[15] Fused nitre, with a further rise of temperature, disengages oxygen and then nitrogen. The nitrite KNO2 is first formed and then potassium oxide. The admixture of certain metals—for example, of finely-divided copper—aids the last decomposition. The oxygen in this case naturally passes over to the metal.
[16] In China, where the manufacture of gunpowder has long been carried on, 75·7 parts of nitre, 14·4 of charcoal, and 9·9 of sulphur are used. Ordinary powder for sporting purposes contains 80 parts of nitre, 12 of charcoal, and 8 of sulphur, whilst the gunpowder used in heavy ordnance contains 75 of nitre, 15 of charcoal, and 10 of sulphur. Gunpowder explodes when heated to 300°, when struck, or by contact with a spark. A compact or finely-divided mass of gunpowder burns slowly and has but little disruptive action, because it burns gradually. To act properly the gunpowder must have a definite rate of combustion, so that the pressure should increase during the passage of the projectile along the barrel of the fire-arm. This is done by making the powder in large granules or in the shape of six-sided prisms with holes through them (prismatic powder).
The products of combustion are of two kinds: (1) gases which produce the pressure and are the cause of the dynamical action of gunpowder, and (2) a solid residue, usually of a black colour owing to its containing unburnt particles of charcoal. Besides charcoal, the residue generally contains potassium sulphide, K2S, and a whole series of other salts—for instance, carbonate and sulphate. It is apparent from this that the combustion of gunpowder is not so simple as it appears to be from the above formula, and hence the weight of the residue is also greater than indicated by that formula. According to the formula, 270 parts of gunpowder give 110 parts of residue—that is, 100 parts of powder give 37·4 parts of residue, K2S, whilst in reality the weight of the residue varies from 40 p.c. to 70 p.c. (generally 52 p.c.). This difference depends on the fact that so much oxygen (of the nitre) remains in the residue, and it is evident that if the residue varies the composition of the gases evolved by the powder will vary also, and therefore the entire process will be different in different cases. The difference in the composition of the gases and residue depends, as the researches of Gay-Lussac, Shishkoff and Bunsen, Nobel and Abel, Federoff, Debus, &c., show, on the conditions under which the combustion of the powder proceeds. When gunpowder burns in an open space, the gaseous products which are formed do not remain in contact with the residue, and then a considerable portion of the charcoal entering into the composition of the powder remains unburnt, because the charcoal burns after the sulphur at the expense of the oxygen of the nitre. In this extreme case the commencement of the combustion of the gunpowder may be expressed by the equation, 2KNO3 + 3C + S = 2C + K2SO4 + CO2 + N2. The residue in a blank cartridge often consists of a mixture of C, K2SO4, K2CO3, and K2S2O3. If the combustion of the gunpowder be impeded—if it take place in a cartridge in the barrel of a gun—the quantity of potassium sulphate will first be diminished, then the amount of sulphite, whilst the amount of carbonic anhydride in the gases and the amount of potassium sulphide in the residue will increase. The quantity of charcoal entering into the action will then be also increased, and hence the amount in the residue will decrease. Under these circumstances the weight of the residue will be less—for example, 4K2CO3 + 4S = K2SO4 + 3K2S + 4CO2. Besides which, carbonic oxide has been found in the gases, and potassium bisulphide, K2S2, in the residue of gunpowder. The amount of potassium sulphide, K2S, increases with the completeness of the combustion, and is formed in the residue at the expense of the potassium sulphite. In recent times the knowledge of the action of gunpowder and other explosives has made much progress, and has developed into a vast province of artillery science, which, guided by the discoveries of chemistry, has worked out a ‘smokeless powder’ which burns without leaving a residue, and does not therefore give any ‘powder smoke’ (to hinder the rapidity of firing and aiming), and at the same time disengages a greater volume of gas and consequently gives (under proper conditions of combustion) the possibility of communicating to the charge a greater initial velocity, and therefore greater distance, force, and accuracy of aim. Such ‘smokeless powder’ is prepared either from the varieties of nitrocellulose (Chapter VI., Note [37]) or from a mixture of them with nitro-glycerine (ibid). In burning they give, besides steam and nitrogen, generally a large amount of oxide of carbon (this is a very serious drawback in all the present forms of smokeless powder, because carbonic oxide is poisonous), and also CO2, H2, &c.
[17] The substances obtained in this case are mentioned in Chapter IX., Note [31].
[18] A. Scott (1887) determined the vapour densities of many of the alkali elements and their compounds in a platinum vessel heated in a furnace and previously filled with nitrogen. But these, the first data concerning a subject of great importance, have not yet been sufficiently fully described, nor have they received as much attention as could be desired. Taking the density of hydrogen as unity, Scott found the vapour densities of the following substances to be—
| Na | 12·75 | (11·5). | KI | 92 | (84). | |
| K | 19 | (19·5). | RbCl | 70 | (60). | |
| CsCl | 89·5 | (84·2). | CsI | 133 | (130). | |
| FeCl3 | 68. | AgCl | 80 | (71·7). |
In brackets are given the densities corresponding with the formulæ, according to Avogadro-Gerhardt's law. This figure is not given for FeCl3, because in all probability under these conditions (the temperature at which it was determined) a portion of the FeCl3 was decomposed. If it was not decomposed, then a density 81 would correspond with the formula FeCl3, and if the decomposition were Fe2Cl6 = 2FeCl2 + Cl2, then the density should be 54. With regard to the silver chloride, there is reason to think that the platinum decomposed this salt. The majority of Scott's results so closely correspond with the formulæ that a better concord cannot be expected in such determinations. V. Meyer (1887) gives 93 as the density of KI.
[19] The molecules of non-metals are more complex—for instance, H2, O3, Cl2, &c. But arsenic, whose superficial appearance recalls that of metals, but whose chemical properties approach more nearly to the non-metals, has a complex molecule containing As4.
[20] As the atomic weight of potassium is greater than that of sodium, the volumes of the molecules, or the quotients of the molecular weight by the specific gravity, for potassium compounds are greater than those of sodium compounds, because both the denominator and numerator of the fraction increase. We cite for comparison the volumes of the corresponding compounds—
| Na | 24 | NaHO | 18 | NaCl | 28 | NaNO3 | 37 | Na2SO4 | 54 |
| K | 45 | KHO | 27 | KCl | 39 | KNO3 | 48 | K2SO4 | 66 |