It was observed very early that when a metal was converted into a calx its weight was increased. But this, though known to Beecher and Stahl, does not seem to have had any effect on their opinions. Boyle, who does not seem to have been aware of the phlogistic theory, though it had been broached before his death, relates an experiment on tin which he made. He put a given weight of it into an open glass vessel, and kept it melted on the fire till a certain portion of it was converted into a calx: it was now found to have increased considerably in weight. This experiment he relates in order to prove the materiality of heat: in his opinion a certain quantity of heat had united to the tin and occasioned the increase of weight. This opinion of Boyle was incompatible with the Stahlian theory: for the tin had not only increased in weight, but had been converted into a calx. It was therefore the opinion of Boyle that calx of tin was a combination of _tin_ and _heat_. It could not consequently be true that calx of tin was tin deprived of phlogiston. When this difficulty struck the phlogistians, which was not till long after the time of Stahl, they endeavoured to evade it by assigning new properties to phlogiston. According to them it is not only destitute of weight, but endowed with a principle of levity. In consequence of this property, a body containing phlogiston is always lighter than it would otherwise be, and it becomes heavier when the phlogiston makes its escape: hence the reason why calx of tin is heavier than the same tin in the metallic state. The increase of weight is not owing, as Boyle believed, to the fixation of heat in the tin, but to the escape of phlogiston from it. Those philosophic chemists, who thus refined upon the properties of phlogiston, did not perceive that by endowing it with a principle of levity, they destroyed all the other characters which they had assigned to it. What is gravity? Is it not an attraction by means of which bodies are drawn towards each other, and remain united? And is there any reason for supposing that chemical attraction differs in its nature from the other kinds of attraction which matter possesses? If, then, phlogiston be destitute of gravity, it cannot possess any attraction for other bodies; if it be endowed with a principle of levity, it must have the property of repelling other bodies, for that is the only meaning that can be attached to the term. But if phlogiston has the property of repelling all other substances, how comes it to be fixed in combustible bodies? It must be united to the calces or the acids, which constitute the other principle of these bodies; and it could not be united, and remain united, unless a principle of attraction existed between it and these bases; that is to say, unless it possessed a principle the very opposite of levity.