"If equal quantities of the substance produce equal effects of any kind, we may employ these effects as measures of the quantity of the substance. For instance, if we are dealing with sulphuric acid of uniform strength, we may estimate the quantity of a given portion of it in several different ways We may weigh it, we may pour it into a graduated vessel, and so measure its volume, or we may ascertain how much of a standard solution of potash it will neutralize."