First, that a _medium_, whose parts are unequally _dense_, and mov’d by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other _coefficient_ cause. Secondly, that there is in the Air or _Atmosphere_ such a variety in the constituent parts of it, both as to their _density_ and _rarity_, and as to their divers mutations and positions one to another. By _Density_ and _Rarity_, I understand a property of a transparent body, that does either more or less refract a Ray of light (coming obliquely upon its superficies out of a third _medium_) toward its perpendicular: As I call Glass a more dense body then Water, and Water a more rare body then Glass, because of the refractions (more or less deflecting towards the perpendicular) that are made in them, of a Ray of light out of the Air that has the same inclination upon either of their superficies. So as to the business of Refraction, spirit of Wine is a more _dense_ body then Water, it having been found by an accurate Instrument that measures the angles of Refractions to Minutes that for the same refracted angle of 30°.00′. in both those _Mediums_, the angle of incidence in Water was but 41°.35′. but the angle of the incidence in the trial with spirit of Wine was 42°.45′. But as to gravity, Water is a more _dense_ body then spirit of Wine, for the proportion of the same Water, to the same very well rectify’d spirit of Wine was, as 21. to 19. So as to Refraction, Water is more Dense then Ice; for I have found by a most certain Experiment, which I exhibited before divers illustrious Persons of the _Royal Society_, that the Refraction of Water was greater then that of Ice, though some considerable Authors have affirm’d the contrary, and though the Ice be a very hard, and the Water a very fluid body.