log - natural logarithm function
returns the natural logarithm of its argument;
i.e., the power to which e must be raised in order to obtain NumExpr,
where e is the transcendental number whose value is approximately 2.71828.
NumExpr may be any numeric expression whose value is greater than zero.
If the value of NumExpr is zero or negative,
an "Illegal function call" error occurs.
The Basmark QuickBASIC Programmerís Manual