28 



A. W. Witkowski on the 



gaseous and liquid condition of matter alike) would be 

 a linear one, at any density. It is now known that a law 

 of that kind is not generally true, or that it holds good 

 only approximately. Yet it is remarkable how nearly it is 

 fulfilled in the case of atmospheric air at widely different 

 temperatures and densities. The curves of fig. 4 depart 

 only insignificantly from straight lines. But none of them 

 cuts the axis of abscissas at the point —273° — so often 

 spoken of (by a curious confusion of ideas) as absolute zero 

 — except, perhaps, those corresponding to very low densities ; 

 the pressure of dense gas decreases far more rapidly than 

 that. 



The constant-volume relation p = ~F(6) will be perhaps more 

 clearly expressed by introducing the pressure-coefficient /3 of 

 expansion defined by the equation 



p=Po(l+j30), 



p being the pressure exerted at 0° by the gas, when 

 compressed to a density p= — (unit of p = density at 0° 



under atmospheric pressure) 

 follows : — 



The values of /3 are as 



p= 



20. 



40. 



60. 



80. 



100. 



120. 



0. 



100,000 x/3. 



+ 100 



386 



406 



426 



447 







- 78-5 



387 



409 



431 



452 



474 



496 



-103-5 



389 



412 



435 



457 



480 



501 



-130 



392 



416 



439 



462 



484 



505 



-140 



394 



420 



444 



467 



490 



513 



-145 



396 



424 



449 



472 



495 



517 



The pressure coefficient does not vary much through a 

 range of 245°, provided the density be kept constant. An 

 increase of density causes it to augment rapidly. In con- 

 trast with the tortuous curves representing the coefficient 

 a, those of /3 form a narrow nearly straight bundle, con- 

 verging approximately to one point, namely, /3 = 0*00367 

 for p=l, 



§ 14. From what has been just said, it follows that 

 equation (3) is not suitable for calculating c v . I preferred to 



