28 Prof. J. Thomson on the [Dec. 11, 



of pressure and temperature jointly, which may be called the triple 

 point *. 



The curve between gas and liquid, which may be called the boiling-line, 

 will be a separating boundary between the regions of the plane corre- 

 sponding to the ordinary liquid and those corresponding to the ordinary 

 gaseous state. But by consideration of Dr. Andrews's experimental 

 results (Phil. Trans. 1869), we may see that this separating boundary 

 comes to an end at a point of temperature and pressure which, in 

 conformity with his language, may be called the critical point of pressure 

 and temperature jointly ; and we may see that, from any liquid state 

 to any gaseous state, the transition may be gradually effected by an 

 infinite variety of courses passing round the extreme end of the boiling- 

 line f. 



The accompanying figure serves to illustrate these considerations in 

 reference to transitions between the three states, the gaseous, the liquid, 

 and the solid. The figure is intended only as a sketch to illustrate prin- 

 ciples, and is not drawn according to measurements for any particular 

 substance, though the main features of the curves shown in it are meant 

 to relate in a general way to the substance of water, steam, and ice. 

 A X and A T are the axes of coordinates for temperatures and pres- 

 * In making this statement, that it appears that the three curves must all cross each 

 other in one point, I would wish to offer it here (as I previously did in the 1871 British- 

 Association paper) subject to some reserve in respect of conditions not yet known with 

 perfect clearness and certainty. I have to suggest that we might not be quite safe in 

 assuming that, within a cavity containing nothing but pure water-substance partly 

 gaseous, the melting temperature and pressure of ice solidified from the gaseous state 

 would be the same as the melting temperature and pressure of ice frozen from the liquid 

 state, and in making other suppositions, such as that the same quantity of heat would 

 become latent in the melting of equal quantities of ice formed in these two ways, and in 

 neglecting conceivable but, I presume, as yet imperfectly known distinctions of capillary 

 conditions between ice amply wet with water and ice only moistened with the last 

 vestiges of water before the whole liquid may be either evaporated or frozen. It might 

 be a question in like manner whether we can be sure that there can be theoretically a 

 condition of repose in a cavity containing only perfectly pure water-substance in which 

 the three states are present together, each in contact with the other two, so that there 

 would be ice partly wet with water, and partly dry in contact with gaseous water-sub- 

 stance, or steam as it may be called, while the water and steam were also in contact with 

 each other. I offer these remarks by way of caution, as they force themselves into 

 notice when we attempt to sketch out the features of the three curves under considera- 

 tion, and because they may serve to suggest questions for experimental and theoretical 

 investigation which may have been generally overlooked before. In the present paper, 

 however, I proceed on assumptions, such as are usually tacitly made, of identity in the 

 thermal and dynamic conditions of pure ice solidified in different ways, assumptions 

 which, so far as is known, may be, and probably are, perfectly true ; and I proceed on 

 the supposition that there can be theoretically the condition of repose here alluded to, 

 of the solid, liquid, and gaseous states, present together each in contact with the other 

 two and consequently that the three curves would meet or cross each other in one point, 

 which I have called the triple point. 



t Mention of this condition has been already made in a former paper by me in the 

 'Proceedings of the Royal Society,' November 16, 1871, page 2. 



