in these departments have determined, with a great deal of certainty, that the atoms and molecules of matter do not touch each other, and that the various velocities they may assume under different conditions are the causes of all the phenomena of light and heat. And, moreover, it has been determined by the most refined experiments, with special instruments of precision, that these atoms have a definite size and weight, and, under special conditions, a definite velocity and momentum. The pressure of gases has also been defined as the resultant of the molecular bombardment or impact of these flying projectiles against the sides of the containing vessel.
Some molecular data have been tabulated from the calculations of Clausius, Maxwell, Thomson, and others, but the figures given are wholly beyond human comprehension. Thus the number of atoms in one cubic inch of hydrogen-gas, at the temperature of freezing water and under the pressure of one atmosphere, is given in the neighborhood of three hundred millions of millions of millions, each atom possessing an initial velocity of over a mile per second, covering nearly eighteen billions of oscillations in different directions in the same second of time. That is to say, each particle of hydrogen, while moving at the rate of seventy miles per minute, has its course wholly changed something like 17,700,000,000 times in every second. Sir William Thomson concludes, from the data given by Clausius, that the diameter of the gaseous molecule is somewhere between the 1250000000 and the 1500000000 inch, and as the density of known liquids and solids is from 500 to 16,000 times that of common air, he concludes that the distance from centre to centre of contiguous molecules in solids is less than the 1250000000 and greater than the 1500000000 of an inch; and he illustrates by supposing "a drop of water to be magnified up to the size of the earth, each molecule to be amplified in the same proportion, these molecules will then be less in size than cricket-balls, but larger than small lead-shot."
Imagine the particles of the air we breathe flying about at the rate of eighteen miles per minute—a velocity exceeding that of a cannonball—a velocity which, if the particles were all moving in one direction, would constitute a tornado ten times more violent than any terrestrial hurricane! How is it, then, that we can survive the incessant bombardment of such a storm of projectiles? Simply because the particles are moving in all directions, so as nearly to counterbalance each other's momentum. Were it not that the molecules are continually changing their direction by executing a sort of carrom upon their neighbors, the interdiffusion of liquids and gases would be almost instantaneous. If the molecules could project in straight lines without interference with each other, the opening of a bottle of perfumery would permit the diffusion of its odor to the distance of many hundred feet sooner than you could open and recork the bottle, or, in some instances, about one-third of a mile in one second of time.
