The Azure of the Air, arises from its being a dense transparent Body, illumined by the Sun, and placed between the Earth and the darkness of the upper Regions. Of it self, it is intirely void of all qualities, having neither Smell, Tast, nor Colour; yet does it easily assume those of things about it, insomuch that it appears of an Azure, by so much the more perfect as the darkness behind it is more gross, and the Light it receives more vivid; provided however that it be at a suitable distance, and not too much charged with moist Vapours. It may further be observ'd, that towards Mountains, that Air appears of the most perfect Azure which the Eye sees at the greatest distance, and which is interposed between that and the most obscure part of those Mountains; the nearer and more illumined Hills communicating their own Colour to the Air, in their passage to the Eye.