is suited to the present needs of the chemistry of solid and liquid substances1. 1 It is interesting to observe in the applications of the periodic law the survival, in modified and more precise form, of the old conception of the element as an essence or principle, capable of impressing on all substances into which it entered properties sufficiently definite to mark off these substances from all others which did not contain this principle. An interesting and important paper on the periodic law, especially as applied to the classification of elements and compounds, by T. Bayley, will be found in Phil. Mag. (5) 13. 26. An important paper has just appeared by Carnelley (Phil. Mag. for July, 1884), in which the periodic law is illustrated by considering the melting and boiling points, and to some extent also the heats of formation, of the halogen compounds of the elements; and the facts thus obtained are applied to determine the values to be assigned to the atomic weights of various elements, and also the positions of these elements in the general scheme of classification based on the law in question. The case of beryllium is considered in detail in this paper. CHAPTER IV. APPLICATION OF PHYSICAL METHODS TO QUESTIONS OF CHEMICAL STATICS. 116. CHEMISTRY being a more concrete science than physics must of necessity derive help in solving its problems from the use of physical methods of investigation: but while using such methods the chemist ought not to forget that his aim is to find answers to chemical, not to physical questions. Minute descriptions of physical processes, and details of physical experiments are not demanded in a treatise on physical chemistry; much less is there required elaborate enunciations of the methods of calculation employed in physical researches. Such things give it is true an appearance of great accuracy and profound knowledge; but the apparently accurate knowledge and full discussion of physical details too frequently serves as an excuse for loose statements and superficial generalisations regarding those vital chemical questions for answering which so vast a collection of 'precautionary and vehiculatory gear' has been provided. In attempting to give an outline of the more important applications of physical methods to chemistry one is also liable to err in the other direction: vague statements to the effect that the boiling points of homologous hydrocarbons exhibit constant differences, or that the molecular structure of carbon compounds is intimately connected with their optical activity, or that chemical actions which involve a loss of energy in the reacting systems frequently occur,-statements such as these are utterly inadequate. I cannot hope to avoid both dangers: but I may venture to believe that the contents of the present chapter will be of some assistance to those who attempt to gain clear conceptions on the important phenomena forming the subjectmatter of physical chemistry. Of the physical methods employed by the chemist as aids in attempts to solve the questions of chemical statics, I shall consider (1) thermal methods, (2) optical methods, (3) methods which involve measurements of the volumes of reacting substances, and (4) methods based on determinations of 'etherification-values'. SECTION I. Thermal Methods1. 117. The principle of the conservation of energy lies at the root of all thermo-chemical investigation. When two or more chemical substances react so as to produce a new system, or new systems of substances, mechanical work may be done by expansion, electrical currents may be produced, heat may be generated, and energy may be lost in the forms of sound or radiant heat. The sum of these various kinds of energy, together with the energy remaining in the final system, must be equal to the energy which was present in the original system. A very large part of the energy lost during chemical changes generally leaves the changing systems in the form of heat; hence, measurements of the quantities of heat evolved during definite chemical processes afford valuable information with respect to the differences between the amounts of energy possessed by the systems in their original and final states. To measure such differences of energy is the primary aim of thermal chemistry. 1 Principal text-books on the subject are NAUMANN'S Lehr- und Handbuch der Thermochemie (1882). THOMSEN's Thermochemische Untersuchungen, containing in a systematic form the work of many years which has hitherto been scattered through various memoirs: 3 vols. are now (1884) published. BERTHELOT'S Essai de Mécanique Chimique fondée sur la Thermochimie, 2 vols. (1879) with supplement. JAHN'S Die Grundsätze der Thermochemie (1882). We are accustomed to conceive of most chemical changes as divisible broadly into two parts, (1) separation of molecules into atoms, (2) re-arrangement of atoms to form new molecules. We picture to ourselves the final arrangement of the atoms as dependent on the nature of these atoms, and on their relative positions in the molecules which composed the original system, that is to say, we picture the progress of mutual actions and reactions among the separated atoms. As we know little, or nothing, of the causes of this re-arrangement, we are accustomed to say that 'the atoms are attracted towards each other by the force of chemical affinity'. Consideration of the circumstances under which chemical changes proceed will, I think, make it evident that measurements of the quantities of heat evolved during these changes do not represent measurements of the 'chemical affinities"1 of the reacting atoms; but these measurements do enable us to draw conclusions as to the constitution of chemical substances, and the general laws of chemical change. The bearing of thermochemical measurements on the subject of affinity and chemical equilibrium in general will be considered in the second book: in the present section I propose to give a sketch of the methods of thermal chemistry, and a summary of the more important results obtained relating to allotropy, isomerism, nascent state, and other phenomena of chemical statics. 118. The notation of thermal chemistry is very simple: the formulæ of the reacting substances are enclosed in a square bracket, and each formula is separated from the other by a comma. Thomsen writes the figure expressing the number of atoms of each element above the symbol of that element. Thus, the formula [H2, C12] = 44,000 +, means that a quantity of heat sufficient to raise the temperature of 44,000 grams of water from o° to 1° C., is evolved during the chemical process represented in ordinary notation by H2+ Cl=2HCl, the quantities of hydrogen and chlorine being taken in grams'. 1 See post, book II. chap. III. 2 The unit of heat employed in this section is always to be taken as the gram-unit. The symbol Aq, separated by a comma from another symbol, means that a large excess of water is present and that its effect in the total thermal change is taken into account; thus, [HCl, Aq] = 17,320+, means that in the absorption of 365 grams of hydrochloric acid by an unlimited amount of water, 17,320 gram-units of heat are evolved; [H2, Cl2, Aq] = 61,320 +, means that the combination of 2 grams of hydrogen with 71 grams of chlorine in the presence of an unlimited amount of water is attended with the evolution of 61,320 gram-units of heat. [HCIAq, KOHAq] = 13,750+, means that when 365 grams of HCl dissolved in a large excess of water react on 56 grams of KOH, also dissolved in a large excess of water, 13,750 gram-units of heat are evolved. The symbol H2O is used as in ordinary notation to represent 18 grams of water; thus (1) [Mn, O2, SO2, 4H2O]=190,810+; (2) [MnSO 4H O, Aq]= 1770+; mean, (1) that in the formation of the amount, in grams, of crystallised manganous sulphate expressed by the formula MnSO 4H,O, from the amounts, in grams, of manganese, oxygen, sulphur dioxide, and water, expressed by the respective formulæ Mn, O., SO, and 4H,O, 190,810 gram-units of heat are evolved: (2) that in the solution of the foregoing number of grams of crystallised manganous sulphate in an unlimited quantity of water 1770 gram-units of heat are evolved. An ordinary chemical equation may be supplemented by the corresponding thermal symbol. Thus H2+Cl2=2HCl +[H2, CI2]; i.e. 2 grams of hydrogen combine with 71 grams of chlorine to give 73 grams of hydrochloric acid, and the change is attended with the evolution of a definite amount of heat. The fact that the decomposition of 2HCl into H, and Cl, is attended with the absorption of the same quantity of heat as is evolved during the union of H, with Cl, may be expressed thus 2HCl=H2+ C1-[H2, C12]1. 1 This notation is however confused and awkward, and is scarcely used. |