334 



Aggregation and Dissociation 



[CH. xvni 



It would be fruitless to try to evaluate this expression further; we 

 cannot even determine its sign with certainty. We shall simply remark 

 that it depends on both the temperature and the density, and this to an 

 extent which increases as we approach low temperatures and high densities. 



The case is similar as regards the quantities C p C v , C v and 7. We 

 should expect them all to depend on the pressure and density to an extent 

 of which it is hardly possible to form any estimate. 



404. As an illustration we may take the case of steam. Wet steam 

 is steam in which large molecular clusters occur, dry steam is steam in which 

 the molecules are all separate, and our quantity q measures what engineers 

 speak of as the dryness of wet steam. For the value of 7 for wet (saturated) 

 steam, Rankine and Zeuner give respectively the values T0625, 1'0646. 

 For dry steam ("steam gas") the recognised value is 1'30. If we used the 

 formula 



for the calculation of n, we should come to the conclusion that n + 3 had the 

 value 32 for wet steam, and 6'6 for dry steam. From what has been said, 

 it will be seen that we may with some confidence take the value n + 3 = 6 

 for the molecule, this being the value we should naturally expect for a 

 triatomic molecule (cf. 271) and regard the difference between this and the 

 apparent value 32 for wet steam, as caused by molecular aggregation. 



405. A further instance is supplied by carbon-dioxide for which we have 

 already (p. 252) noticed the large influence of pressure on the coefficient of 

 viscosity. The following values of C p are given by Lussana*. 



At normal temperature and pressure, the value of G p is 0'2164. The 

 variations of C p with temperature and pressure are therefore enormous. 



From the value 6^ = 2-1096, we obtain, by the use of the formula (521) 



* Nuovo dm. 1894, xxxvi., 1895, i. 



