"The notion of (end of 15th and beginning of 16th century), that lime when burnt combined with 'matter of fire,' had been accepted by many as an explanation of the difference in the behaviour towards acids of burnt and unburnt lime. If this explanation applied to magnesia it should be possible perhaps to get hold of this 'matter of fire,' which combined with the magnesia alba when that body was heated. But Black found that a given mass of magnesia alba weighed more than the calcined magnesia obtained from it. Hence something was lost instead of gained during the process of heating. This something proved on further quantitative examination to be a gas different from common air; to it Black gave the name of fixed air."