Sentence:  It may well be asked, what is a man to believe, when we find ministers and laymen, those who ought to be at once Christians and gentlemen, giving currency to statements so daring and unfounded as those which I am combating; and aiding and abetting each other in attempts to prove a <WSD>false negative</WSD>, by feigning ignorance! 
Meaning of the word marked with <WSD> tags:  A result of a test that shows as absent something that is present.