The beautiful system of aqueous circulation by means of the atmosphere as sketched above was long thought to explain the whole process, and to require no further elucidation; but about a quarter of a century back a curious experiment was made which indicated that there was another factor in the process which had been entirely overlooked. If a small jet of steam is sent into two large glass receivers, one filled with ordinary air, the other with air which has been filtered by passing through a thick layer of cotton wool so as to keep back all particles of solid matter, the first vessel will be instantly filled with condensed cloudy-looking vapour, while in the other vessel the air and vapour will remain perfectly transparent and invisible. Another experiment was then made to imitate more nearly what occurs in nature. The two vessels were prepared as before, but a small quantity of water was placed in each vessel and allowed to evaporate till the air was nearly saturated with vapour, which remained invisible in both. Both vessels were then slightly cooled, when instantly a dense cloud was formed in that filled with unfiltered air, while the other remained quite clear. These experiments proved that the mere cooling of air below the dew point will not cause the aqueous vapour in it to condense into drops so as to form mist, fog, or cloud, unless small particles of solid or liquid matter are present to act as nuclei upon which condensation begins. The density of a cloud will therefore depend not only on the quantity of vapour in the air, but on the presence of an abundance of minute dust-particles on which condensation can begin. That such dust exists everywhere in the air, even up to great heights, is not a supposition but a proved fact. By exposing glass plates covered with glycerine in different places and at different altitudes the number of these particles in each cubic foot of air has been determined; and it is found that not only are they present everywhere at low levels, but that there are a considerable number even at the tops of the highest mountains. These solid particles also act in another way. By radiation in the higher atmosphere they become very cold, and thus condense the vapour by contact, just as the points of grass-blades condense it to form dew.