«Gaseous Dissociation.»—The discrepancy between the two conclusions and any doubt as to the universal soundness of the great generalization expressed in Avogadro's hypothesis disappear, however, in the light of a closer study of the composition of ammonium chloride vapor. It was suggested simultaneously by Cannizzaro, by Kopp and by Kékulé[52] that the abnormally low result, obtained for the molecular weight of ammonium chloride from a study of its vapor density, is due to the ‹dissociation› of the salt at high temperatures into its components, ammonia and hydrogen chloride, the ‹average› of whose ‹molecular weights› is, in fact, (17 + 36.5) / 2, or 26.75, the value found experimentally for the vapor of ammonium chloride. Proof of the correctness of this interpretation was furnished by Pébal,[53] who showed that ammonium chloride vapor does consist of the two gases, the lighter of which, ammonia, diffuses more rapidly through porous walls (Pébal used an asbestos stopper) than does the heavier, hydrogen chloride. The dissociation may be easily demonstrated by using an air cushion as a porous wall.[54] From the mixture produced by vaporizing ammonium chloride,[55] the ammonia will diffuse more rapidly through the layer of air than will the hydrogen [p036] chloride, and the gases may be recognized in succession by their action on litmus paper (‹exp.›). The ‹gaseous dissociation› of other ammonium salts, of phosphorus pentachloride and pentabromide (PX_{5} ⇄ PX_{3} + X_{2}), and of a number of less common compounds, has been demonstrated in similar ways. As a result of the study of each case, the important conclusion has been reached that, as far as our knowledge goes, there are no exceptions to Avogadro's hypothesis, and this hypothesis seems therefore to represent a universal truth.[56] «Molecular Weight Determinations in Solution.»—Van 't Hoff's extension of the Avogadro Hypothesis, so that it shall apply to solutes in dilute solutions, is the basis of another general method of greatest value for determining molecular weights. Equal volumes of dilute solutions of the same osmotic pressure and the same temperature contain, according to van 't Hoff, the same numbers of dissolved molecules, irrespective of the solvent used. Furthermore, the number of dissolved molecules is identical with that which a gas of the same pressure and at the same temperature would contain in the same volume. To determine the molecular weight of a solute, therefore, we may calculate, from the osmotic pressure, the temperature and the concentration of the solution,[57] that weight of the solute which, in 22.4 liters of the solution, at 0° would give 760 mm. osmotic pressure; the weight found represents, in grams, the ‹molecular weight› of the solute ‹in the solution used›. [p037]