622 favour of the formulae H 2 , C1 2 , O^ or even Al 2 Br 6 and Fe 2 Cl 6 , although chemists would probably have contented them selves with H, Cl, O, AlBr 3 , and FeCl 3 , had it not been for the evidence of gas and vapour density, and certainly with out the latter no one would have thought of P 4 , As 4 , or Sg. 1 (3) There are a number of substances in the case of which there is an apparent disagreement between the results of the two Avays of determining molecular weight. Such substances are said to have an anomalous gas or vapour density. The expression anomalous vapour density is sometimes applied to the case of such substances as phosphorus and arsenic, but not very accurately. It would be better to say that these substances have an unexpected vapour density, because their complex molecular formulae, while not clearly indicated by their chemical character, are not at variance with any established law. We shall therefore reserve the term " anomalous vapour density" for those substances the molecular weight of which as given by their vapour density is not reconcilable with any formula which is chemically admissible. In the case of some substances, such as the oxides of chlorine, it has been shown that the discrepancy was due to errors of observation, impure specimens having been used in the experiments ; but there still remain many substances having, in the sense above indicated, an anomalous vapour density. These substances have therefore been examined Avith special care, with the result of completely vindicating the kinetic theory, and of disclosing a very interesting and theoretically important kind of chemical change. We shall take, as instances of such anomalous vapour densities, the substances in the last division of the table, and show how the anomaly has in these cases been explained. Sal-ammoniac has the composition represented by the formula NH 4 C1. This formula agrees with all the chemical actions of the substance and of all the substances in any way related to it, but it does not agree with the results of vapour density determinations. When sal-ammoniac is heated it is converted into vapour or gas, and this vapour or gas is reconverted into solid sal-ammoniac when it is cooled. This looks exactly like the process of sublimation, and it was universally supposed that the vapour given off when sal-ammoniac is heated was really sal-ammoniac vapour. But its vapour density corresponds, not to the for mula NH 4 C1 and the molecular weight 53 5, but to the half of this. Now this formula does not admit of divi sion, and the explanation at once suggests itself, that the vapour examined was not really the vapour of sal- ammoniac, but of hydrochloric acid and ammonia gases, the products of the decomposition of sal-ammoniac. This would of course completely explain the apparent anomaly ; each molecule NH 4 C1 dividing into two mole cules NH 3 and HC1, the gas from a given weight of sal- ammoniac would of course contain twice as many molecules and occupy twice the space which it would do if no such decomposition had occurred. On this supposition the mixed gases would remain uncombined as long as the temperature was above the decomposing point of sal- ammoniac ; if the temperature fell below this point they 1 It is important as a matter of scientific history to note that this agreement of gas density and chemical molecular weight was first indicated by Gay-Lussac, who showed that the ratio of the densities of two gases stood in a very simple arithmetical relation to the ratio of their chemical equivalents. Avogadro in 1811 brought forward his famous hypothesis, that the number of molecules in a given volume of gas is independent of the nature of the gas, or that the densities of gases (temperature and pressure being the same) are to one another as the masses of their molecules. This hypothesis is now shown to be in accordance with the kinetic theory of gas, and is known as "Avogadro s law." See ATOM, vol. iii. p. 40, where a slight con fusion has been caused by using the word " equivalent " instead of "molecule," and by not sufficiently distinguishing between the discovery of Gay-Lussac and the hypothesis of Avogadro. would unite and reproduce sal-ammoniac. It was neces sary, however, to prove that this decomposition occurs. As has been shown above (p. 618), the rate of diffusion of a gas depends upon its density. In this case the two gases into which the substance may be supposed to break up at the moment of volatilization differ considerably in density ; we ought, therefore, to be able to effect partial separation by means of diffusion, and it has been shown that such partial separation actually does occur. Thus, if we have hydrogen gas on one side of a porous dia phragm and volatilized sal-ammoniac on the other side, we find after a time that, mixed with the hydrogen on the one side, we have what we may for shortness call sal- ammoniac vapour that is, a vapour which when cooled forms solid sal-ammoniac with an excess of ammonia, which, being less dense than hydrochloric acid gas, has diffused faster ; while on the other side, also mixed with hydrogen which has diffused through the diaphragm, we have sal-ammoniac vapour with excess of hydrochloric acid, the denser and more slowly diffusing gas. This of course proves that the decomposition has occurred, but it does not prove that the vapour of sal-ammoniac consists entirely of hydrochloric acid and ammonia mixed with one another. That this in fact is not the case has been shown by an ingenious experiment. The two gases were separately raised to a temperature higher than that at which sal-am moniac volatilizes, and were then allowed to mix in a vessel kept at the same temperature as the two gases. In this vessel a delicate thermometer was placed, and it was found that the mixing of the two gases was accompanied by a small but very decided evolution of heat. This proves that some chemical combination takes place, and that the mixed gases must contain some vapour of NH 4 C1. More over, careful determinations of the vapour density of sal- ammoniac prove that it is a little more than the mean of the densities of ammonia and hydrochloric acid (as compared with air at the same temperature and pres sure, I Ol instead of 9255 at 350C.); and this increase of density on mixing the hot gases is easily explained by supposing that a small proportion is in the condition of NH 4 C1, while the most of the gas consists of separate NH 3 and HC1 molecules. In a similar way it has been shown that the vapour of oil of vitriol is a mixture of two vapours, that of water, H 2 O, and that of sulphuric anhydride, SO 3 ; and that sulphide of ammonium when volatilized breaks up into two volumes of ammonia and one of sulphuretted hy drogen, (NH 4 ) 2 S = 2NH 3 + H 2 S. We find, therefore, that in the former case, as in that of sal-ammoniac, w = 2m, and in the latter, w 3m. This peculiar kind of decomposition is now known by the name " dissociation." (See vol. v. pp. 475, 476.) In the cases we have mentioned the substances undergo nearly complete dissociation at the temperature at which they volatilize, and recombination takes place when they are cooled and again assume the solid, or, as in the case of oil of vitriol, the liquid state. These substances are therefore not suited for the illustration of the whole course of dissociation. This has been carefully studied in the case of some compounds, in which the dissociation is far from complete, at the boiling point of the substance, with the result that, if AB be the compound dissociating into the separate molecules A and JB, we may represent the amount of dissociation as the ratio of the num ber of pairs of separate A and B molecules to the total number of pairs of A and B, both separate and combined. This ratio we may call R, so that when dissociation is complete R. (1) R increases as the temperature rises. (2) dR/dt (where t is temperature) is a maximum when R = |. (3) The presence of excess of either A or B diminishes the value of R. For instance, PC1 5 is nearly completely dissociated into PC1 3 and Cl., at 300 C. ; but if a large excess of PC1 3 is mixed with the vapour it is found to contain scarcely any do, so that dissociation is greatly diminished by the presence of excess of PC1 3 . These experimental results are capable of explanation on the kinetic theory of gas, if we adopt Pfaundler s
hypothesis. This is, that for each case of dissociation there is a