SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 201 



When a gas such as carbonic acid, under a pressure which is not inferior to its 

 critical pressure, is confined in a tube as in Andrews' experiment and has a temperature 

 ever so little higher than its critical temperature, it fills the tube as a homogeneous 

 fluid which no pressure, however high, can liquefy. If, by removal of heat or by sudden 

 relief of pressure, its temperature is reduced ever so little below its critical temperature, 

 the homogeneous fluid begins to exhibit the peculiar appearance of moving or flickering 

 striae throughout its entire mass, as described by Andrews. These moving or flickering 

 striae indicate oscillations of density accompanying the efi"ort on the part of the homo- 

 geneous fluid to shed a portion of its mass in the liquid state before there is a liquid 

 nucleus for it to condense on and to aff"ord the first outlet to the latent heat, the escape 

 of which is an essential condition of liquefaction. 



§ 119. We do not know the temperature at which the dry gas can condense on the 

 dry walls of the envelope, but there can be little doubt that it is lower than that at 

 which it condenses on its own liquid. 



It is only in the conditions of Andrews' experiment that we can witness a substance 

 persisting in the gaseous state under a greater than the critical pressure and having a 

 temperature lower than the critical temperature, because it is only when the gas and 

 the envelope which contains it have been maintained at a temperature higher than 

 the critical temperature of the gas that the inner walls of the envelope have a chance 

 of being perfectly dry. By " perfectly dry " I mean free from every and any trace 

 whatever of the liquid substance. If the cooling process as specified above be then 

 carried out, the temperature may be reduced slightly below the critical temperature, 

 and yet the substance may persist in the gaseous state because there is none of itself 

 in the liquid state for it to begin to condense on. 



I have defined * the boiling point of a substance, under a particular pressure, to be 

 the temperature at which it as a liquid evaporates into itself as a gas, arid as a gas or 

 vapour condenses on itself as a liquid. When the gas condenses on any other substance, 

 or in a space filled only by itself, the temperature at which liquefaction commences is 

 uncertain. In the moment, however, that the first, even the minutest trace of liquid 

 appears, whether in the gas or on the walls, the temperature of condensation is defined, 

 because the gas is then condensing on itself as a liquid. How, in such an experiment, 

 the first element of liquid appears, we do not know. We say, it is by accident ; but 

 we may with equal right say, it is by an act of creation — because in the process some- 

 thing appears where there was nothing of the kind before. 



We thus see that there is a close analogy, in all important particulars, between the 

 state of unrest which exists in a supersaturated saline solution before crystallisation 

 commences and that indicated by the flickering striae in a supersaturated gas before 

 liquefaction takes place. 



* "Chemical and Physical Notes," by J. Y. Buchanan, F.R.S , The Antarctic Manual, 1901, p. 97. 

 TRANS. ROY. SOC. EDIN., VOL. XLIX., PART I. (NO. 1). 26 



