4 THE TRUE VALUE Oï a OF VAN DER WAALS' 



extremely interesting relationship holds, I believe, everywhere, at 

 least in all normal substances, from hydrogen on the one hand to 

 such complex substances as methyl butyrate and diisoamyl on the 

 other. It appears to me to mean, simply, from one point of view, 

 that the cohesional attraction is the gravitational attraction inten- 

 sified in the immediate vicinity of the molecules by the action of 

 the valence electrons. It thus confirms an opinion expressed some 

 time ago, I believe by Lord Kelvin, that cohesional attraction is 

 gravitational attraction intensified in some way in the immediate 

 vicinity of the molecules. We may also, however, as I shall point 

 out later in the paper, look at the matter the ■ other way around 

 and regard gravitational attraction as that part of the cohesional 

 attraction toward which the molecules and ether happen to be trans- 

 parent so it can penetrate them. 



This equation may also be written in this way : a = N 2 C 

 (M X VcWf^ , in which the constant C is the gravitational mass 

 factor of two molecules of unity molecular weight and valence 

 raised to the two thirds power, or (w 2 /,-) 2li . C has the value of 

 3.201 X 10 ~" :r7 when a is expressed in dynes. N for a gram mol 

 is 6.06 X 10 2i . 



After the appearance of my papers *) van Laar announced that 

 if he assigned arbitrary values to the various chemical elements it 

 was possible by addition to compute the square root of a for 

 various substances. The additivity of a had been sought by many 

 observers, by Sutherland among the first, by Kleeman, by Leduc 

 and Amagat and others also, but before the publication of my 

 paper no one had found a general method applicable to all kinds 

 of compounds. Van Laar's 2 ) method is simply to assign arbitrary 

 values to carbon, nitrogen, oxygen, chlorine and the other elements. 

 These values multiplied by the number of atoms there are of that 

 particular kind in the molecule, when added to the sums derived 

 in a similar way _ from the other atoms, give the square root 

 of a. However in this simple form the law would not hold, so it 

 became necessary to assume further that the arbitrary values were 

 not always the same for any element, but that carbon, for example, 

 sometimes had zero value, and sometimes various other values 

 depending on the kind of carbon compound, whether ring or ali- 



1) Mathews: Journal of phys. chem., 17 pp. 154, 181, 252, 320, 331, 337, 481, 520, 

 603, 1913; Jour. chim. physique, 12, 1914, p. 428. Jour. phys. cliem., 18, p. 474,1914; 

 20, p. 551, 1916. Science, 36, p. 92, 1912 (Preliminary statement). 



2 ) Van Laar: Proceedings, Konink. Akad. te Amsterdam, 18, 1220, 1916; Jour. chim. 

 phys., 11, 1916, p. 3. 



