MOLECULAR MOTIONS AND TEMPERATURE. 171 



When the absolute temperature is zero (i.e. T = 0) the 

 pressure is zero (P,=0). Substituting these values in 

 (5) and solving for T gives TO = 273. The zero of the 

 absolute scale of temperature is then 273 below the 

 arbitrary zero of the Centigrade scale. In other words 

 the absolute zero is -273 C. 



This absolute zero is probably never to be reached 

 experimentally, although in recent years remarkably 

 low temperatures 1 have been obtained. As the tem- 

 perature of a gas is decreased the molecules may be 

 crowded closer and closer together, until they are so 

 close that they are in the liquid rather than in the 

 aeriform state. This crowding together and consequent 

 liquefaction may not, however, be brought about 

 merely by an increase of pressure. There is for each 

 gas a certain definite temperature above which it is 

 impossible to produce liquefaction by compression. 



The liquid gas may be solidified by a further reduc- 

 tion of its temperature. The temperature at which 

 this occurs is called the freezing point. We see, then, 

 that gases like hydrogen or oxygen may assume any 

 of the three forms solid, liquid, or aeriform, depend- 

 ing upon the temperature and pressure to which their 

 molecules are subjected. This is the familiar action 

 of water ; but in the case of water the freezing point 

 and the boiling point (that is, the temperature at which 

 steam condenses under atmospheric pressure) are at 

 temperatures within the range which we meet in our 

 daily lives instead of only under laboratory conditions. 

 In fact, from our ideas of the molecular construction 



1 The gas helium was liquefied at a temperature of -271.3 C., 

 that is, within two degrees of the absolute zero. 



