[jQuery] Help with book example - alternating row colors

[Priest, James (NIH/NIEHS) [C]]

In the book - there is an example for alternating row colors in a table,
and an expanded version for alternating triplets.

I've got that working - I have a table with two rows of chemical data:

CHEMICAL NAME: BA (GROWTH STIMULANT)
AMOUNT: 1       UNITS: FL.OZ.   ISOTOPE:        ACTIVITY:
ACTIVITY/UNITS:
--------------------------------------------------------------------
CHEMICAL NAME: GADOLINIUM(III)CHLORIDE HEXAHYDRATE
AMOUNT: 1       UNITS: FL.OZ.   ISOTOPE:        ACTIVITY:
ACTIVITY/UNITS:
--------------------------------------------------------------------

Using the book code - I've got the background striped every other row:

        $(document).ready(
                function() {
                        // $('table#chemtable tr:odd').addClass('odd');
                        // $("#chemtable tr:even").addClass("even");
                        var rowClass = "even";
                        var rowIndex = 0;
                        $('table#chemtable tr').each(function(index) {
                        
                        if (rowIndex %2==0){
                        rowClass = (rowClass == "even" ? "odd" :
"even");
                        };
                        
                        $(this).addClass(rowClass);
                        rowIndex++;
                        });
                });

But now I'd like to add a bottom border to each 'set' (like the txt
example above) but so far my attempts haven't worked... I either get the
bottom border correct - or my striping on each 'set' gets thrown off..

I'm thinking I need to setup up another index to loop over within the
main loop?

Thanks,
Jim