PRESSURE OF GASES 



15 



This table shows that Boyle's law does not hold exactly 

 for any gas, but that for the gases named it holds with suffi- 

 cient approximation to be considered for most purposes an 

 exact law of nature. The vapours of liquids, indeed, depart 

 from the law more widely than gases ; but from a theoretical 

 calculation to which Clausius l has subjected Kegnault's 

 observations on saturated steam, it appears that for this 

 vapour from to 100 the values of the magnitudes, which 

 according to the law should be constant, vary by not more 

 than 5 per cent, at the most. Similar relations were found 

 by Herwig 2 for the unsaturated vapours of alcohol, chloro- 

 form, and carbon disulphide. 



Accordingly, therefore, the departures of even the vapours 

 of liquid bodies from the law inferred from theory are suffi- 

 ciently unimportant to be provisionally neglected when we 

 attempt to investigate the other properties of these bodies 

 on the basis of this theory. Strictly speaking, our further 

 conclusions will only be rigorously true for such media as 

 obey Boyle's law exactly; these, however, exist only in the 

 imagination, and are therefore called ideal (or perfect) gases. 

 But, in the main, our considerations are also true for real 

 gases, and the variations between the theory and observation 

 will be of no greater importance than that of the differences 

 in the foregoing numbers. 



8. Defects of the Hypothesis 



We shall, however, not rest content with this approxima- 

 tion, but try to get nearer the truth by inquiring into the 



T.Ann. Ixxix. 1850, p. 513, Clausius, Mechan isc he Warmetheorie, 

 2nd and 3rd ed. i. p. 151 ; transl. Phil. Mag. [4] ii. 1851, p. 102. 

 2 Fogg. Ann. cxxxvii. 1869, pp. 19, 592. 



