92 



He discovered that each gas has a critical temperature, as he called 

 it. For carbon dioxide, this temperature can be observed by 

 placing a heavy-walled, glass tube (Fig. 38), half -filled with 

 liquid carbon dioxide, in a beaker of water, and gradually raising 

 the temperature of the latter. At 31.35, the surface between 

 the liquid and gas becomes hazy and vanishes. At this tem- 

 perature the liquid state disappears, merging into the gaseous. 

 When the temperature falls once more, the surface marking the 

 boundary between liquid and gas reappears. 



The critical temperature of oxygen is 118, of hydrogen 

 -234, of sulphur dioxide 156, of water 358. 



Another Deviation from the Laws of Gases. A Perfect 

 Gas. It is also found by experiment that when a gas is already 

 under very high pressure, and therefore very closely packed, 

 an increase in the pressure does not produce quite as great a diminu- 

 tion in volume as Boyle's law leads us to expect This reminds us 

 that we are diminishing only the space between the molecules, 

 and not the volumes of the molecules themselves, and therefore 

 not the total volume of the gas. When, on severe compression, 

 the volume occupied by the molecules themselves has become 

 an appreciable fraction of the whole volume, additional compres- 

 sion does not affect the whole volume, and the contraction is 

 smaller than Boyle's law would indicate. Thus, 200 liters of 

 hydrogen, at 16 and under one atmosphere pressure, when sub- 

 jected to 200 atmospheres pressure, give 1.134 liters, instead of 

 1 liter. 



The last two effects (namely, those due to the tendency to 

 cohesion of, and the space occupied by the molecules) are called 

 deviations from the laws of gases. In consequence of these in- 

 dividual deviations, there are not exactly equal numbers of mole- 

 cules in equal volumes of any two different gases, at the same 

 temperature and pressure. An imaginary gas, which exhibits 

 neither deviation, called a perfect gas, is often referred to in 

 discussing the behavior of gases (see footnote, .p. 84). 



