with glycerine, and placed at the bottom of the cylinder, receives the jet of air which is produced by the aspirator. The glycerine retains the corpuscles which are brought in by the current, and it is then easy to observe them. This apparatus has been modified in various ways,
but with it, or something like it, the earlier investigators, MM. Pasteur, F. Pouchet, Maddox, Douglas Cunningham, and others, have explored the atmosphere. It did not take long to discover that the air around us contains remnants of articles that we use existing in the condition of impalpable dust. Wool from our clothing, cotton, silk, starch, are floating in it, associated with fragments of various kinds—butterflies' scales, dried tarsuses of insects, feather-barbs, and skeletons of little worms. Pollens of the coniferæ and of numerous plants are abundant in it during the floral season. Particles of mineral matter are also found there, among them those curious spherules of meteoric air which have been described by M. Gaston Tissandier.
Attention is, however, most strongly fixed upon the number and variety of spores of cryptogams of which the air operates as a vehicle of dissemination. Germs of the common molds, and the reproductive cells of the algae that live on the roofs of houses, on walls, and on damp earth, are nearly always abundant. M. Miquel has tried to determine the laws that govern the appearance of these plants in the atmosphere. He first counted them, by disposing his aëroscopes so that they should register the volume of the air passing through them,
- See "Popular Science Monthly," July, 1880, article "Atmospheric Dust."