[1] [Greek: Hatalante], fr. 1, Meineke, Poetarum comicorum Graecorum
frag. (1855), p. 292.
[2] [Plut.] _Vita Isocr._, and the anonymous biographer. Dionysius
does not mention the story, though he makes Isocrates a pupil of
Theramenes.
[3] Some would refer the sojourn of Isocrates at Chios to the years
398-395 B.C., others to 393-388 B.C. The reasons which support the
view given in the text will be found in Jebb's _Attic Orators_, vol.
ii. (1893), p. 6, note 2.
[4] Partim in pompa, partim in acie illustres (_De orat._ ii. 24).
[5] P. Sanneg, _De schola Isocratea_ (Halle, 1867).
[6] _De falsa legat._ p. 426 [Greek: ouch opos orgizonto e kolazein
exioun tous tauta poiountas, all' apeblepon, ezeloun, etimon, andras
hegounto.]
[7] [Greek: ekeinous gar homologeitai ... ede egkrateis dokountas
einai ton pragmaton dia ten Kyrou propeteian atychesai] (_Philippus_,
90; cp. _Panegyr._ 149).
[8] _Philippus_, 346 B.C.; _Epist._ ii. end of 342 B.C. (?).
[9] The views of several modern critics on the tradition of the
suicide are brought together in the _Attic Orators_, ii. (1893) p.
31, note 1.
[10] Isocrates, a loyal and genuine Hellene, can yet conceive of
Hellenic culture as shared by men not of Hellenic blood (_Panegyr._
50). He is thus, as Ernst Curtius has ably shown, a forerunner of
Hellenism--analogous, in the literary province, to Epameinondas and
Timotheus in the political (_History of Greece_, v. 116, 204, tr.
Ward).
[11] [Greek: to ton Hellenon genos ... dunamenon archein panton, mias
tugchanon politeias] (_Polit._ iv. [vii.] 6, 7).
[12] _De Alex. virt._ i. 6.
[13] The word [Greek: philosophia] seems to have come into Athenian
use not much before the time of Socrates; and, till long after the
time of Isocrates, it was commonly used, not in the sense of
"philosophy," but in that of "literary taste and study--culture
generally" (see Thompson on _Phaedrus_, 278 D). Aristeides, ii. 407
[Greek: philokalia tis kai diatribe peri logous, kai ouch ho nun
tropos houtos, alla paideia koinos]. And so writers of the 4th
century B.C. use [Greek: philosophein] as simply = "to study"; as
e.g. an invalid "studies" the means of relief from pain, Lys. _Or._
xxiv. 10; cf. Isocr. _Or._ iv. 6, &c.
[14] Plato, _Gorg._ p. 463; _Euthyd._ 304-306.
[15] These allusions are discussed in the _Attic Orators_, vol. ii.
ch. 13.
[16] Isocr. _Or._ xv. 271.
[17] A. Cartelier, _Le Discours d'Isocrate sur lui-meme_, p. lxii.
(1862).
[18] Totum Isocratis [Greek: myrothekion] atque omnes ejus
discipulorum arculas (_Ad Att._ ii. 1).
[19] Idque princeps Isocrates instituisse fertur, ... ut inconditam
antiquorum dicendi consuetudinem ... numeris astringeret (_De or._
iii. 44, 173).
[20] The dates here given differ to some extent from those in F.
Blass, _Die attische Beredsamkeit_ (2nd ed., 1887-1898).
[21] Some authorities consider the _Ad Demonicum_ spurious.
[22] This was shown by R. C. Jebb in a paper on "The Sixth Letter of
Isocrates," _Journal of Philology_, v. 266 (1874). The fact that
Thebe, widow of Alexander of Pherae, was the daughter of Jason is
incidentally noticed by Plutarch in his life of Pelopidas, c. 28. It
is this fact which gives the clue to the occasion of the letter; cf.
Diod. Sic. xvi. 14.
ISODYNAMIC LINES (Gr. [Greek: isodynamos], equal in power), lines
connecting those parts of the earth's surface where the magnetic force
has the same intensity (see MAGNETISM, TERRESTRIAL).
ISOGONIC LINES (Gr. [Greek: isogonios], equiangular), lines connecting
those parts of the earth's surface where the magnetic declination is the
same in amount (see MAGNETISM, TERRESTRIAL).
ISOLA DEL LIRI, a town of Campania, in the province of Caserta, Italy,
15 m. by rail N.N.W. of Roccasecca, which is on the main line from Rome
to Naples, 10 m. N.W. of Cassino. Pop. (1901), town, 2384; commune,
8244. The town consists of two parts, Isola Superiore and Isola
Inferiore; as its name implies it is situated between two arms of the
Liri. The many waterfalls of this river and of the Fibreno afford motive
power for several important paper-mills. Two of the falls, 80 ft. in
height, are especially fine. About 1 m. to the N. is the church of San
Domenico, erected in the 12th century, which probably marks the site of
the villa of Cicero (see ARPINO).
ISOMERISM, in chemistry. When Wohler, in 1825, analysed his cyanic acid,
and Liebig his quite different fulminic acid in 1824, the composition of
both compounds proved to be absolutely the same, containing each in
round numbers 28% of carbon, 33% of nitrogen, 37% of oxygen and 2% of
hydrogen. This fact, inconsistent with the then dominating conception
that difference in qualities was due to difference in chemical
composition, was soon corroborated by others of analogous nature, and so
Berzelius introduced the term _isomerism_ (Gr. [Greek: isomeres],
composed of equal parts) to denominate the existence of the property of
substances having different qualities, in chemical behaviour as well as
physical, notwithstanding identity in chemical composition. These
phenomena were quite in accordance with the atomic conception of matter,
since a compound containing the same number of atoms of carbon,
nitrogen, oxygen and hydrogen as another in the same weight might differ
in internal structure by different arrangements of those atoms. Even in
the time of Berzelius the newly introduced conception proved to include
two different groups of facts. The one group included those isomers
where the identity in composition was accompanied by identity in
molecular weight, i.e. the vapour densities of the isomers were the
same, as in butylene and isobutylene, to take the most simple case; here
the molecular conception admits that the isolated groups in which the
atoms are united, i.e. the molecules, are identical, and so the molecule
of both butylene and isobutylene is indicated by the same chemical
symbol C4H8, expressing that each molecule contains, in both cases, four
atoms of carbon (C) and eight of hydrogen (H). This group of isomers was
denominated metamers by Berzelius, and now often "isomers" (in the
restricted sense), whereas the term _polymerism_ (Gr. [Greek: polys],
many) was chosen for compounds like butylene, C4H8, and ethylene, C2H4,
corresponding to the same composition in weight but differing in
molecular formula, and having different densities in gas or vapour, a
litre of butylene and isobutylene weighing, for instance, under ordinary
temperature and pressure, about 2.5 gr., ethylene only one-half as much,
since density is proportional to molecular weight.
A further distinction is necessary to a survey of the subdivisions of
isomerism regarded in its widest sense. There are subtle and more subtle
differences causing isomerism. In the case of metamerism we can imagine
that the atoms are differently linked, say in the case of butylene that
the atoms of carbon are joined together as a continuous chain, expressed
by --C--C--C--C--, _normally_ as it is called, whereas in isobutylene the
fourth atom of carbon is not attached to the third but to the second
carbon atom, i.e.
C--
/
--C--C .
\
C--
Now there are cases in which analogy of internal structure goes so far
as to exclude even that difference in linking, the only remaining
possibility then being the difference in relative position. This kind
of isomerism has been denominated _stereoisomerism_ (q.v.) often
stereomerism. But there is a last group belonging here in which identity
of structure goes farthest. There are substances such as sulphur,
showing difference of modification in crystalline state--the ordinary
rhombic form in which sulphur occurs as a mineral, while, after melting
and cooling, long needles appear which belong to the monosymmetric
system. These differences, which go hand in hand with those in other
properties, e.g. specific heat and specific gravity, are absolutely
confined to the crystalline state, disappearing with it when both
modifications of sulphur are melted, or dissolved in carbon disulphide
or evaporated. So it is natural to admit that here we have to deal with
identical molecules, but that only the internal arrangement differs from
case to case as identical balls may be grouped in different ways. This
case of difference in properties combined with identical composition is
therefore called _polymorphism_.
To summarize, we have to deal with polymerism, metamerism,
stereoisomerism, polymorphism; whereas phenomena denominated
tautomerism, pseudomerism and desmotropism form different particular
features of metamerism, as well as the phenomena of allotropy, which is
merely the difference of properties which an element may show, and can
be due to polymerism, as in oxygen, where by the side of the ordinary
form with molecules O2 we have the more active ozone with O3.
Polymorphism in the case of an element is illustrated in the case of
sulphur, whereas metamerism in the case of elements has so far as yet
not been observed; and is hardly probable, as most elements are built
up, like the metals, from molecules containing only one atom per
molecule; here metamerism is absolutely excluded, and a considerable
number of the rest, having diatomic molecules, are about in the same
condition. It is only in cases like sulphur with octatomic molecules,
where a difference of internal structure might play a part.
Before entering into detail it may be useful to consider the nature of
isomerism from a general standpoint. It is probable that the whole
phenomenon of isomerism is due to the possibility that compounds or
systems which in reality are unstable yet persist, or so slowly change
that practically one can speak of their stability; for instance, such
systems as explosives and a mixture of hydrogen and oxygen, where the
stable form is water, and in which, according to some, a slow but until
now undetected change takes place even at ordinary temperatures.
Consequently, of each pair of isomers we may establish beforehand which
is the more stable; either in particular circumstances, a direct change
taking place, as, for instance, with maleic acid, which when exposed to
sunlight in presence of a trace of bromine, yields the isomeric fumaric
acid almost at once, or, indirectly, one may conclude that the isomer
which forms under greater heat-development is the more stable, at least
at lower temperatures. Now, whether a real, though undetected, change
occurs is a question to be determined from case to case; it is certain,
however, that a substance like aragonite (a mineral form of calcium
carbonate) has sensibly persisted in geological periods, though the
polymorphous calcite is the more stable form. Nevertheless, the
theoretical possibility, and its realization in many cases, has brought
considerations to the front which have recently become of predominant
interest; consequently the possible transformations of isomers and
polymers will be considered later under the denomination of reversible
or dynamical isomerisms.
Especially prominent is the fact that polymerism and metamerism are
mainly reserved to the domain of organic chemistry, or the chemistry of
carbon, both being discovered there; and, more especially, the
phenomenon of metamerism in organic chemistry has largely developed our
notions concerning the structure of matter. That this particular feature
belongs to carbon compounds is due to a property of carbon which
characterizes the whole of organic chemistry, i.e. that atoms attached
to carbon, to express it in the atomic style, cling more intensely to it
than, for instance, when combined with oxygen. This explains a good
deal of the possible instability; and, from a practical point of view,
it coincides with the fact that such a large amount of energy can be
stored in our most intense explosives such as dynamite, the explanation
being that hydrogen is attached to carbon distant from oxygen in the
same molecule, and that only the characteristic resistance of the carbon
linkage prevents the hydrogen from burning, which is the main occurrence
in the explosion of dynamite. The possession of this peculiar property
by carbon seems to be related to its high valency, amounting to four;
and, generally, when we consider the most primitive expression of
isomerism, viz. the allotropy of elements, we meet this increasing
resistance with increasing valency. The monovalent iodine, for instance,
is transformed by heating into an allotropic form, corresponding to the
formula I, whereas ordinary iodine answers to I2. Now these
modifications show hardly any tendency to persist, the one stable at
high temperatures being formed at elevated temperatures, but changing in
the reverse sense on cooling. In the divalent oxygen we meet with the
modification called ozone, which, although unstable, changes but slowly
into oxygen. Similarly the trivalent phosphorus in the ordinary white
form shows such resistance as if it were practically stable; on the
other hand the red modification is in reality also stable, being formed,
for instance, under the influence of light. In the case of the
quadrivalent carbon, diamond seems to be the stable form at ordinary
temperatures, but one may wait long before it is formed from graphite.
This connexion of isomerism with resistant linking, and of this with
high valency, explains, in considerable measure, why inorganic compounds
afforded, as a rule, no phenomena of this kind until the systematic
investigation of metallic compounds by Werner brought to light many
instances of isomerism in inorganic compounds. Whereas carbon renders
isomerism possible in organic compounds, cobalt and platinum are the
determining elements in inorganic chemistry, the phenomena being
exhibited especially by complex ammoniacal derivatives. The constitution
of these inorganic isomers is still somewhat questionable; and in
addition it seems that polymerism, metamerism and stereoisomerism play a
part here, but the general feature is that cobalt and platinum act in
them with high valency, probably exceeding four. The most simple case is
presented by the two platinum compounds PtCl2(NH3)2, the
platosemidiammine chloride of Peyrone, and the platosammine chloride of
Jules Reiset, the first formed according to the equation PtCl4K2 + 2NH3
= PtCl2(NH3)2 + 2KCl, the second according to Pt(NH3)4Cl2 = PtCl2(NH3)2
+ 2NH3, these compounds differing in solubility, the one dissolving in
33, the other in 160 parts of boiling water. With cobalt the most simple
case was discovered in 1892 by S. Jorgensen in the second
dinitrotetramminecobalt chloride, [Co(NO2)2(NH3)4]Cl, designated as
flavo--whereas the older isomer of Gibbs was distinguished as
croceo-salt. An interesting lecture on the subject was delivered by A.
Werner before the German chemical society (_Ber._, 1907, 40, p. 15).
(See COBALT; PLATINUM.)
Dealing with organic compounds, it is metamerism that deserves chief
attention, as it has largely developed our notions as to molecular
structure. Polymerism required no particular explanation, since this was
given by the difference in molecular magnitude. One general remark,
however, may be made here. There are polymers which have hardly any
inter-relations other than identity in composition; on the other hand,
there are others which are related by the possibility of mutual
transformation; examples of this kind are cyanic acid (CNOH) and
cyanuric acid (CNOH)3, the latter being a solid which readily transforms
into the former on heating as an easily condensable vapour; the reverse
transformation may also be realized; and the polymers methylene oxide
(CH2O) and trioxymethylene (CH2O)3. In the first group we may mention
the homologous series of hydrocarbons derived from ethylene, given by
the general formula C_nH_(2n), and the two compounds methylene-oxide and
honey-sugar C6H12O6. The cases of mutual transformation are generally
characterized by the fact that in the compound of higher molecular
weight no new links of carbon with carbon are introduced, the
trioxymethylene being probably
CH2--O
/ \
O CH2,
\ /
CH2--O
whereas honey-sugar corresponds to CH2OH.CHOH.CHOH.CHOH.CHOH.CHO, each
point representing a linking of the carbon atom to the next. This
observation is closely related to the above-mentioned resistivity of the
carbon-link, and corroborates it in a special case. As carbon tends to
hold the atom attached to it, one may presume that this property
expresses itself in a predominant way where the other element is carbon
also, and so the linkage represented by --C--C-- is one of the most
difficult to loosen.
The conception of metamerism, or isomerism in restricted sense, has been
of the highest value for the development of our notions concerning
molecular structure, i.e. the conception as to the order in which the
atoms composing a molecule are linked together. In this article we shall
confine ourselves to the fatty compounds, from which the fundamental
notions were first obtained; reference may be made to the article
CHEMISTRY: _Organic_, for the general structural relations of organic
compounds, both fatty and aromatic.
A general philosophical interest is attached to the phenomena of
isomerism. By Wilhelm Ostwald especially, attempts have been made to
substitute the notion of atoms and molecular structure by less
hypothetical conceptions; these ideas may some day receive thorough
confirmation, and when this occurs science will receive a striking
impetus. The phenomenon of isomerism will probably supply the crucial
test, at least for the chemist, and the question will be whether the
Ostwaldian conception, while substituting the Daltonian hypothesis, will
also explain isomerism. An early step accomplished by Ostwald in this
direction is to define ozone in its relation to oxygen, considering the
former as differing from the latter by an excess of energy, measurable
as heat of transformation, instead of defining the difference as
diatomic molecules in oxygen, and triatomic in ozone. Now, in this case,
the first definition expresses much better the whole chemical behaviour
of ozone, which is that of "energetic" oxygen, while the second only
includes the fact of higher vapour-density; but in applying the first
definition to organic compounds and calling isobutylene "butylene with
somewhat more energy" hardly anything is indicated, and all the
advantages of the atomic conception--the possibility of exactly
predicting how many isomers a given formula includes and how you may get
them--are lost.
To Kekule is due the credit of taking the decisive step in introducing
the notion of tetravalent carbon in a clear way, i.e. in the property of
carbon to combine with four different monatomic elements at once,
whereas nitrogen can only hold three (or in some cases five), oxygen two
(in some cases four), hydrogen one. This conception has rendered
possible a clear idea of the linking or internal structure of the
molecule, for example, in the most simple case, methane, CH4, is
expressed by
H
|
H--C--H
|
H
It is by this conception that possible and impossible compounds are at
once fixed. Considering the hydrocarbons given by the general formula
C_xH_y, the internal linkages of the carbon atoms need at least x - 1
bonds, using up 2(x - 1) valencies of the 4x to be accounted for, and
thus leaving no more than 2(x + 1) for binding hydrogen: a compound C3H9
is therefore impossible, and indeed has never been met. The second
prediction is the possibility of metamerism, and the number of metamers,
in a given case among compounds, which are realizable. Considering the
predicted series of compounds C_nH_(2n + 2), which is the well-known
homologous series of methane, the first member, the possible of
isomerism lies in that of a different linking of the carbon atoms. This
first presents itself when four are present, i.e. in the difference
between C--C--C--C and
C--C--C
| .
C
With this compound C4H10, named butane, isomerism is actually observed,
being limited to a pair, whereas the former members ethane, C2H6, and
propane, C3H8, showed no isomerism. Similarly, pentane, C5H12, and
hexane, C6H14, may exist in three and five theoretically isomeric forms
respectively; confirmation of this theory is supplied by the fact that
all these compounds have been obtained, but no more. The third most
valuable indication which molecular structure gives about these isomers
is how to prepare them, for instance, that normal hexane, represented by
CH3.CH2.CH2.CH2.CH2.CH3, may be obtained by action of sodium on propyl
iodide, CH3.CH2.CH2I, the atoms of iodine being removed from two
molecules of propyl iodide, with the resulting fusion of the two systems
of three carbon atoms into a chain of six carbon atoms. But it is not
only the formation of different isomers which is included in their
constitution, but also the different ways in which they will decompose
or give other products. As an example another series of organic
compounds may be taken, viz. that of the alcohols, which only differ
from the hydrocarbons by having a group OH, called hydroxyl, instead of
H, hydrogen; these compounds, when derived from the above methane series
of hydrocarbons, are expressed by the general formula C_nH_(2n + 1)OH.
In this case it is readily seen that isomerism introduces itself in the
three carbon atom derivative: the propyl alcohols, expressed by the
formulae CH3.CH2.CH2OH and CH3.CHOH.CH3, are known as propyl and
isopropyl alcohol respectively. Now in oxidizing, or introducing more
oxygen, for instance, by means of a mixture of sulphuric acid and
potassium bichromate, and admitting that oxygen acts on both compounds
in analogous ways, the two alcohols may give (as they lose two atoms of
hydrogen) CH3.CH2.COH and CH3CO.CH3. The first compound, containing a
group COH, or more explicitly O = C - H, is an _aldehyde_, having a
pronounced reducing power, producing silver from the oxide, and is
therefore called propylaldehyde; the second compound containing the
group --C.CO.C-- behaves differently but just as characteristically, and
is a _ketone_, it is therefore denominated propylketone (also acetone or
dimethyl ketone). And so, as a rule, from isomeric alcohols, those
containing a group --CH2.OH, yield by oxidation aldehydes and are
distinguished by the name primary; whereas those containing CH.OH,
called secondary, produce ketones. (Compare CHEMISTRY: _Organic_.)
The above examples may illustrate how, in a general way, chemical
properties of isomers, their formation as well as transformation, may be
read in the structure formula. It is different, however, with physical
properties, density, &c.; at present we have no fixed rules which enable
us to predict quantitatively the differences in physical properties
corresponding to a given difference in structure, the only general rule
being that those differences are not large.
Perhaps a satisfactory point of view may be here obtained by applying
the van der Waals' equation A(P + a/V^2)(V - b) = 2T, which connects
volume V, pressure P and temperature T (see CONDENSATION OF GASES). In
this equation a relates to molecular attraction; and it is not
improbable that in isomeric molecules, containing in sum the same
amount of the same atoms, those mutual attractions are approximately
the same, whereas the chief difference lies in the value of b, that
is, the volume occupied by the molecule itself. For what reason this
volume may differ from case to case lies close at hand; in connexion
with the notion of negative and positive atoms, like chlorine and
hydrogen, experience tends to show that the former, as well as the
latter, have a mutual repulsive power, but the former acts on the
latter in the opposite sense; the necessary consequence is that, when
those negative and positive groups are distributed in the molecule,
its volume will be smaller than if the negative elements are heaped
together. An example may prove this, but before quoting it, the
question of determining b must be decided; this results immediately
from the above quotation, b being the volume V at the absolute zero (T
= 0); so the volume of isomers ought to be compared at the absolute
zero. Since this has not been done we must adopt the approximate rule
that the volume at absolute zero is proportional to that at the
boiling-point. Now taking the isomers H3C.CCl3(M_v = 108) and
ClH2.CHCl2(M_v = 103), we see the negative chlorine atoms heaped up in
the left hand formula, but distributed in the second; the former
therefore may be presumed to occupy a larger space, the molecular
volume, that is, the volume in cubic centimetres occupied by the
molecular weight in grams, actually being 108 in the former, and 103
in the latter case (compare CHEMISTRY: _Physical_). An analogous
remark applies to the boiling-point of isomers. According to the above
formula the critical temperature is given by 8aA/54b, and as the
critical temperature is approximately proportional to the
boiling-point, both being estimated on the absolute scale of
temperature, we may conclude that the larger value of b corresponds to
the lower boiling-point, and indeed the isomer corresponding to the
left-hand formula boils at 74 deg., the other at 114 deg. Other
physical properties might be considered; as a general rule they depend
upon the distribution of negative and positive elements in the
molecule.
_Reversible (dynamical) Isomerism._--Certain investigations on isomerism
which have become especially prominent in recent times bear on the
possibility of the mutual transformation of isomers. As soon as this
reversibility is introduced, general laws related to thermodynamics are
applicable (see CHEMICAL ACTION; ENERGETICS). These laws have the
advantage of being applicable to the mutual transformations of isomers,
whatever be the nature of the deeper origin, and so bring polymerism,
metamerism and polymorphism together. As they are pursued furthest in
the last case, this may be used as an example. The study of polymorphism
has been especially pursued by Otto Lehmann, who proved that it is an
almost general property; the variety of forms which a given substance
may show is often great, ammonium nitrate, for instance, showing at
least four of them before melting. The general rule which correlates
this polymorphic change is that its direction changes at a given
temperature. For example, sulphur is stable in the rhombic form till
95.4 deg., from then upwards it tends to change over into the prismatic
form. The phenomenon absolutely corresponds to that of fusion and
solidification, only that it generally takes place less quickly;
consequently we may have prismatic sulphur at ordinary temperature for
some time, as well as rhombic sulphur at 100 deg. This may be expressed
in the chosen case by a symbol; "rhombic sulphur <--95.4 deg.-->
prismatic sulphur," indicating that there is equilibrium at the so-called
"transition-point," 95.4 deg., and opposite change below and above.
This comparison with fusion introduces a second notion, that of the
"triple-point," this being in the melting-phenomenon the only
temperature at which solid, liquid and vapour are in equilibrium, in
other words, where three phases of one substance are co-existent. This
temperature is somewhat different from the ordinary melting-point, the
latter corresponding to atmospheric pressure, the former to the maximum
vapour-pressure; and so we come to a third relation for polymorphism.
Just as the melting-point changes with pressure, the transition-point
also changes; even the same quantitative relation holds for both, as L.
J. Reicher proved with sulphur: aT/aP = AvT/q, v being the change in
volume which accompanies the change from rhombic to prismatic sulphur,
and q the heat absorbed. Both formula and experiment proved that an
increase of pressure of one atmosphere elevated the transition point for
about 0.04 deg. The same laws apply to cases of more complicated nature,
and one of them, which deserves to be pursued further, is the mutual
transformation of cyanuric acid, C3H3N3O3, cyanic acid, CHNO, and
cyamelide (CHNO)_x; the first corresponding to prismatic sulphur, stable
at higher temperatures, the last to rhombic, the equilibrium-symbol
being: cyamelide <--150 deg.--> cyanuric acid; the cyanic acid
corresponds to sulphur vapour, being in equilibrium with either cyamelide
or cyanuric acid at a maximum pressure, definite for each temperature.
A second law for these mutual transformations is that when they take
place without loss of homogeneity, for example, in the liquid state, the
definite transition point disappears and the change is gradual. This
seems to be the case with molten sulphur, which, when heated, becomes
dark-coloured and plastic; and also in the case of metals, which obtain
or lose magnetic properties without loss of continuous structure. At the
same time, however, the transition point sometimes reappears even in the
liquid state; in such cases two layers are formed, as has been recently
observed with sulphur, and by F. M. Jager in complicated organic
compounds. Thus the introduction of heterogeneity, or the appearance of
a new phase, demands the existence of a fixed temperature of
transformation.
On the basis of the relation between physical phenomena and
thermodynamical laws, properties of the polymorphous compounds may be
predicted. The chief consideration here is that the stable form must
have the lower vapour pressure, otherwise, by distillation, it would
transform in opposite sense. From this it follows that the stable form
must have the higher melting-point, since at the melting-point the
vapour of the solid and of the liquid have the same pressure. Thus
prismatic sulphur has a higher melting-point (120 deg.) than the rhombic
form (116 deg.), and it is even possible to calculate the difference
theoretically from the thermodynamic relations. A third consequence is
that the stable form must have the smaller solubility: J. Meyer and J.
N. Bronstedt found that at 25 deg., 10 c.c. of benzene dissolved 0.25
and 0.18 gr. of prismatic and rhombic sulphur respectively. It can be
easily seen that this ratio, according to Henry's law, must correspond
to that of vapour-pressures, and so be independent of the solvent; in
fact, in alcohol the figures are 0.0066 and 0.0052. Recently Hermann
Walther Nernst has been able to deduce the transition-point in the case
of sulphur from the specific heat and the heat developed in the
transition only. This best studied case shows that a number of mutual
relations are to be found between the properties of two modifications
when once the phenomenon of mutual transformation is accessible.
In ordinary isomers indications of mutual transformation often occur;
and among these the predominant fact is that denoted as tautomerism or
pseudomerism. It exhibits itself in the peculiar behaviour of some
organic compounds containing the group --C.CO.C--, e.g.
CH3CO.CHX.CO2C2H5, derivatives of acetoacetic ester. These compounds
generally behave as ketones; but at the same time they may act as
alcohols, i.e. as if containing the OH group; this leads to the formula
H3C.C(OH):CX.CO2C2H5. In reality such tautomeric compounds are
apparently a mixture of two isomers in equilibrium, and indeed in some
cases both forms have been isolated; then one speaks of _desmotropy_
(Gr. [Greek: desmos], a bond or link, and [Greek: trope], a turn or
change). Nevertheless, the relations obtained in reversible cases such
as sulphur have not yet found application in the highly interesting
cases of ordinary irreversible isomerism.
A further step in this direction has been effected by the introduction
of reversibility into a non-reversible case by means of a catalytic
agent. The substance investigated was acetaldehyde, C2H4O, in its
relation to paraldehyde, a polymeric modification. The phenomena were
first observed without mutual transformation, aldehyde melting at -118
deg., paraldehyde at 13 deg., the only mutual influence being a lowering
of melting-point, with a minimum at -120 deg. in the eutectic point.
When a catalytic agent, such as sulphurous acid, is added, which
produces a mutual change, the whole behaviour is different; only one
melting-point, viz. 7 deg., is observed for all mixtures; this has been
called the "natural melting-point." It corresponds to one of the
melting-points in the series without catalytic agents, viz. in that
mixture which contains 88% of paraldehyde and 12% of acetaldehyde, which
the catalytic agent leaves unaffected. Such an introduction of
reversibility is also possible by allowing sufficient time to permit the
transformation to be produced by itself. By R. Rothe and Alexander
Smith's interesting observations on sulphur, results have been obtained
which tend to prove that the melting-point, as well as the appearance of
two layers in the liquid state, correspond to unstable conditions.
(J. H. van't H.)
ISOTHERM (Gr. [Greek: isos], equal, and [Greek: therme], heat), a line
upon a map connecting places where the temperature is the same at
sea-level on the earth's surface. These isothermal lines will be found
to vary from month to month over the two hemispheres, or over local
areas, during summer and winter, and their position is modified by
continental or oceanic conditions.
ISOXAZOLES, monazole chemical compounds corresponding to furfurane, in
which the [-=]CH group adjacent to the oxygen atom is replaced by a
nitrogen atom, and therefore they contain the ring system
HC = N
| \
| O.
| /
HC = CH
They may be prepared by the elimination of water from the monoximes of
[beta]-diketones, [beta]-ketone aldehydes or oxymethylene ketones (L.
Claisen, _Ber._, 1891, 24, p. 3906), the general reaction proceeding
according to the equation
R.CO.CH2.CO.R + H2N.OH = 2H2O + R.C = N
| \
| O.
| /
HC = C--R
W. Dunstan and T. S. Dymond (_Jour. Chem. Soc._, 1891, 49, p. 410) have
also prepared isoxazoles by the action of alkalis on nitroparaffins, but
have not been able to obtain the parent substance. Those isoxazoles in
which the carbon atom adjacent to nitrogen is substituted are stable
compounds, but if this is not the case, rearrangement of the molecule
takes place and nitriles are formed. The isoxazoles are feebly basic.
The _isoxazolones_ are the keto derivatives of the as yet unknown
dihydroisoxazole, and are compounds of strongly acid nature,
decomposing the carbonates of the alkaline earth metals and forming
salts with metals and with ammonia. Their constitution is not yet
definitely fixed and they may be regarded as derived from one of the
three types
CH2--C HC--CO HC = C(OH)
| \ || \ | \
| O; || O; | O.
| / || / | /
CH = N HC--NH HC = N---
By the action of nitrous acid on the oxime of o-aminobenzophenone as
[alpha]-phenyl indoxazene,
C--C6H5
/ \\
C6H4 N,
\ /
O
is obtained; this is a derivative of benzisoxazole.
ISRAEL (Hebrew for "God strives" or "rules"; see Gen. xxxii. 28; and the
allusion in Hosea xii. 4), the national designation of the Jews. Israel
was a name borne by their ancestor Jacob the father of the twelve
tribes. For some centuries the term was applied to the northern kingdom,
as distinct from Judah, although the feeling of national unity extended
it so as to include both. It emphasizes more particularly the position
of the Hebrews as a religious community, bound together by common aims
and by their covenant-relation with the national God, Yahweh.
See further JACOB, HEBREW LANGUAGE, HEBREW RELIGION, JEWS: _History_
and _Palestine_.
ISRAELI, ISAAC BEN SOLOMON (9th-10th centuries), Jewish physician and
philosopher. A contemporary of Seadiah (q.v.), he was born and passed
his life in North Africa. He died c. 950. At Kairawan, Israeli was court
physician; he wrote several medical works in Arabic, and these were
afterwards translated into Latin. Similarly his philosophical writings
were translated, but his chief renown was in the circle of Moslem
authors.
ISRAELS, JOSEF (1824- ), Dutch painter, was born at Groningen, of
Hebrew parents, on the 27th of January 1824. His father intended him to
be a man of business, and it was only after a determined struggle that
he was allowed to enter on an artistic career. However, the attempts he
made under the guidance of two second-rate painters in his native
town--Buys and van Wicheren--while still working under his father as a
stockbroker's clerk, led to his being sent to Amsterdam, where he became
a pupil of Jan Kruseman and attended the drawing class at the academy.
He then spent two years in Paris, working in Picot's studio, and
returned to Amsterdam. There he remained till 1870, when he moved to The
Hague for good. Israels is justly regarded as one of the greatest of
Dutch painters. He has often been compared to J. F. Millet. As artists,
even more than as painters in the strict sense of the word, they both,
in fact, saw in the life of the poor and humble a motive for expressing
with peculiar intensity their wide human sympathy; but Millet was the
poet of placid rural life, while in almost all Israels' pictures we find
some piercing note of woe. Duranty said of them that "they were painted
with gloom and suffering." He began with historical and dramatic
subjects in the romantic style of the day. By chance, after an illness,
he went to recruit his strength at the fishing-town of Zandvoort near
Haarlem, and there he was struck by the daily tragedy of life.
Thenceforth he was possessed by a new vein of artistic expression,
sincerely realistic, full of emotion and pity. Among his more important
subsequent works are "The Zandvoort Fisherman" (in the Amsterdam
gallery), "The Silent House" (which gained a gold medal at the Brussels
Salon, 1858) and "Village Poor" (a prize at Manchester). In 1862 he
achieved great success in London with his "Shipwrecked," purchased by Mr
Young, and "The Cradle," two pictures of which the _Athenaeum_ spoke as
"the most touching pictures of the exhibition." We may also mention
among his maturer works "The Widower" (in the Mesdag collection), "When
we grow Old" and "Alone in the World" (Amsterdam gallery), "An Interior"
(Dordrecht gallery), "A Frugal Meal" (Glasgow museum), "Toilers of the
Sea," "A Speechless Dialogue," "Between the Fields and the Seashore,"
"The Bric-a-brac Seller" (which gained medals of honour at the great
Paris Exhibition of 1900). "David Singing before Saul," one of his
latest works, seems to hint at a return on the part of the venerable
artist to the Rembrandtesque note of his youth. As a water-colour
painter and etcher he produced a vast number of works, which, like his
oil paintings, are full of deep feeling. They are generally treated in
broad masses of light and shade, which give prominence to the principal
subject without any neglect of detail.
See Jan Veth, _Mannen of Beteckenis: Jozef Israels_; Chesneau,
_Peintres francais et etrangers_; Ph. Zilcken, _Peintres hollandais
modernes_ (1893); Dumas, _Illustrated Biographies of Modern Artists_
(1882-1884); J. de Meester, in Max Rooses' _Dutch Painters of the
Nineteenth Century_ (1898); Jozef Israels, _Spain: the Story of a
Journey_ (1900).
ISSACHAR (a Hebrew name meaning apparently "there is a hire," or
"reward"), Jacob's ninth "son," his fifth by Leah; also the name of a
tribe of Israel. Slightly differing explanations of the reference in the
name are given in Gen. xxx. 16 (J) and v. 18 (E).[1] The territory of
the tribe (Joshua xix. 17-23) lay to the south of that allotted to
Zebulun, Naphtali, Asher and Dan, and included the whole of the great
plain of Esdraelon, and the hills to the east of it, the boundary in
that direction extending from Tabor to the Jordan, apparently along the
deep gorge of Wadi el Bireh. In the rich territory of Issachar,
traversed by the great commercial highway from the Mediterranean and
Egypt to Bethshean and the Jordan, were several important towns which
remained in the hands of the Canaanites for some time (Judges i. 27),
separating the tribe from Manasseh. Although Issachar is mentioned as
having taken some part in the war of freedom under Deborah (Judges v.
15), it is impossible to misunderstand the reference to its tributary
condition in the blessing of Jacob (Gen. xlix. 14 seq.), or the fact
that the name of this tribe is omitted from the list given in Judges i.
of those who bestirred themselves against the earlier inhabitants of the
country. In the "blessing upon Zebulun and Issachar" in Deut. xxxiii. 18
seq., reference is made to its agricultural life in terms suggesting
that along with its younger, but more successful "brother," it was the
guardian of a sacred mountain (Carmel, Tabor?) visited periodically for
sacrificial feasts.