bodies taking part in this change have been gasified we do not know the molecular weight of any of them. We might read the equation thus; atomic aggregates of barium chloride and sodium sulphate interact to produce atomic aggregates of barium sulphate and sodium chloride.

The definition of molecule is a physical definition; it is stated in terms which have an accurate meaning only when used of gaseous elements and compounds. If we choose to use the term in speaking of the phenomena of liquid and solid bodies we must not forget that the term cannot then be accurately defined. The definition of reacting weight is a chemical definition; but the term reacting weight is much vaguer than the term molecule. The reacting weight of a solid or liquid compound is doubtless an aggregation of atoms which interacts, as a whole, with other aggregations of atoms; but whether the number of atoms in this aggregation is the same in all chemical changes we do not know.

=

We have seen in Chap. xv. that the atomic weights of 316 most of the elements have been determined, either by the method based on the law of Avogadro, or by that founded on the generalisation 'atomic weight into spec. heat a constant.' But the molecular weights of only a few elements have been determined. About 70 elements are known; 14 of these have been gasified; therefore the molecular weights of only 14 are known. The specific gravities in the gaseous state, and hence the molecular weights, of some of these 14 elements are constant through a wide range of temperature; the specific gravities, and hence the molecular weights, of others have very different values at different temperature-intervals. The most probable explanation of the changes in the values of the molecular weights of certain elements is that the atomic compositions of the molecules of these elements are different at different temperatures. Thus sulphur-gas from about 450° to about 600°, is 96 times heavier than an equal volume of hydrogen at the same temperature; therefore the molecular weight of sulphur-gas between 450° and about 600° is approximately 96 × 2=192. But from about 800° and upwards sulphur-gas is only 32 times heavier than hydrogen; therefore the molecular weight of sulphur-gas at temperatures above 800° is approximately 32 x 2 = 64. The atomic weight of sulphur is 31.98; this number is determined by applying Avogadro's law to many gaseous compounds of sulphur, and it is verified by determinations of the spec. heat of sulphur.

317

Now 31.98 x 6 = 191.88, and 31.98 × 2 = 63.96; therefore we conclude; (1) that sulphur-gas at 450° to 600° has the molecular weight 191.88, and at 800° and upwards the molecular weight 63.96; and (2) that the molecule of gaseous sulphur at 4500 to 600° is composed of 6 atoms, or is hexatomic, and that the molecule at 800° and upwards is composed of 2 atoms, or is diatomic.

The expression atomicity of a molecule is used to denote the number of atoms which form the gaseous molecule of an element or compound. The data for classifying the molecules of elements in accordance with their atomicities are presented in the following table.

Monatomic

Atomicity of elementary gaseous molecules.

Hexatomic

318

319

Diatomic

Sulphur ((about 4000 to 600°)

Selenion

(7000 to about 8000)

Arsenic

(at temps. below
white heat)

The molecular weights of some gaseous compounds also vary with variations of temperature. Thus nitrogen tetroxide at very low temperatures is about 46 times heavier than hydrogen, but at higher temperatures it is only 23 times heavier than hydrogen; therefore this gas has two molecular weights which are approximately equal to 92 and 46 respectively. Determinations of the atomic weights of nitrogen and oxygen, and accurate analyses of the compound nitrogen tetroxide, shew that the composition of the molecule of this gas at very low temperatures is represented by the formula NO, (N = 28·02, 063.84)=91 86, and at higher temperatures by the formula NO, 45.93. We shall examine the relations between changes of molecular weight and changes of temperature in more detail later (s. pars. 334 to 337).

4

The conception expressed in the term atomic weight is now

seen to be much more definite than that expressed in the term combining weight. The former term brings before the mind the picture of a small definite portion of matter with definite properties; the latter term merely expresses a ratio. The values of atomic weights are determined by two methods, of general applicability, which are deduced from the principles of a theory of the structure of matter which gives a fairly simple explanation of most, if not all, of the observed physical properties of matter. The value given to the combining weight of an element is a purely empirical value; it must be determined for each element by methods specially applicable to that element; it is one of several possible values, and it is selected on the ground of general convenience and expediency.

The conception expressed in the term molecular weight is 320 also much more definite than that underlying the expression reacting weight. The molecule of a gas is a perfectly definite quantity of matter with defined properties; it is a physical conception, deduced by dynamical reasoning from a physical theory of the structure of matter. This theory presents us with one generally applicable method for determining the relative weights of gaseous molecules. The reacting weight of a body is said to be 'the smallest relative mass of it which takes part in chemical interactions'; but this statement involves terms which cannot, at present at any rate, be accurately defined. The value which, under the circumstances, is the best to be given to the reacting weight of a substance must be deduced for each substance by methods which to a very great extent are empirical, and many of which are applicable only to the special case under consideration. It is true that the molecular and atomic theory has not yet enabled us to define the term molecule as applied to a liquid or solid body; but the theory has thrown light on the chemical conception of reacting weight, and in place of regarding the values of reacting weights merely as numbers, we may now look on them as expressing the relative weights of certain aggregations of atoms which interact with each other to produce new aggregations of atoms.

The molecular and atomic theory then regards the properties 321 of a gas as the properties of the molecules of that gas; and the properties of the molecules as dependent, among other conditions, on the nature and number of the atoms which form these molecules. But if this view is correct, and if it is true that the number of atoms in the molecule of a gaseous element may

322

323

vary, we should expect sometimes to find differences in the properties of one and the same element.

The descriptions given in Chap. XI. pars. 173, 220, 221, of the properties of sulphur and phosphorus shewed that each of these elements differs very considerably in properties under different conditions. But as both elements exhibit differences in the solid state we cannot say whether these differences are, or are not, connected with differences in the atomicities of the molecules of the elements.

Some of the prominent properties of oxygen were described in Chap. VIII. pars. 118 to 121. If a quantity of pure dry oxygen is confined over sulphuric acid and a series of electric induction-sparks is passed through the oxygen, the volume of the gas diminishes until it has become about less than it was at the beginning of the experiment. The properties of the gas after the diminution of volume has ceased are markedly different from those of oxygen; nevertheless it has been conclusively proved that nothing has combined with the oxygen during the change which has occurred. The new gas is called ozone (because of its smell). The whole of a specified quantity of oxygen cannot be changed into ozone, so that pure ozone has not yet been obtained. But experiments have proved; (1) that the relation between the volume of that portion of a quantity of oxygen which is changed into ozone, and the volume of the ozone formed is expressed by the ratio 3 : 2; and (2) that the weight of the ozone produced is equal to the weight of the oxygen which has been changed into ozone. If the gas through which electric sparks have been sent until diminution of volume has ceased is heated to 360° or so, expansion occurs, and the original volume of oxygen is reproduced.

The outcome of the experiments on the volume- and massrelations between oxygen and ozone is that the change of oxygen to ozone, or vice versa, is attended with no change of mass, but that 3 volumes of oxygen condense to 2 vols. of ozone, and 2 volumes of ozone are changed by heat to 3 volumes of oxygen. Now as oxygen is 16 times heavier than hydrogen, it follows that ozone is 24 times heavier than hydrogen; therefore, as the molecular weight of a gas is twice its specific gravity referred to hydrogen, it follows that the molecular weight of ozone is 24 × 2 = 48. As ozone is only modified oxygen, and as the atomic weight of oxygen is 16, the molecule of ozone must be composed of 3 atoms of oxygen.

Therefore the atomic composition of the molecule of ozone is expressed by the symbol O,, that of the molecule of oxygen being expressed by the symbol O..

Oxygen and ozone are both colourless gases; oxygen is odourless, ozone has a very pronounced odour; ozone is a very energetic oxidiser, e. g. when passed into mercury at ordinary temperatures it produces mercuric oxide, and it interacts with lead sulphide (PbS) to produce lead sulphate (PbSO); oxygen does not react with an aqueous solution of potassium iodide (KI), ozone interacts with this salt in aqueous solution and produces potassium oxide, iodine, and oxygen; thus

2KIAq +02 = K2OAq+I2+0.

3

2

The existence of the two kinds of molecules, O, and O̟,, one diatomic and the other triatomic, each characterised by its own properties yet each composed of the same kind of atoms, and of atoms all of which are alike, shews that the properties of some molecules at any rate are conditioned (among other circumstances) by the number of atoms which are combined to form these molecules.

324

The other cases of allotropy (s. Chap. xi. par. 173) exhibited 325 by elements are exhibited by those elements in the solid state, The numbers of atoms in the atomic aggregates which compose the reacting weights of the different solid forms of phosphorus, sulphur, arsenic, &c. may perhaps be different; but we cannot at present decide whether this is so or not.

12

If the properties of a gas are dependent only on the 326 nature and number of the atoms which form the molecule of that gas, then the existence of more than one gaseous compound having a specified composition must be impossible. But, as a matter of fact, several compounds frequently exist, all having the same composition and the same molecular weight. For instance 3 compounds having the composition CH1, are known; these bodies have all been gasified; their specific gravities as gases, and therefore their molecular weights, are identical. But the molecule of each compound is composed of 5 atoms of carbon united with 12 atoms of hydrogen. Therefore we conclude that the properties of some gaseous molecules are conditioned by other circumstances besides the nature and number of their constituent atoms, and that one of these other circumstances probably is the arrangement of the parts of the molecule relatively to each other.

The existence of more than one compound with the same

M. E. C.

15