2 8o THE POPULAR SCIENCE MONTHLY. 



warming the gas, and also the law discovered by Charles, that the 

 proportional expansion of all gases between given temperatures is the 

 same. 



The dynamical theory also tells us what will happen if molecules of 

 different masses are allowed to knock about together. The greater 

 masses will go slower than the smaller ones, so that, on an average, 

 every molecule, great or small, will have the same energy of motion. 



The proof of this dynamical theorem, in which I claim the 

 priority, has recently been greatly developed and improved by Dr. 

 Ludwig Boltzmann. The most important consequence which flows 

 from it is, that a cubic centimetre of every gas at standard tempera- 

 ture and pressure contains the same number of molecules. This is 

 the dynamical explanation of Gay-Lussac's law of the equivalent 

 volumes of gases. But we must now descend to particulars, and cal- 

 culate the actual velocity of a molecule of hydrogen. 



A cubic centimetre of hydrogen, at the temperature of melting ice 

 and at a pressure of one atmosphere, weighs 0.00008954 gramme. 

 We have to find at what rate this small mass must move (whether 

 altogether or in separate molecules makes no difference) so as to pro- 

 duce the observed pressure on the sides of the cubic centimetre. This 

 is the calculation which was first made by Dr. Joule, and the result is 

 1,859 metres per second. This is what we are accustomed to call a 

 great velocity. It is greater than any velocity obtained in artillery 

 practice. The velocity of other gases is less, as you will see by the ta- 

 ble, but in all cases it is very great as compared with that of bullets. 



We have now to conceive the molecules of the air in this hall 

 flying about in all directions, at a rate of about seventeen miles in a 

 minute. 



If all these molecules were flying in the same direction, they would 

 constitute a wind blowing at the rate of seventeen miles a minute, 

 and the only wind which approaches this velocity is that which pro- 

 ceeds from the mouth of a cannon. How, then, are you and I able to 

 stand here ? Only because the molecules happen to be flying in dif- 

 ferent directions, so that those which strike against our backs enable 

 us to support the storm which is beating against our faces. Indeed, 

 if this molecular bombardment were to cease, even for an instant, our 

 veins would swell, our breath would leave us, and we should, literally, 

 expire. But it is not only against us or against the walls of the room 

 that the molecules are striking. Consider the immense number of 

 them, and the fact that they are flying in every possible direction, and 

 you will see that they cannot avoid striking each other. Every time 

 that two molecules come into collision, the paths of both are changed, 

 and they go off in new directions. Thus each molecule is continually 

 getting its course altered, so that in spite of its great velocity it may 

 be a long time before it reaches any great distance from the point at 

 which it set out. 



