Sentence:  It was possible to make estimates of the quantum yield by observing the extent of reduction of a uranyl oxalate actinometer <WSD>solution</WSD> illuminated for a known time in a typical reaction cell and making appropriate conversions based on the differences in the absorption spectra of uranyl oxalate and of chlorine , and considering the spectral distribution of the light source . 
Meaning of the word marked with <WSD> tags:  a homogeneous mixture of two or more substances; frequently (but not necessarily) a liquid solution