Besides these gases which have been enumerated, and which play an important part in supporting life in both the kingdoms of nature, we find a great many solids. Every housewife knows how dust settles upon everything about the house. This dust has recently been the subject of most active study, and it proves to be quite as important as the vital oxygen that actually supports life. When we examine this dust--and it falls everywhere, not only in the city streets, but upon the tops of mountains, upon the deck of the ocean steamer, and the Arctic snow--we find some of it does not belong to the earth, and, as it is not terrestrial, we call it cosmical. And when it falls in large pieces we call it a meteorite or shooting star. When the Challenger crossed the Atlantic, and soundings were made in the deep sea, in the mud that was brought up and examined there were found various little particles that were not terrestrial. They were dust particles that were dropped into the atmosphere of the earth from outer space. Then we have terrestrial dust, and we divide that into mineral and organic. The mineral consists chiefly of clay, sand, and, near the ocean, salt. Then we have organic matter. Some of this is dead leaves which have been ground to powder. Animal matter has also become dry and reduced to powder, and we actually find the remains of animals and plants floating upon the atmosphere, especially in the city. Examinations of the dust which had collected upon the basement and higher windows of a Fifth avenue residence showed that the dust upon the basement floor was chiefly composed of sand. And the higher up I went, the smaller proportion of sand and a larger proportion of animal matter, so that the dust that blows into our faces is largely decomposing animal substance.