42 



Dr. A. Ferguson on the Variation of 

 Table IV. 



Substance. 



K 



a. 



1-21 K. 



094 

 K 



1-16 

 a 



Mean. 



obs. 



Ether.. 



Methyl formate 



Ethyl aeetate 



•00516 

 •00435 

 •00392 

 •00337 

 •00329 

 00266 

 ; -00331 

 •00298 



•00603 

 •00553 

 00463 

 00418 

 •00416 

 •00311 



•00381 



•00624 



•00526 

 ■00474 

 •00408 

 00398 

 •00322 

 •00400 

 •00361 



182° 



216 



240 



279 



286 



353 



284 



315 



192° 



210 



250 



278 



279 



373 



305 



187° 



213 



245 



278-5 



283 



363 



284 



310 



194° 

 214 



250 



283 



288-5 



359 



273 



302 



Carbon tetrachloride . 

 Benzene 



Ohloro-benzene 



Ethyl propionate 



Bromine 





The meaning of the column headed 1/21 K — which is an 

 addition to Walden's table — will be explained later. It is 

 evident that the agreement between the numbers in the last 

 two columns is only approximate, and equation (iii.) is so 

 much more exact in use, that it removes any further 

 necessity for the employment of the very approximate 

 equations (viii.). 



The reason for the variability of the " constants " of 

 equations (viii.) is not far to seek. The coefficient a is not 

 really a constant, but is a function of the temperature 6. 

 If we expand equation (iii.) we see that, very approximately, 

 and for low temperatures 



a = nh = l'21b, 



where b is accurately the reciprocal of the critical tempera- 

 ture. We thus obtain 



*ft,= l-21, (ix.) 



in fair agreement with Walden's empirically found relation 

 a6 c = l'16. Neither of these expressions, however, can be 

 considered as anything but a rough approximation on account 

 of the neglect of squares and higher powers in the expansion 

 of (iii.). 



Ramsay and Shields have remarked that for many sub- 

 stances capillary-rise is approximately a linear function of 

 the temperature right up to the critical point. In which 

 case we have 



h=:ho(l-bd), (X.) 



and therefore, remembering that at ordinary temperatures 

 (and assuming a zero contact-angle) we may write a 2 = rh r 

 we have 



a t ; 2 = h^ { 



-a -to). 



