Abstract: For use in measuring the extent or rate of optical density change produced by a reaction a rigid or semi-rigid disposable reaction container is disclosed of the type comprising a windowed reaction chamber adapted for the transmission of light therethrough and an auxiliary chamber that is separated from the reaction chamber except for an opening which is small enough to prevent substantial diffusion or movement of liquid from one chamber to the other when under static conditions but through which a solution of a reagent in the auxiliary chamber may be forced so as to enter the reaction chamber and commingle with a solution of another reagent and thereby trigger a reaction, improvements comprising the provision in a receptacle having side walls, end walls and a bottom wall of a projection which extends a substantial distance upwardly from the bottom wall so as to separate the lower part of the receptacle into two portions adapted to separate liquid in one of said portions from the other portion in combination w