366 MOLECULES. 



recently been greatly developed and improved by Dr Ludwig Boltzmann. The 

 moat important consequence which flows from it is that a cubic centimetre of 

 every gas at standard temperature 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 

 calculate 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 grammes. We have to find 

 at what rate this small mass must move (whether altogether or in separate 

 molecules makes no difference) so as to produce 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 table, 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 con- 

 stitute a wind blowing at the rate of seventeen miles a minute, and the only 

 wind which approaches this velocity is that which proceeds 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 different 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 hall 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. 



I have here a bottle containing ammonia. Ammonia is a gas which you 

 can recognise by its smell. Its molecules have a velocity of six hundred metres 

 per second, so that if their course had not been interrupted by striking against 



