What really happened???

early alchemists used to do an experiment in which water was boiled for several days in a sealed gl container. eventually some solid residue would appear in bottom of the flask which was interpreted to mean that some of the water in flask had been converted into "earth". when lavoisier repeated this experiment, he found that water weighed the same before and after heating and the m of the flask plus solid residue equaled original m of flask. were the alchemists correct? explain what really happened.