On the Limit of the Liquid State. 



295 



surface tension or capillarity ; in fact, that the state of a fluid above 

 that temperature coincided with the properties we call gaseous. The 

 paper concluded, "The difference between the liquid and the gaseous 

 states is not then entirely dependent upon the length of the mean free 

 path ; but also upon the mean velocity of the molecule." That is to say, 

 we may compress a gas (when a few degrees above the critical tempe- 

 rature) to a less volume than it might occupy as a liquid, and it will 

 still remain gaseous. In the following paper, therefore, the term 

 liquid will be applied only to such bodies as exhibit surface tension, 

 •either as capillarity or by forming a permanent limiting surface when 

 in contact with a vapour or gas. The term gas will be applied to that 

 state of a fluid which precludes its being reduced to a liquid by 

 pressure alone, in other words, to any fluid above its critical tempera- 

 ture. The term vapour will be applied, as has already been done by 

 Andrews, to fluids which can be reduced to liquid by pressure alone, 

 that is, to any aeriform fluid at a temperature lower than the critical. 

 Thus carbon dioxide is a vapour at ordinary temperature, but is a gas 

 at temperatures over 31°. A further distinction of gas and vapour 

 lies in the fact that, on increasing the pressure, the volume of a gas 

 goes on diminishing in a regular way, whereas there is a part of the 

 curve representing pressure and volume of vapour where the curve is 

 asymptotic, that is, where the vapour is in contact with its liquid. 



In the following paper reasons will be shown for believing that the 

 gaseous state depends entirely upon the mean velocity and not upon 

 the mean free path of the molecule at all. The difference between 

 vapour and liquid, on the other hand, is entirely one of the length of 

 the mean free path. The methods of experiments used were similar 

 to those detailed in the paper above referred to, but a larger appa- 

 ratus was employed, so that the results might be more distinctly 

 visible. It was soon noticed that the readings of pressures of mano- 

 meters varied a little with the diameter of the tube employed, the 

 smallest bore giving the highest reading, and this was the case to such 

 an extent as to cause an error of two atmospheres in 70, and about 

 five in 100 ; the higher the pressure the greater the difference. The 

 wide tube was about 0'8 millim. in diameter, the smaller 0*1 millim. 

 Now, whether this error was caused by the hydrogen condensing 

 against the glass, and being thus lost as a manometric substance, or 

 whether it was caused by the hydrogen being dissolved in the film of 

 moisture which may be supposed to adhere to the interior of the tube, 

 has not yet been determined. It has been shown by Professors 

 Liveing and Dewar that the moisture adhering to glass is not driven 

 off till nearly a red heat is reached, and we may be sure that the 

 capillarity of the smaller tube would cause it to retain moisture more 

 eagerly than the larger one. Whatever was the cause, it was almost 

 invariably found that manometers with small bores gave higher read- 



