CH3.CH2.CH2.CH2.CH2.CH3

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.