Derived from a curve fit to table 87 of Dorsey (1940).
The fit matches the table to
within 0.2 percent at worst, and with average
absolute error of 0.07 percent.
The maximum
deviation from the table is one unit in the
last decimal place. No pressure dependence was reported by Dorsey (1940).
It would be a good idea to look for other values
to check those used here, and of course to
get an idea of pressure dependence. Therefore,
this subroutine is provisional.
-- Dan Kelley 2006-01-21.