For the second time in a week, some late night or early morning pranksters dumped liquid soap in the Healdsburg Plaza fountain. As the center fountain sprinkler spread the soap, an overflowing cloud of white bubbles and foam billowed up and over the sides of rectangular, shallow fountain. By daylight on Sept. 20, passersby stopped to marvel at the mischief as city workers next arrived to disperse and dilute the non-caustic but unwanted suds.