MOLECULES ANI) ATOMS 801 



reaction or entering into the composition of a compound, may be also 

 determined. 



sodium acetate, melting at +56 or, as is more generally practised, to place the vessel 

 containing the substance to be experimented with in the vapour of a liquid boiling at a 

 definite temperature, and knowing the pressure under which it is boiling, to determine 

 the temperature of the vapour. For this purpose the boiling points of water at different 

 pressures are given in Chapter I., Note 11, and the boiling points of certain easily procurable 

 liquids at various pressures are given in Chapter II., Note 27. (2) With respect to tempera- 

 tures above 800 (below which mercurial thermometers may be conveniently employed), 

 they are most simply obtained comtant (to give time for the weight and volume of a 

 substance being observed in a given space, and to allow that space to attain the calcu- 

 lated temperature I) by means of substances boiling at a high temperature. Thus, for 

 instance, at the ordinary atmospheric pressure the temperature t of the vapour of 

 sulphur is about 445, of phosphorus pentasulphide 518, of tin chloride 606, of cad- 

 mium 770, of zinc 980 (according to Violle and others), or 1040 (according to Deville), 

 &c. (8) The indications of the hydrogen thermometer must be considered as the most 

 exact (but as hydrogen diffuses . through incandescent platinum, nitrogen is usually 

 employed). (4) The temperature of the vapours used as the bath should in every case 

 be several degrees higher than the boiling point of the liquid whose density is to be 

 determined, in order that no portion should remain in a liquid state. But even in this 

 case, as is seen from the example of nitric peroxide (Chapter VI.), the vapour density 

 does not always remain constant with a change of t, as it should were the law of the 

 expansion of gases and vapours absolutely exact (Chapter II., Note 26). If variations of 

 a chemical and physical nature similar to that which we saw in nitric peroxide take 

 place in the vapours, the main interest is centred in constant densities, which do not 

 vary with t, and therefore the possible effect of t on the density must always be kept 

 in mind in having recourse to this means of investigation. (5) Usually, for the sake of 

 convenience of observation, the vapour density is determined at the atmospheric pres- 

 sure which is read on the barometer; but in the caae of substances which are volati- 

 lised with difficulty, and also of substances which decompose, or, in general, vary at 

 temperatures near their boiling points, it is best or even indispensable to conduct the deter- 

 mination at low pressures, whilst for substances which decompose at low pressures the 

 observations have to be conducted under a more or less considerably increased pressure. 

 (6) In many cases it is convenient to determine the vapour density of a substance in 

 admixture with other gases, and consequently under the partial pressure, which may be 

 calculated from the volume of the mixture and that of the intermixed gas (see Chapter I., 

 Note 1). This method is especially important for substances which are easily decom- 

 posable, because, as shown by the phenomena of dissociation, a substance is able to remain 

 unchanged in the atmosphere of one of its products of decomposition. Thus, Wurtz 

 determined the density of phosphoric chloride, PC1 5 , in admixture with the vapour of 

 phosphorous chloride, PC1 3 . (7) It is evident, from the example of nitric peroxide, that 

 a change of pressure may alter the density and aid decomposition, and therefore identical 

 results are sometimes obtained (if the density be variable) by raising t and lowering h 5 

 but if the density does not vary under these variable conditions (at least, to an 

 extent appreciably exceeding the limits of experimental error), then this constant density 

 indicates the gaseous and invariable state of a substance. The laws hereafter laid down 

 refer only to such vapour densities. But the majority of volatile substances show such 

 a constant density at a certain degree aboye their boiling points up to the starting point 

 of decomposition. Thus, the density of aqueous vapour does not vary for t between 

 the ordinary temperature and 1000 (there are no trustworthy determinations beyond 

 this) and for pressures varying from fractions of an atmosphere up to several atmo- 

 spheres. If, however, the density does vary considerably with a variation of h and t, 

 the fact may serve as a guide for the investigation of the chemical changes which are 

 undergone by the substance in a state of vapour, or at least as an indication of a 

 deviation from the laws of Boyle, Mariotte, and Gay-Lussac (for the expansion of gases 



