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When Archimedes was asked by King Hiero of Syracuse to prove whether a handcrafted crown really consisted of pure gold and not of gold with a proportion silver, he immersed the crown and an equal weight of pure gold, both attached to a beam balance, into a vessel of water. We all know that according to Archimedes's principle the scale beam rose on the side the crown was attached proving that the goldsmith had intermingled some silver. In spite of the simplicity of this experiment, the result convincingly demonstrates the physical integrity of this classical approach which is based on a weighing procedure. The mass sensitive technique we shall report on in this article is based on the principle that a shift in the resonance frequency of an oscillating AT-cut quartz disk can be correlated quantitatively with a change in mass in the nanogram range. One side of the quartz disk is in contact with a liquid, while the other side remains exposed to the atmosphere. The changes in weight involved in testing Hiero's crown were probably several hundred grams which corresponds to about lo2' water molecules. By applying the quartz microbalance, e.g., to solvent migration through thin films, one can sense entrapment of water with a resolution down to I O l 4 molecules per unit surface area. If adsorbate Iayers are in question, fractional coverages down to 0.02 monolayers can be monitored, which only goes to prove the capability of this instrument for investigating interfacial processes on a microscopic level. The option of recording charge flux and interfacial capacitance along with the mass change makes this instrument a powerful in-situ tool for scientific and technological applications in many fields.