TEMPERATURE AND THE PROPERTIES OF GASES 47 



by which the gas is boiled under a pressure which can be kept 

 constant when required or changed very slowly to coincide 

 with the slow change in temperature, which is indicated by some 

 delicate and sensitive thermoscope, while a thermometer is used 

 to make the actual measurements of temperature when this has 

 been constant for a sufficient time for a steady state to have 

 been reached in the gas reservoir and adjacent parts. Although 

 the gas thermometer is the invariable standard, subject to the 

 corrections considered above, it is hardly ever used for the 

 actual measurements, partly because a standard gas thermo- 

 meter is a valuable instrument which might be damaged in the 

 course of the experiments, and partly because the work of 

 reading the pressure and volume and of keeping all the condi- 

 tions suitable for obtaining the best results is so laborious and 

 complicated that the temperatures are better obtained by means 

 of resistance or thermoelectric thermometers which have been 

 carefully calibrated in the neighbourhood of the experimental 

 points by comparison with a standard gas thermometer. These 

 electric methods have also the great advantage that the measure- 

 ments can take place in another room in quiet. 



What has been said about low applies equally to high 

 temperatures, only here the gas reservoir is sometimes immersed 

 in the vapour of a boiling liquid ; but very few isothermal measure- 

 ments have been made at high temperatures except at the 

 comparatively low pressures of gas thermometery. 



There is some reason for this, as there are only a few 

 substances where high temperature measurements are likely to 

 give any very important result. Of these, mercury is pro- 

 bably the most manageable, although other substances, such as 

 zinc and cadmium, which also have monatomic vapours would 

 be of great interest. There are at the present time many 

 measurements on vapour pressures, but these give only very 

 meagre information in comparison with that obtained when the 

 volume is measured also. The normal boiling point is only a 

 special vapour pressure which occurs at different reduced 

 temperatures, as the pressure of 1 atmosphere is a varied 

 fraction of the critical pressure. It is not without interest 

 to consider the relation between the boiling point and the 

 critical data of all the mono-, di-, and tri-atomic substances for 

 which reasonably accurate data are available, as collected in 

 Table III. 



