Well, that last test did not turn out as I hoped. The seedlings' roots became entwined with the paper towels - extracting them proved too traumatic. One survives, and that is still growing well, in a large pot.
This time, there are three groups:
M = plants fed tap water that has been boiled in a microwave oven
T = plants fed untreated tap water
K = plants fed tap water that has been boiled in an electric kettle
I bought packaged seeds - Bonny Best tomatoes - from the local grocery store. In each pot I planted three seeds, assuming that of the three at least one would germinate.
As you can see in the photograph above, taken a few minutes ago (that is, on April 26th) I was wrong. One Tap Water pot grew nothing; the other grew only one plant. The Microwave pot on the left was the first to show growth, and three germinated there. One of the Kettle pots grew two plants. (Clicking on the photo should generate a larger image.)
When the seedlings grow a little more I shall thin them out, so each pot holds a single tomato plant.