Sentence:  The <WSD>fixer</WSD> removes the undeveloped silver salts from the film, rendering it no longer sensitive to light. Pour the <WSD>fixer</WSD> — at a temperature within plus or minus 5° F of the developer's — into the tank, using the amount specified by the tank  directions. Agitate with ten inversions initially and then for 10 seconds out of every minute for the remainder of the fixing time. The fixing time is not as critical as the developing lime, but stay within the time range suggested in the <WSD>fixer</WSD> directions. 
Meaning of the word marked with <WSD> tags:  A chemical (sodium thiosulfate) used in photographic development that fixes the image in place, preventing further chemical reactions.