Page:Popular Science Monthly Volume 51.djvu/695

Rh with ease, the writer to guide his pen at will, and the pedestrian to raise his feet from the ground; in fact, I should never get to an end if I should have to recall the principal examples of the utility of this microscopic cushion of air on the surface of solid bodies.

Long and patient observations by Moser and Waidele have made it extremely probable that every substance has its special gaseous envelope, which depends on the condition of the free surface, the temperature, the pressure, the vapors diffused in the surrounding space, etc. This is so true that it is enough to pass the finger over a plate of glass or metal to modify the minute molecular aggregate covering the surface. We can prove this by tracing, with the finger or any kind of rod or stick, invisible characters on the plate and breathing upon it, when all the tracings will immediately come out on it; for this reason, beyond a doubt, that the vapor of the breath deposits itself in different manners on the surface that has not been touched and on the parts followed by the tracings. Further, if we allow two metallic plates to remain for a considerable time slightly removed from one another, one of which is highly polished, and the other bears engraved characters such as may be found on a presentation watch, on separating them, say after two months, simply breathing on the surface of the smooth plate will cause the characters engraved upon the other to appear revealed. The cause of this appearance is, that the hollowed parts of one of the plates condense more air and moisture, and thus, by frequent changes of temperature and pressure, the parts of the smooth surface opposite the cavities are covered with a gaseous envelope different from that of the parts adjoining, and the difference is marked by a special condensation of the vapor of the breath.

Legions of grains of dust are known to be floating in the atmosphere, not near the ground alone, but miles above the sea level. We may form an estimate of the prodigious number of these solid particles suspended in the air by collecting snow during the earlier moments of a fall; the water resulting from the melting of it is nearly black with the corpuscles of every kind which the little ice crystals have brought down in the cavities of the snow. Later collections of snowflakes give clearer and clearer water. The snow has therefore been called the "broom of the atmosphere." The particles can not be held up in the atmosphere of themselves; for, taken one at a time and thoroughly dried, they will certainly weigh more than the air they displace. To learn the real cause of the phenomenon, we must recollect that the constitution of a solid particle is that of a minute kernel surrounded by a very thin layer of gradually decreasing density, into which the surrounding air infiltrates itself so as to make a