MOLECULAR REFRACTION OF BENZENE 127 ethereal salts, and those compounds derived from them are found to be greater than the calculated values, and in these compounds it is assumed for several reasons that they contain an atom of oxygen united to an atom of carbon by both its affinities. If it be assumed that the refraction equivalent of oxygen when so combined is 2.34, that is, 0·76 greater than its refraction equivalent in the alcohols and other similar compounds, then we obtain calculated results exhibiting a satisfactory agreement with the results of observation. Investigations undertaken at the suggestion of Landolt by J. Brühl have shown that the so-called unsaturated carbon compounds, viz. such compounds as combine directly with chlorine, bromine, or even hydrogen, possess a molecular refraction greater than those obtained by calculation. From the numerous cases investigated it is found that a satisfactory agreement between the calculated and observed results is obtained when to the calculated molecular refraction 1.79 is added, for every pair of carbon atoms united by double combining units, and 1.97 must be added for every pair of carbon atoms united by three combining units. After these rules had been established chemists sought by their aid to determine the mode of atomic linkage in various organic compounds, and more especially to fix the number of the groups of carbon atoms united by two or three combining units. It has thus been shown to be extremely probable that in benzene, toluene, and analogous hydrocarbons there are three pairs of carbon atoms united by two combining units, as is required to satisfy Kekulé's constitutional formula (§ 51):— The molecular refraction of benzene and its derivatives has been found to be about five units greater than the sum, S, of the refraction equivalents of the elements contained in them, as will be seen from the following examples: This and similar applications have combined to make the molecular refraction a very important aid in the investigation of the mode of linkage of the atoms in different compounds. § 74. Interaction of Liquids with other Substances. Wetting and Imbibition.-If a liquid be brought into contact with another liquid or with a solid body upon which it has no chemical action, the change resulting from this contact will depend upon the material composition of both, and determined by this, mixing, or dissolution, or mere contact will result. Liquids which do not mix with one another will arrange themselves in accordance with their specific gravities, liquids of equal densities forming spherical drops in the mass of the other. If one of the immiscible substances is a liquid and the other a solid, then one of two sets of phenomena will be produced; either the liquid wets or moistens the solid, and in that case we have the phenomenon of capillary attraction and the liquid is raised in the solid, or the liquid does not wet the solid, then at the point of contact a depression of the liquid surface is produced. Thus, as is well known, water, spirits of wine, oils, and many other liquids rise on the surfaces of glass, whilst mercury is depressed by glass, just as the surface of water is depressed by contact with fat. In such cases the nature of the solid body is of importance only so far as it determines whether it is moistened or not by the liquid; in other respects the capillary ascent or depression is dependent upon the nature of the liquid alone. There is a class of solids which possess the remarkable property of absorbing liquids, by which they are moistened, without dissolving in these same liquids. This absorption of liquids is usually accompanied by a considerable increase in the volume of the solid, and is described as 'Gelatinisation' or 'Imbibition.' Cellulose, starch, glue, coagu lated albumen, and many other substances swell up when brought into contact with water, and caoutchouc behaves similarly when moistened by ether. To those substances which exhibit this phenomenon is ascribed a large molecular weight and an atomlinkage of such a character that the atoms form a species of network into the spaces between which the smaller molecules of liquids are able to penetrate without destroying the network. It is worthy of remark that frequently phenomena such as diffusion (§ 80) can proceed in the interior of the absorbed liquid as well as in that portion which has not been so taken up. This power of absorption is very different for different substances; whilst in many cases the volume of a solid is considerably increased during the process of imbibition, in some cases the increase is scarcely perceptible. We are acquainted with all the intermediate stages exhibited by this class of bodies, and by substances such as burnt clay, hydrophane, &c. which, possessing a visibly porous structure, take up liquids into their pores, which serve simply to wet the interior of the pores. § 75. Heterogeneous Mixtures of Liquids. Solutions. Many substances are able to form fluid mixtures with other bodies. Such mixtures are usually styled solutions' or 'dissolutions,' one constituent being distinguished as the solvent, the other as the dissolved substance. Such distinctions are entirely arbitrary and have no scientific import. Fluid mixtures may be produced in the following ways:(1) By solid bodies alone. (2) By solids and liquids. (3) By liquids alone. (4) By liquids and gaseous bodies. (5) By gases alone. (6) By gases and solids. The states of aggregation of the constituents of such liquids only affect the nature and properties of the mixture so far as the constituents assume these conditions or states on separating out from the mixture. So long as they exist in the mixture, they must be regarded as liquids. The quantities of the constituents in such a fluid mixture or solution are either quite unlimited, the mixing taking place indefinitely in any proportions, or the proportions are so limited K that the admixture takes place only within certain limits, beyond which it is not possible to pass. The first of these cases is represented by liquids such as: water and alcohol, ethyl alcohol and methyl alcohol, or water and glycerol, which mix with one another in every proportion. In those cases in which such mixing or solution takes place only within certain limits the maximum amount of one of the substances which is taken up by a definite proportion of the other, say 100 or 1000 parts by weight of this substance, is styled the solubility' of the first in the second. When both substances are liquids, then the proportion of one of these may be raised from zero to a certain fixed limit, but this latter cannot be exceeded. If more than this amount be added, then it remains in the liquid state and separates from the rest; it can, on its part, however, take up some of the other constituent. According to Schuncke, water at 20° C. can take up 0.075 of its weight of ether, whilst this latter may take up as much as 0·027 of its weight of water. Mixtures of ether and water, therefore, can only be obtained containing from 0 to 7 or from 97 to 100 parts by weight of ether in 100 parts of the mixture. Consequently at 20° C. mixtures of ether and water can only be produced containing less than 7 or more than 97 parts by weight of ether. When one of the bodies is a liquid and the other a solid, then, whilst the proportion of the former may be raised indefinitely, that of the latter is fixed within a certain maximum limit, any excess above this amount remaining undissolved and generally in the solid state, though in some cases, as with phenol and water, in a fluid condition. Solutions which are incapable of dissolving any more of the solid are said to be 'saturated.' When both constituents are solids, but the mixture formed' by them a liquid, then, as in the case of salt and ice, there is for both of the constituents of the solution an upper and lowerlimit; neither of these limits must be exceeded if the mixture is to remain liquid. $76. Effect of Heat on Solubility.-The dissolution and also the solubility of different bodies are considerably affected by heat, the effects being analogous to those produced on simple unmixed substances. With such simple bodies an increase of EFFECT OF HEAT ON SOLUBILITY 131 volume is associated with an absorption of heat: a reduction in volume, with a loss of heat; so also, as a rule, in the admixture of liquids, a contraction is attended by an evolution of heat, which may be, and often is, considerable. The changes in aggregation associated with dissolution are also frequently accompanied by considerable thermal disturb ances. Just as when a solid body is melted, heat is rendered latent, so also there is a reduction in temperature attending the dissolution of a solid. The reduction in temperature is especially great when both the bodies passing into solution are solids. Thus, by mixing salt and ice in suitable proportions, the temperature of the mass can be lowered by 20° C. The heat so disappearing or becoming latent' is used in the conversion of the solid into the liquid state. Many substances first combine chemically with a portion of the solvent, and the compound so produced is then dissolved. Thus, for instance, anhydrous calcium chloride when brought into contact with water combines with the latter with production of heat to form the crystallisable compound CaCl,6H2O, which dissolves in water with a considerable absorption of heat. For the production of cold, therefore, hydrated, and not anhydrous, substances are best adapted. The limits of solubility are extended by changes of temperature, and in the majority of cases a rise in temperature increases the solubility. Still there are exceptions to this rule, and more especially in the case of liquids. Thus, for example, ether is less soluble in warm water than in cold, and consequently a cold saturated solution of ether in water becomes turbid when heated, owing to the separation of ether from the water. According to Alexejeff, in the case of aniline and water the mutual solubility of each is increased by rise in temperature. At low temperatures solutions can be obtained containing only but little water and very little aniline. As the temperature rises their solubility in each other increases, so that at 167° C. these substances may be mixed with each other in any propor tion. A reduction in the solubility with rise in temperature has been observed only in the case of a few solids, and in these cases |