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PROFESSOR W. RAMSAY AND DR. S. YOUNG 
and at a given temperature it reaches a maximum, owing to equilibrium being estab¬ 
lished between dissociation and recombination, as well as between evaporation and 
condensation, if the latter occurs. We have already shown (‘ Transactions,’ Part I., 
1884, p. 37) that in the case of solids, where unlike molecules are not formed when 
change to a gaseous condition occurs, the curve which represents the maximum 
pressure attainable by the vapour of a solid at different temperatures represents also 
the maximum temperature to which the solid can be heated under different pressures, 
when there is free surface for evaporation. This has long been known to hold for 
liquids. If the work done in dissociating complex molecules of a solid or liquid into 
unlike simpler gaseous molecules is analogous to that expended in converting a stable 
solid or liquid into gas, it seems not unnatural to expect that the above relation 
between temperature and pressure should hold good for such bodies as dissociate 
in passing from the solid or liquid into the gaseous state. 
In the present memoir, the relations between temperature and pressure in the case 
of dissociating substances will be studied. 
Description of General Methods. 
§ 8. Temperature of Volatilization under different Pressures. — The method 
employed has already been described in the ‘Transactions,’ Part II., 1884, p. 461. It 
was often found more convenient to heat the substance in the still, by means of a 
vapour-jacket instead of a paraffin or sulphuric-acid bath. Care must be taken that 
the temperature of the vapour is at least 30° to 40° higher than that registered by 
the thermometer. For high temperatures, the vapour of aniline, methyl-salicylate, 
bromonaphthalene, or mercury was used as a jacket; and for very high temperatures, 
it was preferable to use a still with a short tube, with the stem of the thermometer 
outside of the apparatus. This, of course, necessitated a somewhat troublesome 
correction, in order to ascert ain the true temperature; for different parts of the stem 
of the thermometer were heated to different temperatures. In many cases, air was 
introduced into the previously calibrated thermometer to prevent sublimation of 
mercury in the stem. Although the results are uniformly given in arithmetical 
order, yet readings were taken sometimes after raising, sometimes after lowering the 
pressure. 
§ 9. Vapour-pressure (Pressure of Dissociation) at different Temperatures. —The 
customary method was employed of introducing the substance into a barometer-tube, 
heating the tube, and comparing the height of the column of mercury with that in a 
barometer placed in the same trough. The methods of introducing liquids or solids 
into the experimental tube, so as to exclude air and moisture, have been already 
described in the memoir referred to above. As it was necessary to heat the experi¬ 
mental tube to any desired temperature, and to maintain that temperature constant 
for any length of time, the following method was resorted to as the only practicable 
