preface to fifteenth edition

PHYSICAL PROPERTIES 5.137
where the summation extends over all charges (electrons and nuclei) in the molecule. The numerical
values ofthe dipole moment, expressed in the c.g.s. system ofunits, are in debye units, D, where
1D 1018
esu ofThe charge centimeters. conversion factor to SI units is
30
1D 3.335 64 10 C · m [coulomb-meter]
Tables 5.17 and 5.18 contain a selected group ofcompounds for which the dipole moment is
given. An extensive collection ofdipole moments (approximately 7000 entries) is contained in
A. L. McClellan, Tables of Experimental Dipole Moments, W. H. Freeman, San Francisco, 1963. A
critical survey of500 compounds in the gas phase is given by Nelson, Lide, and Maryott, NSRDS-
NBS 10, Washington, D.C., 1967.
5.6.4 Dielectric Constants
Iftwo oppositely charged plates exist in a vacuum, there is a certain force ofattraction between
them, as stated by Coulomb’s law:
1 qq 1 2
F ·
2
4 r
where F is the force, in newtons, acting on each of the charges q 1 and q 2 , r is the distance between
the charges, is the dielectric constant ofthe medium between the plates, and 0 is the permittivity
offree space. q 1 , q 2 are expressed in coulombs and r in meters. Ifanother substance, such as a
solvent, is in the space separating these charges (or ions in a solution), their attraction for each other
is less. The dielectric constant is a measure of the relative effect a solvent has on the force with
which two oppositely charged plates attract each other. The dielectric constant is a unitless number.
Dielectric constants for a selected group of inorganic and organic compounds are included in
Tables 5.17 and 5.18. An extensive list has been compiled by Maryott and Smith, National Bureau
Standards Circular 514, Washington, D.C., 1951.
For gases the values of the dielectric constant can be adjusted to somewhat different conditions
oftemperature and pressure by means ofthe equation
( 1) t,p
p
( 1) 760[1 0.003 411(t 20)]
20,1 atm
where p is the pressure (in mmHg) and t is the temperature (in C). The errors associated with this
equation probably do not exceed 0.02% for gases between 10 and 30C and for pressures between
700 and 800 mm. The dielectric constants ofselected gases will be found in Table. 5.18.
0
5.6.5 Viscosity
The dynamic viscosity, or coefficient of viscosity, ofa Newtonian fluid is defined as the force per
unit area necessary to maintain a unit velocity gradient at right angles to the direction offlow between
two parallel planes a unit distance apart. The SI unit is pascal-second or newton-second per meter
2
2
squared [N·s·m ]. The c.g.s. unit ofviscosity is the poise [P]; 1cP 1mN·s·m . The dynamic
viscosity decreases with the temperature approximately according to the equation: log
A B/T. Values of A and B for a large number of liquids are given by Barrer, Trans. Faraday Soc.
39:48 (1943).