"[T]he blue which is seen in the atmosphere is not its own colour, but is caused by the heated moisture having evaporated into the most minute imperceptible particles, which the beams of the solar rays attract and cause to seem luminous against the deep intense darkness of the region of fire that forms a covering above them. ...As a further example of the colour of the atmosphere, we may take the case of the smoke produced by old dry wood, for as it comes out of the chimneys it seems to be a pronounced blue when seen between the eye and a dark space, but as it rises higher and comes between the eye and the luminous atmosphere, it turns immediately to an ashen grey hue, and this comes to pass because it no longer has darkness beyond it... But if this smoke comes from new green wood, then it will not assume a blue colour, because... it is not transparent... [I]f the atmosphere had this transparent blue as its natural colour, it would follow that wherever a greater quantity of atmosphere came between the eye and the fiery element, it would appear of a deeper shade of blue, as is seen with blue glass and with sapphires, which appear darker in proportion as they are thicker. ...We may also observe the difference between the atoms of dust and those of the smoke seen in the sun’s rays as they pass through the chinks of the walls in dark rooms, that the one seems the colour of ashes, and the other—the thin smoke—seems of a most beautiful blue. We may see also, in the dark shadows of mountains, far from the eye, that the atmosphere which is between the eye and these shadows will appear very blue, and in the portion of these mountains which is in light, it will not vary much from its first colour."