199.8 mercury Mercury diethide 9'97 287.8 257'7 +47.88 5 Mercurous chloride 8.3 239'6 235'17 V Analysis, stated in parts per molecule, hydrogen being taken as unity +23'94 carbon +6 hydrogen +35 37 chlorine + 10 Notes to the preceding Table. 1 The density of hydrofluoric acid was determined indirectly by Gore (Phil. Trans. for 1869. 173) at 100°. Mallet (Amer. Chem. Journal 3. 189) by directly weighing litre of the gas at 30° found the specific gravity to be 142, which gives a molecular weight of 4102. The molecular weight of this gas therefore decreases as temperature increases. 2 and 3 Indirectly determined by Bineau (Ann. Chim. Phys. [2] 68. 424); two volumes of each hydride when decomposed by metal yielded 2 vols. of hydrogen, 78 parts by weight of selenion in one case, and 128 parts by weight of tellurium in the other being produced. At a temperature slightly above its boiling point the specific gravity of gaseous stannous chloride points to the molecular weight 377; but at 200° higher the specific gravity is as given in the table; this gas therefore, like hydrofluoric acid, has two molecular weights: see Meyer and Züblin (Ber. 13. 811). 5 There is some doubt whether the vapour of mercurous chloride does or does not contain mercury and mercuric chloride: the number in the table is from a paper by Fileti, who states that by vaporising a mixture of the two chlorides of mercury, the protochloride remains undissociated (see abstract of Fileti's paper in C. S. Journal Abstracts for 1882. 466). 6 Chromium hexfluoride (CrF) is frequently mentioned in text-books as a gaseous compound of chromium; the evidence in favour of the existence of a definite fluoride of chromium is meagre, no determinations of its density (if it exists) have been made; see Unverdorben (Pogg. Ann. 7. 311). 7 Odling [Phil. Mag. [4] 29. 316) gave the specific gravity of aluminium tetramethide at temperatures above 200° as 25, and at 130° as 5'0; but it is undecided whether the gas at 200° was homogeneous or a mixture of the products of decomposition by heat of molecules existing at lower temperatures (see Wanklyn loc. cit. 313 and Williamson do. 395). If the gas at 200° was really homogeneous, we should have 2.5 × 28.87=725 as the molecular weight of aluminium tetramethide, and this quantity of the gas contains 27'3 aluminium +35°91 carbon +9 hydrogen (=72·21). 8 At 450° the sp. gr. of the vapour of gallic chloride is 7.8, and at the same temperature in presence of an indifferent gas acting as diluent, it is 66: the gas dissociates under these conditions. (See. Lecoq de Boisbaudran, Compt. rend. 93. 294, 329 and 815.) The maximum atomic weights deduced from these data may in many cases be regarded with a large degree of probability as the true atomic weights of the elements. The greater the number of gaseous compounds of an element analysed, the greater is the probability that the number which represents the smallest amount of that element in two volumes of any of these compounds is the true atomic weight of the element. 7. 20. When the atomic and molecular weights of an element are known, the atomicity of the molecule, i.e. the number of atoms in the molecule, is known. In the following table the molecules of the elements, so far as the relative weights of these have been determined by the method founded on Avogadro's law, are classified in accordance with their atomicity. Atomicity of Elementary Molecules. The molecules of the majority of the elements in this table are diatomic, but inasmuch as the molecular and atomic weights of only 13 elements have been determined it is impossible to say whether a majority of all the elementary molecules contain each two atoms. It ought also to be observed that of the 13 elements in the table, five have more than one molecular weight, and therefore exhibit the phenomenon of varying atomicity. The table contains two well-defined metals, cadmium and mercury; the molecules of these elements are monatomic, and hence are of a simpler structure than the molecules of those elements which possess in a marked manner the properties summed up in the term nonmetal. 1 This table shews that many elementary gases have complex structures; hence arise difficulties in forming accurate physical conceptions of actions and reactions among the parts of these structures. This will be again referred to when dealing with atomic heats (see pp. 64—5). 21. Chemical formulæ for the most part profess to represent not only the elementary composition, but also the relative weights of the molecules, of the bodies formulated: but unless some method for determining molecular weights other than that founded on Avogadro's law is adopted, it is evident from the data in the table on pp. 37-40 that the majority of the formulæ employed in mineral chemistry cannot be said to be certainly molecular formulæ. Thus analysis shews that 17.96 parts by weight of water contain 15.96 parts of oxygen and 2 parts of hydrogen; analysis also shews that 58:37 parts by weight of sodium chloride contain 23 parts of sodium and 35 37 parts of chlorine. The specific gravity of water vapour shews that the molecular weight of this compound is about 18, hence-assuming the atomic weight of oxygen to be 15.96—the molecular formula is written H2O (1796). But no determination of the density of sodium chloride vapour has yet been made; hence the molecular weight may be about 59, or it may be a multiple of this number (assuming the atomic weights of sodium and chlorine to be known), and hence the formula NaCl (58.37) is not necessarily molecular, and is therefore not strictly comparable with the formula H2O. Even if a formula does express the relative weight of the molecule of the body formulated it is well to remember that it is the weight of the gaseous molecule which is thus expressed; the formula does not necessarily also represent the relative weight of the molecule of the same body when solid. As a general rule, the melting and boiling points of substances with large molecular weights are high; thus in any homologous series of hydrocarbons the boiling and melting points increase with increase of molecular weight'; the same connection between these constants is noticed in many series of oxides, e.g. the oxides of nitrogen. It would therefore appear probable that the molecular weight of a solid is greater 1 Thus, B. P. = 2 70° 209 CH10 CH12 CH14 CH16, C8H18, CgHa0, C10H22 &c. 1o 38° NO N2O gaseous at 110°, B. P. 88°, about 20°, 22°, M. P. = 30°. 6 than that of the same substance when in the state of gas. So also, as a rule, the action of heat produces molecules of less, from those of greater weight. Thus N,O, which exists at low temperatures becomes NO, when heated (see numbers on p. 32); so S. exists at 500°, but S, at 1000" at temperatures above 300° the molecule O, decomposes into O Reactions are known in which heat appears to favour the production of molecules of greater weight and complexity than those previously existing; but these more complex molecules only mark intermediate stages towards the formation of less complex and comparatively lighter molecules. Thus the action of heat on sodium-hydrogen sulphate is generally formulated in two stages, (1) 2NaHSO, Na,S,O,+ H2O: (2) Na,S,O, Na,SO, +SO,. So also when mercuric cyanide is. decomposed by heat molecules of cyanogen are produced, having the formula (CN)n where n > 2, but at 800°-900° these are dissociated into the lighter molecules C,N. Lead monoxide (PbO)n when heated forms the heavier oxide (Pb,O,), &c.: in many of these cases however we are not certain that the formulæ employed represent the relative weights of molecules. = = The physical phenomena presented by liquids and solids cannot be expressed by such comparatively simple generalisations as those which express the properties of gases; the molecular phenomena of the former classes of bodies are evidently more complex than those of the latter class. Great caution must therefore be used in applying deductions made from the study of the molecular phenomena of gases to solid or liquid bodies1. 22. The following table gathers together the results of the observations recorded in the table on pp. 37-40, so far as regards the maximum atomic weights of elements determined by the application of Avogadro's law: 1 The comparison of the molecular phenomena of gases with those of solids and liquids will be considered more fully in a future chapter. |