"These general considerations are sufficient to convince us, that in nature a substance exists whose properties are different from those of fire, air, water; and which is, like these other substances, one of the elements of compound bodies. But a vague assertion like this does not satisfy chemists. Besides the ascertaining of the exigence of the different substances submitted to their examination, they require to know the properties of these substances in their greatest degree of purity and simplicity; but they have found much difficulty and uncertainty in investigating the essential properties of the purest and simplest terrestrial element."