1873.] Gaseous , Liquid, and Solid States of Water. 29 



sures respectively ; A, the origin, being taken as the zero for pressures 

 and as the zero for temperatures on the Centigrade scale. The curve L 



represents the boiling-line terminating in the critical point E. The line 

 T M represents the line between liquid and solid. It is drawn showing 

 in an exaggerated degree the lowering of the freezing temperature of 

 water by pressure, the exaggeration being necessary to allow small 

 changes of temperature to be perceptible in the diagram. The line T N 

 represents the line between the gaseous and the solid states of water- 

 substance. The line L T N appears to have been generally (in the dis- 

 cussion of experimental, results on the pressure of aqueous vapour above 

 and below the freezing-point) regarded as one continuous curve ; but it 

 was a part of my object in the two British- Association papers referred 

 to, to show that it ought to be considered two distinct curves (L T P 

 and N T Q) crossing each other in the triple point T. 



In the second of the two British-Association papers already referred 

 to (the one read at the Brighton Meeting, 1872), I gave demonstrations 

 showing that these two curves L T and N T should meet, as shown in 

 the accompanying figure, with a re-entrant angle at T, not with a salient 

 angle such as is exemplified in the vertex of a pointed arch, and offered 

 in conclusion the suggestion that the reasoning which had been adduced 

 might be followed up by a quantitative calculation founded on experi- 

 mental data, by w r hich calculation the difference of the pressures of steam 

 with water and steam with ice for any given temperature very near the 

 triple point may be found with a very close approximation to the truth. 



In the month of last October (October 1872) I explained to my 

 brother, Sir William Thomson, the nature of that contemplated quanti- 

 tative calculation : I mentioned to him the method which I had prepared 

 for carrying out the intended investigation, and inquired of him for some 

 of the experimental data, or data already deduced by theory from experi- 



