«Apparent Exceptions.»—Instead of discussing the vast amount of material of this kind, which agrees with van 't Hoff's theory, we may consider, more profitably, typical cases of ‹apparent exceptions›. The most important instance of this kind, the case of solutions of compounds which undergo ‹electrolytic dissociation or ionization›, will be separately discussed in the next chapter, and we shall find that van 't Hoff's great generalization is a vital element in the evidence of this important form of dissociation. Of other apparent exceptions, we may note the fact that some solutes seem to give "abnormally" ‹low› osmotic pressures[23] in certain solutions. For instance, benzoic acid, in benzene solutions, gives only a little more than half as great an osmotic pressure as it does in aqueous solutions of the same concentration and temperature, and as would be calculated on the basis of the Avogadro-van 't Hoff Hypothesis for a compound of the formula C_{ 6}H_{ 5}COOH and the molecular weight 122. But a rigorous study[24] of the distribution of benzoic acid between water and benzene, when solutions of the acid in the two solvents are shaken together until equilibrium is established (Chapter VIII), has proved that the distribution is strictly in accord with the assumption that benzoic acid, in aqueous solution, has the molecular weight 122 and the composition C_{ 6}H_{ 5}COOH, and that, in benzene solution, it has the molecular weight 244 and the composition (C_{ 6}H_{ 5}COOH)_{ 2}; only a ‹small part› of the acid (C_{ 6}H_{ 5}COOH)_{ 2} is decomposed in benzene solution into the simpler molecules, of the composition C_{ 6}H_{ 5}COOH. In other words, the simpler molecules C_{ 6}H_{ 5}COOH are ‹polymerized› or ‹associated› to form larger molecules in benzene solution, much as the gas nitrogen dioxide NO_{ 2} goes over more or less into the gas N_{ 2}O_{ 4}, especially at low temperatures, and as hydrogen fluoride at low temperatures has the composition H_{ 2}F_{ 2}, while at higher temperatures it is HF. The divergence of the benzene solutions of benzoic acid from the Avogadro-van 't Hoff principle is therefore only an ‹apparent one›, not a real one, inasmuch as the osmotic pressure of the solutions agrees perfectly with that calculated for solutions of a substance (C_{ 6}H_{ 5}COOH)_{ 2}, of molecular weight 244. Such associated molecules (of organic acids, alcohols, phenols, etc.) occur [p019] particularly readily in liquids of small dissociating power, like benzene, and such solutions show marked ‹absorption of heat on dilution›,[25] the dilution being accompanied by a ‹chemical change›. The associated molecules are dissociated more and more completely [(C_{ 6}H_{ 5}COOH)_{ 2} ⇄ 2 C_{ 6}H_{ 5}COOH], even in these solvents, as the solutions are diluted. Since dilution results in a ‹chemical increase› in the number of molecules, the osmotic pressure cannot decrease proportionally with the increase of volume in such a case as this. Nor does gas pressure, it must be remembered, decrease proportionally to the volume in the case of gases which show ‹chemical changes› with change of volume, ‹e.g.› in the case of nitrogen tetroxide, for which we have N_{ 2}O_{ 4} ⇄ 2 NO_{ 2}.