TEMPERATURE AND THE PROPERTIES OF GASES 27 



from his chair at Amsterdam just at the time when this 

 result was obtained, which so brilliantly confirmed his pre- 

 diction that all substances which do not decompose could be 

 brought under suitable conditions of pressure and temperature 

 into the states of solid, liquid, vapour, or gas respectively. 



With the latter states, and probably with solids also, the 

 whole thermodynamic condition of a substance is known with 

 the determination of two sets of data. One, the relation between 

 the volume and the pressure at any possible temperature, is 

 commonly spoken of as the determination of the equation of 

 state for the substance. The second, the relation between the 

 change of temperature of the substance and the amounts of heat 

 required to produce that change of temperature under different 

 conditions, is known as the determination of the specific heat. 

 It may be said at once that with no substance is there a complete 

 knowledge of the equation of state or of the variations of the 

 specific heat covering even two out of the four states of matter 

 mentioned above. On the other hand, small ranges are known 

 for various substances with more or less accuracy, and these can 

 be pieced together, by the aid of a principle which will be 

 considered later, into equations of state which represent an ideal 

 substance which occupies an average position among the varia- 

 tions of actual substances. 



This subject has to be attacked from two different sides, one 

 that of thermodynamics, which enunciates general propositions 

 to which all substances in any state must agree, but which 

 is sometimes only applied to actual substances with difficulty 

 owing to the want of knowledge of some of the data which are 

 requisite. On the other hand, an attempt can be made to build 

 up a theory which will account satisfactorily for the behaviour 

 of matter by considering its constitution and attempting to 

 arrive, by as nearly strict mathematical paths as possible, at the 

 probable behaviour of matter with the constitution which has 

 been supposed. We will not consider the various constitutions 

 which have been suggested, nor any one in detail, but shall 

 merely outline the fundamental conceptions on which the one 

 most commonly used — the kinetic theory — has been based. The 

 conceptions on which this theory are grounded have enabled 

 progress to be made in other branches of science also, and 

 the assistance derived from these in return has helped to con- 

 firm the validity of the conceptions of the kinetic theory. 



