Sentence:  A given quantity of the spirits was poured upon a quantity of gunpowder in a dish and set on fire. If at the end of the combustion, the gunpowder continued dry enough, it took fire and exploded; but if it had been wetted by the water in the spirits, the flame of the alcohol went out without setting the powder on fire. This was called the <WSD>"proof</WSD>". 
Meaning of the word marked with <WSD> tags:  An effort, process, or operation designed to establish or discover a fact or truth; an act of testing; a test; a trial.