30 QUEEN'S QUARTERLY. 



boiling, not only at the surface but all through the vessel. The 

 temperature at which this occurs is known as the critical tempera- 

 ture. There is more or less doubt as to the nature of matter at the 

 critical temperature. Some have thought that the liquid state came 

 to an end here; others that the liquid state persisted above this tem- 

 perature but could not be seen on account of being mtimately mixed 

 with the vapor. The later theory is that the liquid and vapor mole- 

 cules are of different kinds, the liquid being perhaps congeries of 

 the vapor ones, and that a liquid always contains some of the vapor 

 molecules and the vapor some of the liquid molecules. As the tem- 

 perature is raised the proportions of the two become more alike until 

 at the critical point vapor and liquid contain the same amounts of 

 each, are precisely alike and mix thoroughly. This theory accounts 

 for many observed facts and will do for the present. 



All liquids undergo the same changes with rise of temperature 

 and, except for the temperature at which visible liquid disappears 

 and the pressure at the time, there is no difference. Here is a unity 

 which has not been fully understood. Air has a critical temperature 

 just as water has, and if the proper temperature could be reached 

 solids, such as the metals, would show the same phenomenon. Some 

 of the more easily fusible solids have been investigated. 



The critical point is an interesting one. Here, a slight increase 

 of pressure causes an enormous decrease in volume, the substance is 

 sometimes spoken of as being " mushy " ; surface tension vanishes 

 and the liquid will not rise in a small tube. It is indeed a critical 

 point. 



Since changes of pressure affect the freezing point slightly, it 

 has been thought that there might be a lower critical point, a tem- 

 perature where the solid and liquid states would become identical, 

 but experimental evidence is against such a view, though the experi- 

 ments of Barus, Tammann and others show that for some substances 

 there is a temperature above which a solid ceases to exist and that 

 no pressure is sufficient to solidify the liquid. Below this tempera- 

 ture the liquid may be solidified by pressure, the pressure necessary 

 in many cases being thousands of atmospheres. The pressures in the 

 interior of the earth of course run up to enormous amounts, and it 

 is probable that many rocks at high temperature may be held in solid 

 form by the terrible pressure only to become liquid by a release in 

 pressure. 



A. L. C. 



