Al Campagna,
To my understanding to calculate the rate of change between two points the
standard rate of change formula is used: (Change in Y) / (Change in X).

In Excel if you have the dates in a column i.e. " A " and the values in a
column i.e. " B " in this case these two columns hold the dates the sample
where taken and the amount of Hydrogen present. so to calculate the rate of
change between consecutive records you can use the formula of:

=(B8-B7)/(A8-A7)

If this formula is copied down the list with the numbers increasing in
increments of one then it will calculate the rate of change per day per for
each record.

Example:

           A            B                     C
         Date      Hydrogen     Rate of Change
6      
7   14/07/2003     20          
8   17/09/2004     27          =(B8-B7)/(A8-A7)    This returns 0.02 of
Hydrogen per day
9   28/10/2005     38          =(B9-B8)/(A9-A8)    This returns 0.03 of
Hydrogen per day
10
    ......etc......

(The Rate of change has been set to 2 d.p so increasing this would give more
accurate results)

This then shows the rate of change between each record over a period of time
allowing you to see how the rate of change has "changed" over time.

To my understanding this is correct, if you or anyone else knows otherwise
please let me know, and if you can solve my initial question using this
information then I would be thankful.

Jonathan