Gentlemen, thanks for the responses.
I've tried the 'experiment'again with plain hot WATER in the following containers: Porcelain cup, single walled glass cup, double walled stainless steel cup but with NO distinguishable change in tone.
I always tap the bottom of the vessel on exactly the same spot, frequency and force.
But, when I again tried hot coffee in a porcelain cup whilst holding it by the handle and not resting it on any surface the tone changed from low to high. Here's my theory:
Due to the soundwaves and vibrations in the suspension during the tapping, the particles start separating (heavy and light) and as a result, the density at the bottom of the cup increases whilst decreasing in the upper regions of the suspension. As a result, the frequency of the vibrating cup changes along with the change in density.
Do you think that I'm on the right track? I value your objective opinions, so don't hesitate to tell me that I'm way off the mark here.
Thanks, in anticipation.