[N]othing gains weight but by the addition of matter, nor loses it but by its subtraction—so inseparably are matter and weight united, as has been sh… - Jean Rey

God, creating the universe, neither made it perfectly like Himself, nor perfectly unlike, for He, being One, has made the world as not one, from the diverse multiplicity of its innumerable parts, ordaining, nevertheless, that they should collect into a certain unity by their exact contiguity. The upper world has no connexion with this subject; the lower, and elementary world, owes this contiguity to the weight divinely impressed on its parts, aided by the subtle fluidity of some of its simple bodies. It is by this quality, with which the matter of the four elements is more or less invested, that they are separated from one another, and each transported to its proper place, as the generation of compounds, and the beauty of the universe requires.

But if we investigate the subject by the balance, a case occurs in which, without any addition or subtraction of matter, a substance will appear more or less heavy; namely, by its contraction or expansion.

[W]e must not only inquire whence these seven ounces are derived, but, moreover, whence that which has replaced the loss of weight, necessarily arising from the enlargement of volume of the tin, by its conversion into calx, and from the vapours and exhalations that have escaped.