis_ (which plays an important part in the
vital processes of organisms and also in technical processes),
presents, according to Graham's researches, a sharply defined
change in passing from such crystallisable substances as the
majority of salts and acids to substances which are capable
of giving jellies (gum, gelatin, &c.) The former diffuse
into solutions and pass through membranes much more rapidly
than the latter, and Graham therefore distinguishes between
_crystalloids_, which diffuse rapidly, and _colloids_, which
diffuse slowly. On breaking solid colloids into pieces, a total
absence of cleavage is remarked. The fracture of such substances
is like that of glue or glass. It is termed a 'conchoidal'
fracture. Almost all the substances of which animal and vegetable
bodies consist are colloids, and this is, at all events, partly
the reason why animals and plants have such varied forms,
which have no resemblance to the crystalline forms of the
majority of mineral substances. The colloid solid substances in
organisms--that is, in animals and plants--almost always contain
water, and take most peculiar forms, of networks, of granules,
of hairs, of mucous, shapeless masses, &c., which are quite
different from the forms taken by crystalline substances. When
colloids separate out from solutions, or from a molten state,
they present a form which is similar to that of the liquid from
which they were formed. Glass may he taken as the best example of
this. Colloids are distinguishable from crystalloids, not only
by the absence of crystalline form, but by many other properties
which admit of clearly distinguishing both these classes of
solids, as Graham showed. Nearly all colloids are capable of
passing, under certain circumstances, from a soluble into an
insoluble state. The best example is shown by white of eggs
(albumin) in the raw and soluble form, and in the hard-boiled
and insoluble form. The majority of colloids, on passing into
an insoluble form in the presence of water, give substances
having a gelatinous appearance, which is familiar to every one
in starch, solidified glue, jelly, &c. Thus gelatin, or common
carpenter's glue, when soaked in water, swells up into an
insoluble jelly. If this jelly be heated, it melts, and is then
soluble in water, but on cooling it again forms a jelly which
is insoluble in water. One of the properties which distinguish
colloids from crystalloids is that the former pass very slowly
through a membrane, whilst the latter penetrate very rapidly.
This may be shown by taking a cylinder, open at both ends, and
by covering its lower end with a bladder or with vegetable
parchment (unsized paper immersed for two or three minutes in a
mixture of sulphuric acid and half its volume of water, and then
washed), or any other membranous substance (all such substances
are themselves colloids in an insoluble form). The membrane must
be firmly tied to the cylinder, so as not to leave any opening.
Such an apparatus is called a _dialyser_ (fig. 15), and the
process of separation of crystalloids from colloids by means of
such a membrane is termed _dialysis_. An aqueous solution of a
crystalloid or colloid, or a mixture of both, is poured into the
dialyser, which is then placed in a vessel containing water, so
that the bottom of the membrane is covered with water. Then,
after a certain period of time, the crystalloid passes through
the membrane, whilst the colloid, if it does pass through at all,
does so at an incomparably slower rate. The crystalloid naturally
passes through into the water until the solution attains the same
strength on both sides of the membrane. By replacing the outside
water with fresh water, a fresh quantity of the crystalloid may
be separated from the dialyser. While a crystalloid is passing
through the membrane, a colloid remains almost entirely in the
dialyser, and therefore a mixed solution of these two kinds of
substances may be separated from each other by a dialyser. The
study of the properties of colloids, and of the phenomena of
their passage through membranes, should elucidate much respecting
the phenomena which are accomplished in organisms.

Hence, if it be desired to increase the rate of solution, recourse must be had to stirring, shaking, or some such mechanical motion. But if once a uniform solution is formed, it will remain uniform, no matter how heavy the dissolved substance is, or how long the solution be left at rest, which fact again shows the presence of a force holding together the particles of the body dissolved and of the solvent.[19]

[19] The formation of solutions may be considered in two aspects,
from a physical and from a chemical point of view, and it is
more evident in solutions than in any other department of
chemistry how closely these provinces of natural science are
allied together. On the one hand solutions form a particular
case of a physico-mechanical interpenetration of homogeneous
substances, and a juxtaposition of the molecules of the substance
dissolved and of the solvent, similar to the juxtaposition which
is exhibited in homogeneous substances. From this point of view
this diffusion of solutions is exactly similar to the diffusion
of gases, with only this difference, that the nature and store of
energy are different in gases from what they are in liquids, and
that in liquids there is considerable friction, whilst in gases
there is comparatively little. The penetration of a dissolved
substance into water is likened to evaporation, and solution to
the formation of vapour. This resemblance was clearly expressed
even by Graham. In recent years the Dutch chemist, Van't Hoff,
has developed this view of solutions in great detail, having
shown (in a memoir in the _Transactions of the Swedish Academy
of Science_, Part 21, No. 17, 'Lois de l'équilibre chimique
dans l'état dilué, gazeux ou dissous,' 1886), that for dilute
solutions the _osmotic pressure_ follows the same laws of Boyle,
Mariotte, Gay-Lussac, and Avogadro-Gerhardt as for gases. The
osmotic pressure of a substance dissolved in water is determined
by means of membranes which allow the passage of water, but not
of a substance dissolved in it, through them. This property is
found in animal protoplasmic membranes and in porous substances
covered with an amorphous precipitate, such as is obtained by the
action of copper sulphate on potassium ferrocyanide (Pfeffer,
Traube). If, for instance, a one p.c. solution of sugar he placed
in such a vessel, which is then closed and placed in water, the
water passes through the walls of the vessel and increases the
pressure by 50 mm. of the barometric column. If the pressure be
artificially increased inside the vessel, then the water will be
expelled through the walls. De Vries found a convenient means
of determining _isotonic_ solutions (those presenting a similar
osmotic pressure) in the cells of plants. For this purpose a
portion of the soft part of the leaves of the _Tradescantis
discolor_, for instance, is cut away and moistened with the
solution of a given salt and of a given strength. If the osmotic
pressure of the solution taken be less than that of the sap
contained in the cells they will change their form or shrink; if,
on the other hand, the osmotic pressure be greater than that of
the sap, then the cells will expand, as can easily be seen under
the microscope. By altering the amount of the different salts in
solution it is possible to find for each salt the strength of
solution at which the cells begin to swell, and at which they
will consequently have an equal osmotic pressure. As it increases
in proportion to the amount of a substance dissolved per 100
parts of water, it is possible, knowing the osmotic pressure
of a given substance--for instance, sugar at various degrees
of concentration of solution--and knowing the composition of
isotonic solutions compared with sugar, to determine the osmotic
pressure of all the salts investigated. The osmotic pressure of
dilute solutions determined in this manner directly or indirectly
(from observations made by Pfeffer and De Vries) was shown to
follow the same laws as those of the pressure of gases; for
instance, by doubling or increasing the quantity of a salt (in
a given volume) _n_ times, the pressure is doubled or increases
_n_ times. So, for example, in a solution containing one part
of sugar per 100 parts of water the osmotic pressure (according
to Pfeffer) = 58·5 cm. of mercury, if 2 parts of sugar = 101·6,
if 4 parts = 208·2 and so on, which proves that the ratio is
true within the limits of experimental error. (2) Different
substances for equal strengths of solutions, show very different
osmotic pressures, just as gases for equal parts by weight in
equal volumes show different tensions. (3) If, for a given dilute
solution at 0°, the osmotic pressure equal _p_°, then at _t_°
it will be greater and equal to _p_°(1 + 0·00367_t_), _i.e._ it
increases with the temperature in exactly the same manner as the
tension of gases increases. (4) If in dilute solutions of such
substances as do not conduct an electric current (for instance,
sugar, acetone, and many other organic bodies) the substances
be taken in the ratio of their molecular weights (expressed by
their formulæ, see Chapter VII.), then not only will the osmotic
pressure be equal, but its magnitude will be determined by
that tension which would be proper to the vapours of the given
substances when they would be contained in the space occupied by
the solution, just as the tension of the vapours of molecular
quantities of the given substances will be equal, and determined
by the laws of Gay-Lussac, Mariotte, and Avogadro-Gerhardt. Those
formulæ (Chapter VII., Notes 23 and 24) by which the gaseous
state of matter is determined, may also be applied in the present
case. So, for example, the osmotic pressure _p_, in centimetres
of mercury, of a one per cent. solution of sugar, may be
calculated according to the formula for gases:

M_p_ = 6200_s_(273 + _t_),

where M is the molecular weight, _s_ the weight in grams of a
cubic centimetre of vapour, and _t_ its temperature. For sugar M
= 342 (because its molecular composition is C_{12}H_{22}O_{11}).
The specific gravity of the solution of sugar is 1·003, hence the
weight of sugar _s_ contained in a 1 per cent. solution = 0·01003
gram. The observation was made at _t_ = 14°. Hence, according
to the formula, we find _p_ = 52·2 centimetres. And experiments
carried on at 14° gave 53·5 centimetres, which is very near to
the above. (5) For the solutions of salts, acids, and similar
substances, which conduct an electric current, the calculated
pressure is usually (but not always in a definite or multiple
number of times) less than the observed by _i_ times, and this
_i_ for dilute solutions of MgSO_{4} is nearly 1, for CO_{2} = 1,
for KCl, NaCl, KI, KNO_{3} greater than 1, and approximates to 2,
for BaCl_{2}, MgCl_{2}, K_{2}CO_{3}, and others between 2 and 3,
for HCl, H_{2}SO_{4}, NaNO_{3}, CaN_{2}O_{6}, and others nearly
2 and so on. It should be remarked that the above deductions are
only applicable (and with a certain degree of accuracy) to dilute
solutions, and in this respect resemble the generalisations of
Michel and Kraft (see Note 44). Nevertheless, the arithmetical
relation found by Van't Hoff between the formation of vapours
and the transition into dilute solutions forms an important
scientific discovery, which should facilitate the explanation of
the nature of solutions, while the osmotic pressure of solutions
already forms a very important aspect of the study of solutions.
In this respect it is necessary to mention that Prof. Konovaloff
(1891, and subsequently others also) discovered the dependence
(and it may be a sufficient explanation) of the osmotic pressure
upon the differences of the tensions of aqueous vapours and
aqueous solutions; this, however, already enters into a special
province of physical chemistry (certain data are given in Note
49 and following), and to this physical side of the question
also belongs one of the extreme consequences of the resemblance
of osmotic pressure to gaseous pressure, which is that the
concentration of a uniform solution varies in parts which are
heated or cooled. Soret (1881) indeed observed that a solution
of copper sulphate containing 17 parts of the salt at 20° only
contained 14 parts after heating the upper portion of the tube
to 80° for a long period of time. This aspect of solution, which
is now being very carefully and fully worked out, may be called
the _physical_ side. Its other aspect is purely _chemical_, for
solution does not take place between any two substances, but
requires a special and particular attraction or affinity between
them. A vapour or gas permeates any other vapour or gas, but a
salt which dissolves in water may not be in the least soluble
in alcohol, and is quite insoluble in mercury. In considering
solutions as a manifestation of chemical force (and of chemical
energy), it must be acknowledged that they are here developed to
so feeble an extent that the definite compounds (that is, those
formed according to the law of multiple proportions) formed
between water and a soluble substance dissociate even at the
ordinary temperature, forming a homogeneous system--that is,
one in which both the compound and the products into which it
decomposes (water and the aqueous compound) occur in a liquid
state. The chief difficulty in the comprehension of solutions
depends on the fact that the mechanical theory of the structure
of liquids has not yet been so fully developed as the theory of
gases, and solutions are liquids. The conception of solutions
as liquid dissociated definite chemical compounds is based on
the following considerations: (1) that there exist certain
undoubtedly definite chemical crystallised compounds (such as
H_{2}SO_{4},H_{2}O; or NaCl,2H_{2}O; or CaCl_{2},6H_{2}O; &c.)
which melt on a certain rise of temperature, and then form true
solutions; (2) that metallic alloys in a molten condition are
real solutions, but on cooling they often give entirely distinct
and definite crystallised compounds, which are recognised by
the properties of alloys; (3) that between the solvent and the
substance dissolved there are formed, in a number of cases,
many undoubtedly definite compounds, such as compounds with
water of crystallisation; (4) that the physical properties of
solutions, and especially their specific gravities (a property
which can be very accurately determined), vary with a change in
composition, and in such a manner as would be required by the
formation of one or more definite but dissociating compounds.
Thus, for example, on adding water to fuming sulphuric acid its
density is observed to decrease until it attains the definite
composition H_{2}SO_{4}, or SO_{3} + H_{2}O, when the specific
gravity increases, although on further diluting with water it
again falls. Moreover (Mendeléeff, _The Investigation of Aqueous
Solutions from their Specific Gravities_, 1887), the increase
in specific gravity (_ds_), varies in all well-known solutions
with the proportion of the substance dissolved (_dp_), and this
dependence can be expressed by a formula (_ds_/_dp_ = A + B_p_)
between the limits of definite compounds whose existence in
solutions must be admitted, and this is in complete accordance
with the dissociation hypothesis. Thus, for instance, from
H_{2}SO_{4} to H_{2}SO_{4} + H_{2}O (both these substances exist
as definite compounds in a free state), the fraction _ds_/_dp_
= 0·0729-0·000749_p_ (where _p_ is the percentage amount of
H_{2}SO_{4}). For alcohol C_{2}H_{6}O, whose aqueous solutions
have been more accurately investigated than all others, the
definite compound C_{2}H_{6}O + 3H_{2}O, and others must be
acknowledged in its solutions.

The two aspects of solution above mentioned, and the hypotheses
which have as yet been applied to the examination of solutions,
although they have somewhat different starting points, will
doubtless in time lead to a general theory of solutions,
because the same common laws govern both physical and chemical
phenomena, inasmuch as the properties and motions of molecules,
which determine physical properties, depend on the motions and
properties of atoms, which determine chemical reactions. For
details of the questions dealing with theories of solution,
recourse must now be had to special memoirs and to works on
theoretical (physical) chemistry; for this subject forms one of
special interest at the present epoch of the development of our
science. In working out chiefly the chemical side of solutions, I
consider it to be necessary to reconcile the two aspects of the
question; this seems to me to be all the more possible, as the
physical side is limited to dilute solutions only, whilst the
chemical side deals mainly with strong solutions.

In the consideration of the process of solution, besides the conception of diffusion, another fundamental conception is necessary--namely, that of the _saturation of solutions_.

Just as moist air may be diluted with any desired quantity of dry air, so also an indefinitely large quantity of a liquid solvent may be taken, and yet a uniform solution will be obtained. But more than a definite quantity of aqueous vapour cannot be introduced into a certain volume of air at a certain temperature. The excess above the point of saturation will remain in the liquid state.[20] The relation between water and substances dissolved in it is similar. More than a definite quantity of a substance cannot, at a certain temperature, dissolve in a given quantity of water; the excess does not unite with the water. Just as air or a gas becomes saturated with vapour, so water becomes saturated with a substance dissolved in it. If an excess of a substance be added to water which is already saturated with it, it will remain in its original state, and will not diffuse through the water. The quantity of a substance (either by volume with gases, or by weight with solids and liquids) which is capable of saturating 100 parts of water is called the _co-efficient of solubility_ or the _solubility_. In 100 grams of water at 15°, there can be dissolved not more than 35·86 grams of common salt. Consequently, its solubility at 15° is equal to 35·86.[21] It is most important to turn attention to the _existence of the solid insoluble substances of nature_, because on them depends the shape of the substances of the earth's surface, and of plants and animals. There is so much water on the earth's surface, that were the surface of substances formed of soluble matters it would constantly change, and however substantial their forms might be, mountains, river banks and sea shores, plants and animals, or the habitations and coverings of men, could not exist for any length of time.[22]

[20] A system of (chemically or physically) re-acting substances in
different states of aggregation--for instance, some solid, others
liquid or gaseous--is termed a heterogeneous system. Up to now it
is only systems of this kind which can be subjected to detailed
examination in the sense of the mechanical theory of matter.
Solutions (_i.e._ unsaturated ones) form fluid homogeneous
systems, which at the present time can only be investigated with
difficulty.

In the case of limited solution of liquids in liquids, _the
difference between the solvent and the substance dissolved_ is
clearly seen. The former (that is, the solvent) may be added in
an unlimited quantity, and yet the solution obtained will always
be uniform, whilst only a definite saturating proportion of the
substance dissolved can be taken, We will take water and common
(sulphuric) ether. On shaking the ether with the water, it will
be remarked that a portion of it dissolves in the water. If the
ether be taken in such a quantity that it saturates the water
and a portion of it remains undissolved, then this remaining
portion will act as a solvent, and water will diffuse through it
and also form a saturated solution of water in the ether taken.
Thus two saturated solutions will be obtained. One solution
will contain ether dissolved in water, and the other solution
will contain water dissolved in ether. These two solutions will
arrange themselves in two layers, according to their density;
the ethereal solution of water will be on the top. If the upper
ethereal solution be poured off from the aqueous solution,
any quantity of ether may be added to it; this shows that the
dissolving substance is ether. If water be added to it, it is
no longer dissolved in it; this shows that water saturates the
ether--here water is the substance dissolved. If we act in the
same manner with the lower layer, we shall find that water is the
solvent and ether the substance dissolved. By taking different
amounts of ether and water, the degree of solubility of ether in
water, and of water in ether, may be easily determined. Water
approximately dissolves 1/10 of its volume of ether, and ether
dissolves a very small quantity of water. Let us now imagine that
the liquid poured in dissolves a considerable amount of water,
and that water dissolves a considerable amount of the liquid. Two
layers could not be formed, because the saturated solutions would
resemble each other, and therefore they would intermix in all
proportions. This is, consequently, a case of a phenomenon where
two liquids present considerable co-efficients of solubility
in each other, but where it is impossible to say what these
co-efficients are, because it is impossible to obtain a saturated
solution.

[21] The solubility, or co-efficient of solubility, of a substance is
determined by various methods. Either a solution is expressly
prepared with a clear excess of the soluble substance and
saturated at a given temperature, and the quantity of water and
of the substance dissolved in it determined by evaporation,
desiccation, or other means; or else, as is done with gases,
definite quantities of water and of the soluble substance are
taken and the amount remaining undissolved is determined.

The solubility of a gas in water is determined by means of an
apparatus called an _absorptiometer_ (fig. 16). It consists of an
iron stand _f_, on which an india-rubber ring rests. A wide
glass tube is placed on this ring, and is pressed down on it by
the ring _h_ and the screws _i i_. The tube is thus firmly fixed
on the stand. A cock _r_, communicating with a funnel _r_, passes
into the lower part of the stand. Mercury can be poured into the
wide tube through this funnel, which is therefore made of steel,
as copper would be affected by the mercury. The upper ring _h_
is furnished with a cover _p_, which can be firmly pressed down
on to the wide tube, and hermetically closes it by means of an
india-rubber ring. The tube _r r_ can be raised at will, and
so by pouring mercury into the funnel the height of the column
of mercury, which produces pressure inside the apparatus, can
be increased. The pressure can also be diminished at will, by
letting mercury out through the cock _r_. A graduated tube _e_,
containing the gas and liquid to be experimented on, is placed
inside the wide tube. This tube is graduated in millimetres
for determining the pressure, and it is calibrated for volume,
so that the number of volumes occupied by the gas and liquid
dissolving it can be easily calculated. This tube can also be
easily removed from the apparatus. The lower portion of this
tube when removed from the apparatus is shown to the right of
the figure. It will be observed that its lower end is furnished
with a male screw _b_, fitting in a nut _a_. The lower surface of
the nut _a_ is covered with india-rubber, so that on screwing up
the tube its lower end presses upon the india-rubber, and thus
hermetically closes the whole tube, for its upper end is fused
up. The nut _a_ is furnished with arms _c c_, and in the stand
_f_ there are corresponding spaces, so that when the screwed-up
internal tube is fixed into stand _f_, the arms _c c_ fix into
these spaces cut in _f_. This enables the internal tube to be
fixed on to the stand _f_. When the internal tube is fixed in the
stand, the wide tube is put into its right position, and mercury
and water are poured into the space between the two tubes, and
communication is opened between the inside of the tube _e_ and
the mercury between the interior and exterior tubes. This is done
by either revolving the interior tube _e_, or by a key turning
the nut about the bottom part of _f_. The tube _e_ is filled with
gas and water as follows: the tube is removed from the apparatus,
filled with mercury, and the gas to be experimented on is passed
into it. The volume of the gas is measured, the temperature and
pressure determined, and the volume it would occupy at 0° and
760 mm. calculated. A known volume of water is then introduced
into the tube. The water must be previously boiled, so as to be
quite freed from air in solution. The tube is then closed by
screwing it down on to the india-rubber on the nut. It is then
fixed on to the stand _f_, mercury and water are poured into
the intervening space between it and the exterior tube, which
is then screwed up and closed by the cover _p_, and the whole
apparatus is left at rest for some time, so that the tube _e_,
and the gas in it, may attain the same temperature as that of
the surrounding water, which is marked by a thermometer _k_
tied to the tube _e_. The interior tube is then again closed by
turning it in the nut, the cover _p_ again shut, and the whole
apparatus is shaken in order that the gas in the tube _e_ may
entirely saturate the water. After several shakings, the tube
_e_ is again opened by turning it in the nut, and the apparatus
is left at rest for a certain time; it is then closed and again
shaken, and so on until the volume of gas does not diminish after
a fresh shaking--that is, until saturation ensues. Observations
are then made of the temperature, the height of the mercury in
the interior tube, and the level of the water in it, and also
of the level of the mercury and water in the exterior tube. All
these data are necessary in order to calculate the pressure under
which the solution of the gas takes place, and what volume of gas
remains undissolved, and also the quantity of water which serves
as the solvent. By varying the temperature of the surrounding
water, the amount of gas dissolved at various temperatures may
be determined. Bunsen, Carius, and many others determined the
solution of various gases in water, alcohol, and certain other
liquids, by means of this apparatus. If in a determination of
this kind it is found that _n_ cubic centimetres of water at
a pressure _h_ dissolve _m_ cubic centimetres of a given gas,
measured at 0° and 760 mm., when the temperature under which
solution took place was _t_°, then it follows that at the
temperature _t the co-efficient of solubility of the gas_ in 1
volume of the liquid will be equal to _m_/_n_ × 760/_h_.