An apparatus is disclosed which delivers a dosage of a chemical substance preferably to a biological tissue in such a controlled manner that the concentration affecting the biological tissue is known at each and every point in time. The apparatus includes a plurality of vessels with each vessel containing a different concentration of the chemical substance. A valve has a plurality of inlets and a single outlet wherein each inlet is connected to a corresponding vessel. The biological tissue is preferably held within a chamber for treatment with the chemical substance. A pump positioned preferably between the valve and the biological tissue provides a transport force for delivering the chemical substance to the tissue. The valve and the pump are controlled by a computer control system which determines which inlet of the valve is fluidly connected to the outlet for providing a preselected concentration of the chemical substance to the tissue such that the concentration of the chemical substance affecting the tissue is controlled at each and every point in time.