FUSELI, HENRY (1741-1825), English painter and writer on art, of
German-Swiss family, was born at Zurich in Switzerland on the 7th of
February 1741; he himself asserted in 1745, but this appears to have
been a mere whim. He was the second child in a family of eighteen. His
father was John Caspar Fussli, of some note as a painter of portraits
and landscapes, and author of Lives of the _Helvetic Painters_. This
parent destined his son for the church, and with this view sent him to
the Caroline college of his native town, where he received an excellent
classical education. One of his schoolmates there was Lavater, with whom
he formed an intimate friendship.
After taking orders in 1761 Fuseli was obliged to leave his country for
a while in consequence of having aided Lavater to expose an unjust
magistrate, whose family was still powerful enough to make its vengeance
felt. He first travelled through Germany, and then, in 1765, visited
England, where he supported himself for some time by miscellaneous
writing: there was a sort of project of promoting through his means a
regular literary communication between England and Germany. He became in
course of time acquainted with Sir Joshua Reynolds, to whom he showed
his drawings. By Sir Joshua's advice he then devoted himself wholly to
art. In 1770 he made an art-pilgrimage to Italy, where he remained till
1778, changing his name from Fussli to Fuseli, as more Italian-sounding.
Early in 1779 he returned to England, taking Zurich on his way. He found
a commission awaiting him from Alderman Boydell, who was then organizing
his celebrated Shakespeare gallery. Fuseli painted a number of pieces
for this patron, and about this time published an English edition of
Lavater's work on physiognomy. He likewise gave Cowper some valuable
assistance in preparing the translation of Homer. In 1788 Fuseli married
Miss Sophia Rawlins (who it appears was originally one of his models,
and who proved an affectionate wife), and he soon after became an
associate of the Royal Academy. Two years later he was promoted to the
grade of Academician. In 1799 he exhibited a series of paintings from
subjects furnished by the works of Milton, with a view to forming a
Milton gallery corresponding to Boydell's Shakespeare gallery. The
number of the Milton paintings was forty-seven, many of them very large;
they were executed at intervals within nine years. This exhibition,
which closed in 1800, proved a failure as regards profit. In 1799 also
he was appointed professor of painting to the Academy. Four years
afterwards he was chosen keeper, and resigned his professorship; but he
resumed it in 1810, and continued to hold both offices till his death.
In 1805 he brought out an edition of Pilkington's _Lives of the
Painters_, which, however, did not add much to his reputation. Canova,
when on his visit to England, was much taken with Fuseli's works, and on
returning to Rome in 1817 caused him to be elected a member of the first
class in the Academy of St Luke. Fuseli, after a life of uninterrupted
good health, died at Putney Hill on the 16th of April 1825, at the
advanced age of eighty-four, and was buried in the crypt of St Paul's
cathedral. He was comparatively rich at his death, though his
professional gains had always appeared to be meagre.
As a painter, Fuseli had a daring invention, was original, fertile in
resource, and ever aspiring after the highest forms of excellence. His
mind was capable of grasping and realizing the loftiest conceptions,
which, however, he often spoiled on the canvas by exaggerating the due
proportions of the parts, and throwing his figures into attitudes of
fantastic and over-strained contortion. He delighted to select from the
region of the supernatural, and pitched everything upon an ideal scale,
believing a certain amount of exaggeration necessary in the higher
branches of historical painting. "Damn Nature! she always puts me out,"
was his characteristic exclamation. In this theory he was confirmed by
the study of Michelangelo's works and the marble statues of the Monte
Cavallo, which, when at Rome, he used often to contemplate in the
evening, relieved against a murky sky or illuminated by lightning. But
this idea was by him carried out to an excess, not only in the forms,
but also in the attitudes of his figures; and the violent and
intemperate action which he often displays destroys the grand effect
which many of his pieces would otherwise produce. A striking
illustration of this occurs in his famous picture of "Hamlet breaking
from his Attendants to follow the Ghost": Hamlet, it has been said,
looks as though he would burst his clothes with convulsive cramps in all
his muscles. This intemperance is the grand defect of nearly all
Fuseli's compositions. On the other hand, his paintings are never either
languid or cold. His figures are full of life and earnestness, and seem
to have an object in view which they follow with rigid intensity. Like
Rubens he excelled in the art of setting his figures in motion. Though
the lofty and terrible was his proper sphere, Fuseli had a fine
perception of the ludicrous. The grotesque humour of his fairy scenes,
especially those taken from _A Midsummer-Night's Dream_, is in its way
not less remarkable than the poetic power of his more ambitious works.
As a colourist Fuseli has but small claims to distinction. He scorned to
set a palette as most artists do; he merely dashed his tints recklessly
over it. Not unfrequently he used his paints in the form of a dry
powder, which he rubbed up with his pencil with oil, or turpentine, or
gold size, regardless of the quantity, and depending for accident on the
general effect. This recklessness may perhaps be explained by the fact
that he did not paint in oil till he was twenty-five years of age.
Despite these drawbacks he possessed the elements of a great painter.
Fuseli painted more than 200 pictures, but he exhibited only a minority
of them. His earliest painting represented "Joseph interpreting the
Dreams of the Baker and Butler"; the first to excite particular
attention was the "Nightmare," exhibited in 1782. He produced only two
portraits. His sketches or designs numbered about 800; they have
admirable qualities of invention and design, and are frequently superior
to his paintings.
His general powers of mind were large. He was a thorough master of
French, Italian, English and German, and could write in all these
tongues with equal facility and vigour, though he preferred German as
the vehicle of his thoughts. His writings contain passages of the best
art-criticism that English literature can show. The principal work is
his series of _Lectures_ in the Royal Academy, twelve in number,
commenced in 1801.
Many interesting anecdotes of Fuseli, and his relations to
contemporary artists, are given in his _Life_ by John Knowles, who
also edited his works in 3 vols. 8vo, London, 1831. (W. M. R.)
FUSEL OIL (from the Ger. _Fusel_, bad spirits), the name applied to the
volatile oily liquids, of a nauseous fiery taste and smell, which are
obtained in the rectification of spirituous liquors made by the
fermentation of grain, potatoes, the marc of grapes, and other
material, and which, as they are of higher boiling point than ethyl
alcohol, occur in largest quantity in the last portions of the
distillate. Besides ethyl or ordinary alcohol, and amyl alcohol, which
are present in them all, there have been found in fusel oil several
other bodies of the C_n H_(2n + 1).OH series, also certain ethers, and
members of the C_n H_(2n + 1).CO2H series of fatty acids. Normal propyl
alcohol is contained in the fusel oil of the marc brandy of the south of
France, and isoprimary butyl alcohol in that of beet-root molasses. The
chief constituent of the fusel oil procured in the manufacture of
alcohol from potatoes and grain, usually known as fusel oil and
potato-spirit, is isoprimary amyl alcohol, or isobutylcarbinol. Ordinary
fusel oil yields also an isomeric amyl alcohol (active amyl alcohol)
boiling at about 128 deg. Variable quantities of fusel oil, less or
greater according to the stage of ripening, exist in commercial spirits
(see SPIRITS).
Fusel oil and its chief constituent, amyl alcohol, are direct nerve
poisons. In small doses it causes only thirst and headache, with furred
tongue and some excitement. In large doses it is a convulsent poison.
Impure beverages induce all the graver neurotic and visceral disorders
in alcoholism; and, like fusel oil, furfurol and the essence of
absinthe, are convulsent poisons. Pure ethyl alcohol intoxication,
indeed, is rarely seen, being modified in the case of spirits by the
higher alcohols contained in fusel oil. According to Rabuteau the toxic
properties of the higher alcohols increase with their molecular weight
and boiling point. Richet considers that the fusel oil contained in
spirits constitutes the chief danger in the consumption of alcoholic
beverages. The expert can immediately detect the peculiarly virulent
characters of the mixed intoxication due to the consumption of spirits
containing a large percentage of fusel oil.
FUSIBLE METAL, a term applied to certain alloys, generally composed of
bismuth, lead and tin, which possess the property of melting at
comparatively low temperatures. Newton's fusible metal (named after Sir
Isaac Newton) contains 50 parts of bismuth, 31.25 of lead and 18.75 of
tin; that of Jean Darcet (1725-1801), 50 parts of bismuth with 25 each
of lead and tin; and that of Valentin Rose the elder, 50 of bismuth with
28.1 of lead and 24.1 of tin. These melt between 91 deg. and 95 deg. C.
The addition of cadmium gives still greater fusibility; in Wood's metal,
for instance, which is Darcet's metal with half the tin replaced by
cadmium, the melting point is lowered to 66 deg.-71 deg. C.; while
another described by Lipowitz and containing 15 parts of bismuth, 8 of
lead, 4 of tin and 3 of cadmium, softens at about 55 deg. and is
completely liquid a little above 60 deg. By the addition of mercury to
Darcet's metal the melting point may be reduced so low as 45 deg. These
fusible metals have the peculiarity of expanding as they cool; Rose's
metal, for instance, remains pasty for a considerable range of
temperature below its fusing point, contracts somewhat rapidly from 80
deg. to 55 deg., expands from 55 deg. to 35 deg., and contracts again
from 35 deg. to 0 deg. For this reason they may be used for taking casts
of anatomical specimens or making _cliches_ from wood-blocks, the
expansion on cooling securing sharp impressions. By suitable
modification in the proportions of the components, a series of alloys
can be made which melt at various temperatures above the boiling point
of water; for example, with 8 parts of bismuth, 8 of lead and 3 of tin
the melting point is 123 deg., and with 8 of bismuth, 30 of lead and 24
of tin it is 172 deg. With tin and lead only in equal proportions it is
241 deg. Such alloys are used for making the fusible plugs inserted in
the furnace-crowns of steam boilers, as a safeguard in the event of the
water-level being allowed to fall too low. When this happens the plug
being no longer covered with water is heated to such a temperature that
it melts and allows the contents of the boiler to escape into the
furnace. In automatic fire-sprinklers the orifices of the pipes are
closed with fusible metal, which melts and liberates the water when,
owing to an outbreak of fire in the room, the temperature rises above a
predetermined limit.
FUSILIER, originally (in French about 1670, in English about 1680) the
name of a soldier armed with a light flintlock musket called the fusil;
now a regimental designation. Various forms of flintlock small arms had
been used in warfare since the middle of the 16th century. At the time
of the English civil war (1642-1652) the term "firelock" was usually
employed to distinguish these weapons from the more common matchlock
musket. The special value of the firelock in armies of the 17th century
lay in the fact that the artillery of the time used open powder barrels
for the service of the guns, making it unsafe to allow lighted matches
in the muskets of the escort. Further, a military escort was required,
not only for the protection, but also for the surveillance of the
artillerymen of those days. Companies of "firelocks" were therefore
organized for these duties, and out of these companies grew the
"fusiliers" who were employed in the same way in the wars of Louis XIV.
In the latter part of the Thirty Years' War (1643) fusiliers were simply
mounted troops armed with the fusil, as carabiniers were with the
carbine. But the escort companies of artillery came to be known by the
name shortly afterwards, and the regiment of French Royal Fusiliers,
organized in 1671 by Vauban, was considered the model for Europe. The
general adoption of the flintlock musket and the suppression of the pike
in the armies of Europe put an end to the original special duties of
fusiliers, and they were subsequently employed to a large extent in
light infantry work, perhaps on account of the greater individual
aptitude for detached duties naturally shown by soldiers who had never
been restricted to a fixed and unchangeable place in the line of battle.
The senior fusilier regiment in the British service, the (7th) Royal
Fusiliers (City of London Regiment), was formed on the French model in
1685; the 5th foot (now Northumberland Fusiliers), senior to the 7th in
the army, was not at that time a fusilier regiment. The distinctive
head-dress of fusiliers in the British service is a fur cap, generally
resembling, but smaller than and different in details from, that of the
Foot Guards.
In Germany the name "fusilier" is borne by certain infantry regiments
and by one battalion in each grenadier regiment.
FUSION, the term generally applied to the melting of a solid substance,
or the change of state of aggregation from the solid to the liquid. The
term "liquefaction" is frequently employed in the same sense, but is
often restricted to the condensation of a gas or vapour. The converse
process of freezing or solidification, the change from the liquid to the
solid state, is subject to the same laws, and must be considered
together with fusion. The solution of a solid in a foreign liquid, and
the deposition or crystallization of a solid from a solution, are so
closely related to the fusion of a pure substance, that it will also be
necessary to consider some of the analogies which they present.
1. _General Phenomena._--There are two chief varieties of the process of
fusion, namely, crystalline and amorphous, which are in many ways
distinct, although it is possible to find intermediate cases which
partake of the characteristics of both. The melting of ice may be taken
as a typical case of crystalline fusion. The passage from rigid solid to
mobile liquid occurs at a definite surface without any intermediate
stage or plastic condition. The change takes place at a definite
temperature, the fusing or freezing point (abbreviated F.P.), and
requires the addition of a definite quantity of heat to the solid, which
is called the latent heat of fusion. There is also in general a
considerable change of volume during fusion, which amounts in the case
of ice to a contraction of 9%. Typical cases of amorphous solidification
are those of silica, glass, plastic sulphur, pitch, alcohol and many
organic liquids. In this type the liquid gradually becomes more and more
viscous as the temperature falls, and ultimately attains the rigidity
characteristic of a solid, without any definite freezing point or latent
heat. The condition of the substance remains uniform throughout, if its
temperature is uniform; there is no separation into the two distinct
phases of solid and liquid, and there is no sudden change of volume at
any temperature.
A change or transition from one crystalline form to another may occur in
the solid state with evolution or absorption of heat at a definite
temperature, and is analogous to the change from solid to liquid, but
usually takes place more slowly owing to the small molecular mobility of
the solid state. Thus rhombic sulphur when heated passes slowly at 95.6
deg. C. into the monosymmetric form which melts at 120 deg., but if
heated rapidly the rhombic form melts at 114.5. The two forms, rhombic
and monosymmetric, can exist in equilibrium at 95.6 deg., the transition
point at which they have the same vapour pressure. Similarly a solid
solution of carbon in iron, when cooled slowly, passes at about 700 deg.
C., with considerable evolution of heat, into the form of "pearlite,"
which is soft when cold, but if rapidly chilled the carbon remains in
solution and the steel is very hard (see also ALLOYS).
In the case of crystalline fusion it is necessary to distinguish two
cases, the homogeneous and the heterogeneous. In the first case the
composition of the solid and liquid phases are the same, and the
temperature remains constant during the whole process of fusion. In the
second case the solid and liquid phases differ in composition; that of
the liquid phase changes continuously, and the temperature does not
remain constant during the fusion. The first case comprises the fusion
of pure substances, and that of eutectics, or cryohydrates; the second
is the general case of an alloy or a solution. These have been very
fully studied and their phenomena greatly elucidated in recent years.
There is also a sub-variety of amorphous fusion, which may be styled
colloid or gelatinous, and may be illustrated by the behaviour of
solutions of water in gelatin. Many of these jellies melt at a fairly
definite temperature on heating, and coagulate or set at a definite
temperature on cooling. But in some cases the process is not reversible,
and there is generally marked hysteresis, the temperature of setting and
other phenomena depending on the rate of cooling. This case has not yet
been fully worked out; but it appears probable that in many cases the
jelly possesses a spongy framework of solid, holding liquid in its
meshes or interstices. It might be regarded as a case of "heterogeneous"
amorphous fusion, in which the liquid separates into two phases of
different composition, one of which solidifies before the other. The two
phases cannot, as a rule, be distinguished optically, but it is
generally possible to squeeze out some of the liquid phase when the
jelly has set, which proves that the substance is not really
homogeneous. In very complicated mixtures, such as acid lavas or slags
containing a large proportion of silica, amorphous and crystalline
solidification may occur together. In this case the crystals separate
first during the process of cooling, the mother liquor increases
gradually in viscosity, and finally sets as an amorphous ground-mass or
matrix, in which crystals of different kinds and sizes, formed at
different stages of the cooling, remain embedded. The formation of
crystals in an amorphous solid after it has set is also of frequent
occurrence. It is termed devitrification, but is a very slow process
unless the solid is in a plastic state.
2. _Homogeneous Crystalline Fusion._--The fusion of a solid of this type
is characterized most clearly by the perfect constancy of temperature
during the process. In fact, the law of constant temperature, which is
generally stated as the first of the so-called "laws of fusion," does
not strictly apply except to this case. The constancy of the F.P. of a
pure substance is so characteristic that change of the F.P. is often one
of the most convenient tests of the presence of foreign material. In the
case of substances like ice, which melt at a low temperature and are
easily obtained in large quantities in a state of purity, the point of
fusion may be very accurately determined by observing the temperature of
an intimate mixture of the solid and liquid while slowly melting as it
absorbs heat from surrounding bodies. But in the majority of cases it is
more convenient to observe the freezing point as the liquid is cooled.
By this method it is possible to ensure perfect uniformity of
temperature throughout the mass by stirring the liquid continuously
during the process of freezing, whereas it is difficult to ensure
uniformity of temperature in melting a solid, however gradually the heat
is supplied, unless the solid can be mixed with the liquid. It is also
possible to observe the F.P. in other ways, as by noting the temperature
at the moment of the breaking of a wire, of the stoppage of a stirrer,
or of the maximum rate of change of volume, but these methods are
generally less certain in their indications than the point of greatest
constancy of temperature in the case of homogeneous crystalline solids.
_Fusing Points of Common Metals_
Mercury -38.8 deg. Antimony 630 deg.
Potassium 62.5 deg. Aluminium 655 deg.
Sodium 95.6 deg. Silver 962 deg.
Tin 231.9 deg. Gold 1064 deg.
Bismuth 269.2 deg. Copper 1082 deg.
Cadmium 320.7 deg. Nickel 1427 deg.
Lead 327.7 deg. Palladium 1535 deg.
Zinc 419.0 deg. Platinum 1710 deg.
The above table contains some of the most recent values of fusing points
of metals determined (except the first three and the last three) with
platinum thermometers. The last three values are those obtained by
extrapolation with platinum-rhodium and platinum-iridium couples. (See
Harker, _Proc. Roy. Soc._ A 76, p. 235, 1905.) Some doubt has recently
been raised with regard to the value for platinum, which is much lower
than that previously accepted, namely 1775 deg.
3. _Superfusion, Supersaturation._--It is generally possible to cool a
liquid several degrees below its normal freezing point without a
separation of crystals, especially if it is protected from agitation,
which would assist the molecules to rearrange themselves. A liquid in
this state is said to be "undercooled" or "superfused." The phenomenon
is even more familiar in the case of solutions (e.g. sodium sulphate or
acetate) which may remain in the "metastable" condition for an
indefinite time if protected from dust, &c. The introduction into the
liquid under this condition of the smallest fragment of the crystal,
with respect to which the solution is supersaturated, will produce
immediate crystallization, which will continue until the temperature is
raised to the saturation point by the liberation of the latent heat of
fusion. The constancy of temperature at the normal freezing point is due
to the equilibrium of exchange existing between the liquid and solid.
Unless both solid and liquid are present, there is no condition of
equilibrium, and the temperature is indeterminate.
It has been shown by H.A. Miers (_Jour. Chem. Soc._, 1906, 89, p. 413)
that for a supersaturated solution in metastable equilibrium there is an
inferior limit of temperature, at which it passes into the "labile"
state, i.e. spontaneous crystallization occurs throughout the mass in a
fine shower. This seems to be analogous to the fine misty condensation
which occurs in a supersaturated vapour in the absence of nuclei (see
VAPORIZATION) when the supersaturation exceeds a certain limit.
4. _Effect of Pressure on the F.P._--The effect of pressure on the
fusing-point depends on the change of volume during fusion. Substances
which expand on _freezing_, like ice, have their freezing points
lowered by increase of pressure; substances which expand on _fusing_,
like wax, have their melting points raised by pressure. In each case
the effect of pressure is to retard increase of volume. This effect
was first predicted by James Thomson on the analogy of the effect of
pressure on the boiling point, and was numerically verified by Lord
Kelvin in the case of ice, and later by Bunsen in the case of paraffin
and spermaceti. The equation by which the change of the F.P. is
calculated may be proved by a simple application of the Carnot cycle,
exactly as in the case of vapour and liquid. (See THERMODYNAMICS.) If
L be the latent heat of fusion in mechanical units, v' the volume of
unit mass of the solid, and v" that of the liquid, the work done in
an elementary Carnot cycle of range d[theta] will be dp(v"- v'), if
dp is the increase of pressure required to produce a change d[theta]
in the F.P. Since the ratio of the work-difference or cycle-area to
the heat-transferred L must be equal to d[theta]/[theta], we have the
relation
d[theta]/dp = [theta](v" - v')/L. (1)
The sign of d[theta], the change of the F.P., is the same as that of
the change of volume (v" - v'). Since the change of volume seldom
exceeds 0.1 c.c. per gramme, the change of the F.P. per atmosphere is
so small that it is not as a rule necessary to take account of
variations of atmospheric pressure in observing a freezing point. A
variation of 1 cm. in the height of the barometer would correspond to
a change of .0001 deg. C. only in the F.P. of ice. This is far beyond
the limits of accuracy of most observations. Although the effect of
pressure is so small, it produces, as is well known, remarkable
results in the motion of glaciers, the moulding and regelation of ice,
and many other phenomena. It has also been employed to explain the
apparent inversion of the order of crystallization in rocks like
granite, in which the arrangement of the crystals indicates that the
quartz matrix solidified subsequently to the crystals of felspar,
mica or hornblende embedded in it, although the quartz has a higher
melting point. It is contended that under enormous pressure the
freezing points of the more fusible constituents might be raised above
that of the quartz, if the latter is less affected by pressure. Thus
Bunsen found the F.P. of paraffin wax 1.4 deg. C. below that of
spermaceti at atmospheric pressure. At 100 atmospheres the two melted
at the same temperature. At higher pressures the paraffin would
solidify first. The effect of pressure on the silicates, however, is
much smaller, and it is not so easy to explain a change of several
hundred degrees in the F.P. It seems more likely in this particular
case that the order of crystallization depends on the action of
superheated water or steam at high temperatures and pressures, which
is well known to exert a highly solvent and metamorphic action on
silicates.
5. _Variation of Latent Heat._--C.C. Person in 1847 endeavoured to
show by the application of the first law of thermodynamics that the
increase of the latent heat per degree should be equal to the
difference (s"- s') between the specific heats of the liquid and
solid. If, for instance, water at 0 deg. C. were first frozen and then
cooled to -t deg. C., the heat abstracted per gramme would be (L'+
s't) calories. But if the water were first cooled to -t deg. C., and
then frozen at -t deg.C., by abstracting heat L", the heat abstracted
would be L" + s"t. Assuming that the heat abstracted should be the
same in the two cases, we evidently obtain L'- L" = (s"- s')t. This
theory has been approximately verified by Petterson, by observing the
freezing of a liquid cooled below its normal F.P. (_Jour. Chem. Soc._
24, p. 151). But his method does not represent the true variation of
the latent heat with temperature, since the freezing, in the case of a
superfused liquid, really takes place at the normal freezing point. A
quantity of heat s"t is abstracted in cooling to -t, (L"- s"t) in
raising to 0 deg. and freezing at 0 deg., and s't in cooling the ice
to -t. The latent heat L" at -t does not really enter into the
experiment. In order to make the liquid freeze at a different
temperature, it is necessary to subject it to pressure, and the effect
of the pressure on the latent heat cannot be neglected. The entropy of
a liquid [phi]" at its F.P. reckoned from any convenient zero [phi]0
in the solid state may be represented by the expression
_
/
[phi]" - [phi]0 = | s'd[theta] / [theta] + L / [theta]. (2)
_/
Since [theta]d[phi]"/d[theta] = s", we obtain by differentiation the
relation
dL / d[theta] = s"- s' + L / [theta], (3)
which is exactly similar to the equation for the specific heat of a
vapour maintained in the saturated condition. If we suppose that the
specific heats s' and s'' of the solid and liquid at equilibrium
pressure are nearly the same as those ordinarily observed at constant
pressure, the relation (3) differs from that of Person only by the
addition of the term L/[theta]. Since s" is greater than s' in all
cases hitherto investigated, and L/[theta] is necessarily positive, it
is clear that the latent heat of fusion must increase with rise of
temperature, or diminish with fall of temperature. It is possible to
imagine the F.P. so lowered by pressure (positive or negative) that
the latent heat should vanish, in which case we should probably obtain
a continuous passage from the liquid to the solid state similar to
that which occurs in the case of amorphous substances. According to
equation (3), the rate of change of the latent heat of water is
approximately 0.80 calorie per degree at 0 deg. C. (as compared with
0.50, Person), if we assume s" = 1, and s' = 0.5. Putting (s" - s') =
0.5 in equation (2), we find L = 0 at -160 deg. C. approximately, but
no stress can be laid on this estimate, as the variation of (s" - s')
is so uncertain.
6. _Freezing of Solutions and Alloys._--The phenomena of freezing of
heterogeneous crystalline mixtures may be illustrated by the case of
aqueous solutions and of metallic solutions or alloys, which have been
most widely studied. The usual effect of an impurity, such as salt or
sugar in solution in water, is to lower the freezing point, so that no
crystallization occurs until the temperature has fallen below the normal
F.P. of the pure solvent, the depression of F.P. being nearly
proportional to the concentration of the solution. When freezing begins,
the solvent generally separates out from the solution in the pure state.
This separation of the solvent involves an increase in the strength of
the remaining solution, so that the temperature does not remain constant
during the freezing, but continues to fall as more of the solvent is
separated. There is a perfectly definite relation between temperature
and concentration at each stage of the process, which may be represented
in the form of a curve as AC in fig. 1, called the freezing point curve.
The equilibrium temperature, at the surface of contact between the solid
and liquid, depends only on the composition of the liquid phase and not
at all on the quantity of solid present. The abscissa of the F.P. curve
represents the composition of that portion of the original solution
which remains liquid at any temperature. If instead of starting with a
dilute solution we start with a strong solution represented by a point
N, and cool it as shown by the vertical line ND, a point D is generally
reached at which the solution becomes "saturated." The dissolved
substance or "solute" then separates out as the solution is further
cooled, and the concentration diminishes with fall of temperature in a
definite relation, as indicated by the curve CB, which is called the
solubility curve. Though often called by different names, the two curves
AC and CB are essentially of a similar nature. To take the case of an
aqueous solution of salt as an example, along CB the solution is
saturated with respect to salt, along AC the solution is saturated with
respect to ice. When the point C is reached along either curve, the
solution is saturated with respect to both salt and ice. The
concentration cannot vary further, and the temperature remains constant,
while the salt and ice crystallize out together, maintaining the exact
proportions in which they exist in the solution. The resulting solid was
termed a cryohydrate by F. Guthrie, but it is really an intimate mixture
of two kinds of crystals, and not a chemical compound or hydrate
containing the constituents in chemically equivalent proportions. The
lowest temperature attainable by means of a freezing mixture is the
temperature of the F.P. of the corresponding cryohydrate. In a mixture
of salt and ice with the least trace of water a saturated brine is
quickly formed, which dissolves the ice and falls rapidly in
temperature, owing to the absorption of the latent heat of fusion. So
long as both ice and salt are present, if the mixture is well stirred,
the solution must necessarily become saturated with respect to both ice
and salt, and this can only occur at the cryohydric temperature, at
which the two curves of solubility intersect.
[Illustration: FIG. 1.--F.P. or Solubility Curve: simple case.]
The curves in fig. 1 also illustrate the simplest type of freezing point
curve in the case of alloys of two metals A and B which do not form
mixed crystals or chemical compounds. The alloy corresponding to the
cryohydrate, possessing the lowest melting point, is called the eutectic
alloy, as it is most easily cast and worked. It generally possesses a
very fine-grained structure, and is not a chemical compound. (See
ALLOYS.)
[Illustration: FIG. 2.--Cooling Curves of Alloys: typical case.]
To obtain a complete F.P. curve even for a binary alloy is a laborious
and complicated process, but the information contained in such a curve
is often very valuable. It is necessary to operate with a number of
different alloys of suitably chosen composition, and to observe the
freezing points of each separately. Each alloy should also be analysed
after the process if there is any risk of its composition having been
altered by oxidation or otherwise. The freezing points are generally
best determined by observing the gradual cooling of a considerable mass,
which is well stirred so long as it remains liquid. The curve of cooling
may most conveniently be recorded, either photographically, using a
thermocouple and galvanometer, as in the method of Sir W.
Roberts-Austen, or with pen and ink, if a platinum thermometer is
available, according to the method put in practice by C.T. Heycock and
F.H. Neville. A typical set of curves obtained in this manner is shown
in fig. 2. When the pure metal A in cooling reaches its F.P. the
temperature suddenly becomes stationary, and remains accurately constant
for a considerable period. Often it falls slightly below the F.P. owing
to super-fusion, but rises to the F.P. and remains constant as soon as
freezing begins. The second curve shows the cooling of A with 10% of
another metal B added. The freezing begins at a lower temperature with
the separation of pure A. The temperature no longer remains constant
during freezing, but falls more and more rapidly as the proportion of B
in the liquid increases. When the eutectic temperature is reached there
is a second F.P. or arrest at which the whole of the remaining liquid
solidifies. With 20% of B the first F.P. is further lowered, and the
temperature falls faster. The eutectic F.P. is of longer duration, but
still at the same temperature. For an alloy of the composition of the
eutectic itself there is no arrest until the eutectic temperature is
reached, at which the whole solidifies without change of temperature.
There is a great advantage in recording these curves automatically, as
the primary arrest is often very slight, and difficult to observe in any
other way.
7. _Change of Solubility with Temperature._--The lowering of the F.P.
of a solution with increase of concentration, as shown by the F.P. or
solubility curves, may be explained and calculated by equation (1) in
terms of the osmotic pressure of the dissolved substance by analogy
with the effect of mechanical pressure. It is possible in salt
solutions to strain out the salt mechanically by a suitable filter or
"semi-permeable membrane," which permits the water to pass, but
retains the salt. To separate 1 gramme of salt requires the
performance of work PV against the osmotic pressure P, where V is the
corresponding diminution in the volume of the solution. In dilute
solutions, to which alone the following calculation can be applied,
the volume V is the reciprocal of the concentration C of the solution
in grammes per unit volume, and the osmotic pressure P is equal to
that of an equal number of molecules of gas in the same space, and may
be deduced from the usual equation of a gas,