THE FORCES IN AN AIR BUBBLE. 679 



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 sur- 

 face, the temperature, the pressure, the vapors diffused in the sur- 

 rounding 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 mo- 

 lecular 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 trac- 

 ings 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 breath- 

 ing 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 con- 

 dense more air and moisture, and thus, by frequent changes of 

 temperature and pressure, the parts of the smooth surface oppo- 

 site 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 at- 

 mosphere, 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 dur- 

 ing 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 atmos- 

 phere 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 



