Sentence:  Though a ſingle grain of copper diſſolved in upwards of twenty gallons of clear water may be detected by a viſible blue <WSD>tinge</WSD> appearing on the addition of a few drops of volatile alcali, yet this is by no means the caſe in turbid high-coloured liquors, [...] Hence the neceſſity of diluting ſuch liquors in varying the experiments. In no inſtance did the blue <WSD>tinge</WSD> appear with the alcalis, even after the wine was diluted, and yet its abſence does "not" prove the liquor to be abſolutely "free" from an impregnation of copper. 
Meaning of the word marked with <WSD> tags:  A small added amount of colour;  a small added amount of some other thing.