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III. The Influence of Pressure on the Temperature of Volatilization of Solids. 
By Willtam Ramsay, Ph.D., and Sydney Young, D.Sc. 
Communicated by Sir Andrew C. Ramsay, LL.D., F.R.S. 
Received June 5,—Read June 21, 1883. 
1. The experiments to be described in this paper were undertaken in order to ascertain 
whether solids have definite volatilizing* points under different pressures, as liquids 
have definite boiling-points, and whether these pressures are identical with their 
vapour-tensions at those temperatures. 
It has been long known that arsenic, which volatilizes without melting under 
atmospheric pressure, melts when the pressure is raised; and some years ago 
Carnelley proved that ice, mercuric chloride, and camphor do not melt below certain 
pressures peculiar to each substance; but above these pressures they melt when 
heated. He proposed the term “ critical pressure ” to denote that pressure below 
which a solid cannot melt. Preliminary experiments appeared to show that the solid 
might be raised in temperature above its ordinary melting-point without melting ; 
but it has since been experimentally proved that this is not the case. 
In January, 1881, shortly after the publication of Carnelley’s experiments, one of 
us read a paper before the Chemical Society of the Owens College, in which it was 
pointed out that, theoretically, at pressures below the triple point of Jas. Thomson, 
water should be unable to exist as such. It was at that time experimentally 
undecided whether ice could be heated above 0° C. or not; and the annexed diagram 
(fig. 1) was designed to show the relations of solid, liquid, and gas, to temperature 
and pressure. 
A somewhat similar diagram was subsequently published in ‘ Nature ’ by Petterssen, 
(June 23rd, 1881), which, however, did not show Jas. Thomson’s ice-steam line, and 
in which the triple-point was placed below, instead of above 0°. This was pointed 
out in a letter to ‘ Nature’ (July 14th, 1881). 
2. Although it appeared extremely probable that the temperature of a solid corre¬ 
sponding to a definite vapour-tension is identical with the temperature at which it 
* By the term “ volatilizing” we wish to imply the condition of a solid analogous to that of a liquid 
when it is said to be “ boiling ” ; and not the mere passing oil into vapour analogous to evaporating in 
the case of a liquid; in other words, the “ volatilizing point ” of a solid at a given pressure is the 
maximum temperature to which the solid can attain under that pressure. 
