The present invention provides a method for the real-time continuous monitoring of a change or density and/or viscosity within a test sample. Such methods can be used to determine the occurrence of a chemical reaction within a test sample where the same causes and increase or decrease in the density and/or viscosity of the sample due to, for example, a gelation, precipitation or coagulation occurring within the test sample. There is further provided a multi-resonator apparatus for use in measuring the density and/or viscosity of a test sample in which the multi-beam resonator is immersed.