TEMPERATURE AND THE PROPERTIES OF GASES 31 



of motion which can be measured by observations on the specific 

 heats. Without considering the historical development of 

 knowledge in this direction, the modern position may be 

 summed up as follows, leaving out of account considerations 

 of electrons which can only make very small percentage changes 

 in these relations. 



(1) All chemically elementary substances, and many com- 

 pounds, are capable of existing in the conditions of solid, liquid, 

 vapour or gas under specific conditions of pressure and 

 temperature. 



(2) All pure gases consist of a very large number (n = about 

 io 20 per cubic centimetre under normal conditions) of similar 

 molecules. 



(3) These molecules are moving in straight lines for distances 

 depending on the density of the gas and known as the free path, 

 the mean value being of the order of io -4 mm. at ordinary 

 temperature and pressure ; they move with velocities which 

 are changing at each collision, but continually varying about 

 some mean value, the square of which is proportional to the 

 absolute temperature. These velocities are of the order of 

 1 kilometre per second at the ordinary temperature. 



(4) All molecules of any given pure gas consist of the same 

 number of one or more atoms, these being the smallest particles 

 of the substance which can exist without loss of identity alone 

 or in combination. Each atom occupies a definite volume under 

 definite conditions of temperature and pressure, and each mole- 

 cule of more than one atom another volume which is not the sum 

 of the atomic volumes. There is in each case a limiting volume 

 which would only be reached at the lowest temperatures and 

 highest pressures. Each molecule occupies an effective space 

 which is some small multiple of its real volume and is usually 

 denoted by (b). 



(5) Complex molecules at any rate have some internal motion ; 

 and possibly atoms also, though to a smaller extent. 



(6) The molecules exert an attraction on one another which 

 varies very little with the pressure, but which decreases as the 

 temperature decreases. It is probable that the law of attraction 

 varies with a much higher power than the square (that of 

 gravitation and simple electric or magnetic attraction), some 

 index of the order of 6 being indicated, and hence it is only 

 effective when the molecules are very close together. 



