758 



of our quantity y, which represents the coefficient of direction of 

 the straight line between Va ^o ^^^ ^k in a D — T diagram. 



IV. The BoYLE-point. 



This is another point of great importance for the study of the 

 a and 6-values. Then RThg—a is namely =3 0, so that the accurate 

 knowledge of the temperature of the Boyle point at the same time 

 enables us to know the ratio he,: a there. According to K. Onnes ^j 

 the following value follows from the vahies of the second virial 

 coefficient B calculated by him — by interpolation between the 

 temperature — 164°, 09 lying very close to it, and the somewhat 

 higher temperature — 182°, 75 — for this point: 

 Tb = — 165°,72 C. = 107,37 abs., 



For RTb we therefore find 0,39317 X 0,99941 == 0,39294, so 

 that, therefore, 1 : 0,39294 being = 2,545, is found : 



*"^ = 2,545 , . . . . ... (4) 



«/0 



i. e. entirely the same value as we found above for 7). 

 Between 33° abs. and 107 abs., i.e. between Tk and ^'/^Tjc the 



ratio between b,, and a has reuiained constant, so that we have some 

 reason to suppose that this constancy will also be maintained for 

 higher and lower temperatures '). 



We may further point out that from: 



8 ai 8 fa\ ^,„ (a 



follows : 



Tb_^ 



a \ t a 

 hjE \bgjk_ 



If therefore {a:hg)B = {a:b,,)k, the ratio Tb: Tk is given by: 



Tb 3,375 



Tk hf 



(5), 



in which ;. is the correction factor introduced by us (in the formulae 

 for RTjc and pk), and (p represents the ratio bg:bk for the critical 

 isotherm. For H, ^ is = 0,999 (see § III), but for most substances 

 with normally high critical temperature this factor rises to about 



1) Coram. 100a (These Proc. of Dec. 1907). 



2) We remark also in this connection that the interval between Tk and 3'/4 Tk 

 would mean an enormous temperature range for other substances, e.g. Fluorbenzene. 

 For the substance mentioned from 560° abs. to 1820° abs., i.e. from 287° C to 

 1546° G. 



