Chap. xv. par. 294); therefore a mixture of equal volumes of 18+ 80 = Therefore the observed specific gravity of the gas obtained by heating sulphuric acid is identical with the specific gravity calculated from the experimentally determined composition of this gas. 337 Much discussion at one time took place as to the specific gravity of phosphorus pentachloride in the gaseous state. Analyses of this compound, and determinations of the atomic weights of phosphorus (31) and chlorine (35.5), shewed clearly that the simplest formula which could be given to the solid compound was PC. The specific gravity of this compound in the state of gas, referred to hydrogen as unity, must be 31 + (35·5 × 5) – 104-25; or, referred to air as unity, 2 = gravity must be 7·2. the spec. The following table gives the observed spec. gravities (air = 1) at different temperatures of the gas obtained by heating phosphorus pentachloride, PCI. In each case the pressure was 760 mm. The specific gravity, and therefore probably the molecular weight, of the gaseous compound varies; but at no temperature does the specific gravity approximate to that required by the formula PCI. Here again we have an instance of so-called abnormal vapour-density. Further investigation however shewed that the numbers are not really abnormal, but that they find a ready and simple explanation in terms of the molecular and atomic theory. Experiments proved that phosphorus pentachloride is gradually dissociated by heat into phosphorus trichloride and chlorine; PCI, PCI,+Cl. The gas obtained by heating PCI, is therefore a mixture of more than one substance. The specific gravity of a mixture of phosphorus 3 trichloride and chlorine in the ratio PCI, Cl, must be PC1, + Cl2 3 4 = 5 2 72.125 if hydrogen is taken as unity, and 3.6 if air is taken as unity. The specific gravity of the gas obtained by heating PCI, to 300° is practically identical with that of a mixture of PC, and Cl,; the specific gravity at different temperatures lower than that at which dissociation is complete is that of a mixture of these two gases with some gaseous PC1, in varying proportions. The change of PC1, into PCl, and Cl,, brought about by the action of heat, is therefore a normal process of dissociation. A portion of the PCI, is volatilised without change, but at the same time some of it is dissociated into the simpler molecules PCl, and Cl,; as temperature rises the quantity of unchanged PCI, decreases, and the quantities of PC1, and Cl, increase, until at about 300° the whole of the PCI, is changed to PC1, + Cl,. The numbers in the third columns of the table shew the ratio, at each temperature, of PCI, to the total PCI, possible as the asumption that the whole of the PCI, had been dissociated. 3 3 There are other cases of so-called abnormal vapour-densities which are not quite so easily explained by the theory of atoms and molecules; but we cannot discuss these here. The practical outcome of these facts as bearing on deter- 338 minations of molecular weights is, that the specific gravity of a gas must be constant through a considerable interval of temperature before we are justified in deducing an approximate value for the molecular weight, from the observed specific gravity, of this gas. If the specific gravity varies considerably with temperature-changes, then the gas is probably a mixture; but the expression molecular weight of a mixture has no meaning. In Chap. XI. par. 175 we had examples of chemical changes 339 brought about by oxygen and hydrogen, respectively, when these elements were themselves products of one part of the complete cycles of change. Thus, when chlorine is passed into a warm solution of potash (KOH), potassium hypochlorite (KCIO) is formed, but this compound is quickly decomposed with production of potassium chloride (KCl) and oxygen; lead monoxide (PbO) is suspended in the warm potash solution, the oxygen, or a portion of the oxygen, produced from the decomposition of the hypochlorite combines with the lead monoxide to form dioxide (PbO). But if oxygen is passed into warm potash solution holding lead monoxide in suspension if no lead dioxide is produced; the oxygen must be produced by a chemical reaction in the system of which the body to be oxidised forms a part. Similarly, if hydrogen is produced, by the interaction of zinc and dilute sulphuric acid, in a solution containing sodium sulphite (Na,SO), hydrogen sulphide (HS) is produced; but if hydrogen is passed into a solution of sodium sulphite hydrogen sulphide is not produced: the hydrogen must be produced by a chemical reaction in the system of which the sodium sulphite forms a part. of In Chap. xiv. par. 266 we briefly considered the changes energy which accompany such chemical changes as these. The molecular and atomic theory throws some light on these changes. This theory leads to the view that a system composed of atoms of a specified element, could such a system exist, would differ from a system composed of molecules of the same element. It also leads to the view that in very many, if not most, chemical changes, the formation of the molecules of the products of the change is preceded by the breaking up into atoms, or sometimes into groups of atoms, of the molecules of the interacting substances. And, lastly, the theory almost obliges us to believe that a system composed of atoms of one of those elements the molecules of which are built up of more than a single atom (s. table in par. 317), if it could exist would be extremely unstable, and would almost at once pass into a system composed of molecules of the same element. The application of these conceptions to the class of changes we are considering affords some explanation of the mechanism of these changes. The explanation may be stated as follows. Under ordinary conditions quantities of oxygen or hydrogen consist of molecules of these gases. Oxygen passed into potash holding lead monoxide in suspension does not oxidise the lead oxide, because the affinity between molecules of oxygen and lead monoxide is not sufficient to produce this change, and there is not sufficient energy available in the system for separating the molecules of oxygen into atoms: but when potash and chlorine interact, atoms of oxygen are produced; these atoms combine with the molecules of lead monoxide to form lead dioxide, and in this change more energy is degraded than would be the case if the atoms of oxygen had combined with each other to form molecules of oxygen. A similar explanation would be given of the interaction between sodium sulphite and the atoms of hydrogen produced by the interaction of zinc and sulphuric acid. Chemical changes which are brought about by elements only when these are themselves products of a part of the complete change are sometimes classed together as nascent actions. The name has been useful as marking a class of reactions which have a common feature. If the view here taken of these reactions is correct, there is nothing in any way abnormal about them; they belong to the ordinary type of chemical change. It is evident then that the molecular and atomic theory 340 brings into one point of view, and gives fairly simple explanations of, many classes of chemical occurrences. It also indicates directions in which experiments ought to proceed with the object of discovering and explaining new classes of chemical events. The words law, hypothesis, and theory, have been frequently used in this book. The word law has sometimes been employed as synonymous with a general truth; for instance the laws of chemical combination are general truths, they summarise many facts. The same term, law, is sometimes used as meaning an abstract truth; for instance, Newton's laws of motion are truths involved in many phenomena although actually seen in none. The statement 'equal volumes of gases contain equal numbers of molecules' has been called a law. This statement is really a deduction from a theory. The deduction has a definite meaning when the terms in which it is made are defined, but this can be done only by granting the fundamental assumptions of the theory. The 'law' stands or falls with the theory. The molecular and atomic theory, like all scientific theories, is based on certain assumptions. The fewer, the simpler, and the more binding, the assumptions on which a theory rests the better is the theory. One of the marks of a satisfactory theory is the impossibility of escaping from discrepancies between observed facts and deductions from the theory by the invention of subsidiary hypotheses which do not follow directly from the assumptions on which the theory rests. The molecular and atomic theory, it must be confessed, has been too elastic in this respect. An hypothesis is specially framed to explain a definite occurrence, or a series of occurrences. For instance, when Davy found that nitric acid was formed at the positive electrode during the electrolysis of water, he framed the hypothesis that the air surrounding the decomposing water was the source of the acid: he was able to prove by direct experiment that this hypothesis was correct. An hypothesis is sometimes stated in very general terms, and is used to explain a great many apparently unconnected facts. For instance, very many facts concerning chemical change are generalised in the hypothesis that 'the amount of a chemical change is proportional to the affinities and the active masses of the substances taking part in the change'. A direct and final experimental proof of such an hypothesis as this can scarcely be given. If the terms can be accurately defined, and if after prolonged inquiry no facts are discovered which negative the hypothesis, it is adopted as a trustworthy guide. CHAPTER XVII. ISOMERISM AND STRUCTURAL FORMULAE. 341 342 In the last chapter we had an instance of isomerism; namely, the existence of three different compounds all having the molecular composition expressed by the formula C ̧H12 The prominent fact of isomerism is, that two or more compounds sometimes exist having identical compositions, and identical specific gravities in the state of gases, and yet exhibiting different properties. The statement of this fact in the language of the molecular and atomic theory is, that two or more gaseous molecules may exist composed of the same number of the same atoms, and yet differing from each other in their properties. An instance of isomerism is furnished by the existence of two compounds having the composition CHO. One of these is ethylic alcohol, the other is methylic ether. Ethylic alcohol interacts with potassium or sodium thus, C ̧HO+ K = C ̧H ̧KO+H; methylic ether and potassium (or sodium) do not interact. Phosphorus pentachloride interacts with both isomerides; the interactions are these: 6 = (1) alcohol; CHO+PCI, CH,CI+ POCI, + HCl. 4 = The alcohol is easily oxidised, first to aldehyde C2HO, then to acetic acid C,H,O,; the ether is not easily oxidised. Ethylic alcohol is a colourless, volatile, liquid, boiling at 78°.3; methylic ether is a colourless gas which may be condensed by cold to a liquid boiling at - 21o. Another instance of isomerism is furnished by the existence of four hydrocarbons having the molecular composition CH,,. These bodies are all liquids, boiling at 134, 136°—Ï37o, |