Abstract: In order to determine a property of a given substance, the latter and a reference substance are alternately fed, by a suitable pump, for example, into and through a receiver in which each substance remains for a given time increment before the other substance is delivered thereto. The receiver is permeable to predetermined radiation which travels through the receiver and the substance therein to be absorbed to different extents by the given substance and the reference substance. The radiation travelling beyond the receiver is measured to derive therefrom alternate measurements resulting from the radiation travelling through the given substance and the radiation travelling through the reference substance, this radiation being derived from a suitable source which directs the radiation through the receiver and the substance therein. These alternate measurements are compared to determine the property of the given substance.