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Mr. J. B. Hannay. 



[June 16, 



cavity, in which state the whole substance cannot be determined by 

 sight alone, because no free surface is visible. It might be solid for all 

 our eyes could tell us. Here, again, we see that a free surface is 

 necessary for determining the state. In Andrews' experiments there 

 is no free surface, for at pressures greater than the critical a free 

 surface is impossible. It was towards the procural of this free surface 

 at any pressure that my work tended. The last definition is that of 

 the phrase " free surface." A free surface is that surface of a solid or 

 liquid matter which is in contact with vapour or gas, in fact, a surface 

 in contact with a space where it has freedom of motion. 



The difference between the solid and liquid states being easily 

 recognisable, need not be stated, but that between the liquid and 

 gaseous requires some consideration. Regnault has shown that every 

 liquid has a certain molecular activity corresponding to each tem- 

 perature, in virtue of which it throws off from its free surface with a 

 definite mean velocity, a certain number of molecules per unit of 

 surface. When the space over the liquid is limited, some molecules 

 pass back again into the liquid, and when this number equals that of 

 those outgoing, the space is said to be saturated. The pressure which 

 the molecules by their impacts cause upon the sides of the vessel is 

 called the vapour tension of the liquid. When the temperature rises, 

 a greater number of molecules are thrown off than with the liquid, and 

 the density of the vapour increases until it is nearly equal to that of 

 the liquid. But still there is a limiting surface indicating the lower 

 portion to be liquid. Let the temperature be now raised till the 

 densities of the upper and lower portions are equal, then we have a 

 state of matter in which the substance " assumes no surface nor definite 

 figure, and which is so extensible, that any quantity will distribute 

 itself throughout a space," as we before defined the gaseous state. 



Now as to the proof that it is really in the gaseous state. The 

 general effect of the attraction of the molecules of a liquid for each 

 other and for solids, yields three measurable phenomena. First, the 

 property of tensile strength measurable in liquid films (as soap- 

 bubbles), second, capillary attraction, and third, the cohesion by reason 

 of which the neck of a drop can support it as a wire supports a load. 

 It has been shown by experiment, that all these three manifestations 

 of attraction decrease as the temperature rises, and each may be used 

 as a measurement of the other. As " surface tension " is the feature 

 of the liquid state, it was necessary to investigate how it is affected 

 by the gas or vapour with which it is in contact, and the results 

 showed that the height of a liquid in a capillary tube is not per- 

 ceptibly affected by replacing the air by hydrogen, or removing every- 

 thing except the vapour of the liquid. Even when hydrogen was 

 compressed at 200 atmospheres pressure over a liquid in which it is 

 nearly insoluble, the capillarity only fell very slightly. It appeared 



