Sentence:  The last column shows the rate of <WSD>exchange</WSD> that would have been observed at a relative intensity of 4 ( 14.7 cm. distance ) calculated on the assumptions that the incident light intensity is inversely proportional to the square of the distance of the lamp from the cell and that the rate is directly proportional to the incident light intensity . 
Meaning of the word marked with <WSD> tags:  chemical process in which one atom or ion or group changes places with another