656 THE POPULAR SCIENCE MONTHLY. 



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 condi- 

 tions 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 re- 

 sultant of the molecular bombardment or impact of these flying pro- 

 jectiles 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 neighbor- 

 hood of three hundred millions of millions of millions, each atom pos- 

 sessing an initial velocity of over a mile per second, covering nearly 

 eighteen billions of oscillations in different directions in the same sec- 

 ond 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 diam- 

 eter of the gaseous molecule is somewhere between the ^-g U o ^^^ 

 the -g ooooooou ^^ ^" 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 o^o,?,,,^^ and greater than the stottoVoToo 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 pro- 

 portion, 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 cannon- 

 ball a velocity which, if the particles were all moving in one direc- 

 tion, would constitute a tornado ten times more violent than any ter- 

 restrial 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 continu- 

 ally changing their direction by executing a sort of carrom upon their 

 neighbors, the interdiffusion of liquids and gases would be almost in- 

 stantaneous. 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. 



