In 1881 the writer had occasion to conduct a scientific expedition to the highest point in the territories of the United States, on one of the summits of the Sierra Nevadas of Southern California, which rise even above the Rocky Mountains. The illustration on page 177 represents the camp occupied by this party below the summit, where the tents, which look as if in the bottom of a valley, are yet really above the highest zone of vegetation, and at an altitude of nearly twelve thousand feet. Still above these rise the precipices of barren rock seen in the background, their very bases far above the highest visible dust-clouds, which overspread like a sea the deserts at the mountain’s foot,--precipices which when scaled lift the observer into what is, perhaps, the clearest and purest air to be found in the world. It will be seen from the mere looks of the landscape that we are far away here from ordinary sources of contamination in the atmosphere. Yet even above here on the highest peak, where we felt as if standing on the roof of the continent and elevated into the great aerial currents of the globe, the telescope showed particles of dust in the air, which the geologists deemed to have probably formed part of the soil of China and to have been borne across the Pacific, but which also, as we shall see later, may owe something to the mysterious source of the phenomena already alluded to. It is far from being indifferent to us that the dust is there; for, to mention nothing else, without it, it would be night till the sunrise, and black night again as soon as the sun’s edge disappeared below the horizon. The morning and the evening twilight, which in northern latitudes increase our average time of light by some hours, and add very materially to the actual days of man’s life, are probably due almost wholly to particles scarcely visible in the microscope, and to the presence of such atoms, smaller than the very motes ordinarily seen in the sunbeam, which, as Mr. Aitken has shown, fill the air we breathe,--so minute and remote are the causes on which the habits of life depend. Before we can see that a part of this impalpable, invisible dust is also perhaps a link between our world and other members of the solar system, we must ask how it gets into the atmosphere. Is it blown up from the earth, or does it fall down out of the miscalled “void” of space?