Nature s Operations ivhich Man is competent to Study. 459 



gets close to another molecule. Then an encounter takes 

 place : the molecules struggle together for an excessively 

 brief period, after which they fling asunder in two new direc- 

 tions. The average velocity with which the molecules dart 

 about had been known before Maxwell's investigation. It is 

 about 500 metres per second in the air which we breathe. It 

 was also known that, except in very high vacua, the molecules 

 are so crowded that their journeys between their encounters 

 can be but short ; but the length of these journeys was not 

 known. What Professor Maxwell effected was an actual 

 determination in certain gases of the average length of these 

 " free paths. " He did this by showing that upon this average 

 depends what is called viscosity in a gas — that property which 

 gradually brings a gas to rest after it has been disturbed and 

 currents established in it. He further showed that the average 

 length of the free paths is what determines the rate at which 

 gases diffuse into one another. Accordingly, from experiments 

 on viscosity made by Sir George Stokes, and from Graham's 

 experiments on diffusion, he was able to ascertain what the 

 average length of the free paths must be to produce the ob- 

 served amount of effect. He thus found it to be about six 

 eighthets * of a metre — that which would be represented 

 arithmetically by 000,000,00 of a metre — in atmospheric 

 air at the temperature and pressure of the experiments, which 

 we may take to have been a barometric pressure of 760 milli- 

 metres of mercury and a temperature of about 17° centigrade. 

 This length is smaller than any interval which the microscope 

 can show T , and yet it is a length which must be regarded as 

 very large among molecular magnitudes. 



Nature's Work at Closer Quarters. 



We can, however, extract from Maxwell's determination 

 information about still smaller quantities. In fact, Clausius 

 had previously been able to showf that in the more perfect 

 gases, at ordinary temperatures and pressures, the mean 



regions of an atmosphere the law of the equal diffusion of gases no longer 

 holds. See " On the Physical Constitution of the Sun and Stars,'" Royal 

 Society's Proceedings, No. 105, 1868, pp. 13 and 14 ; or " Of Atmospheres 

 upon Planets and Satellites," Royal Dublin Society's Scientific Transac- 

 tions, vol. vi. 1897, p. 305, or Astrophysical Journal, vol. viii. 1898, p. 25. 



* Subsequent experiments by Maxwell himself on the viscosity of air 

 (Phil. Trans. 1866, p. 258) assign a length of 10'6 eighth-metrets to the 

 average free path. The mean of all the determinations is 7'6 eighth- 

 metrets. 



t Pogg. Ann. 1858, vol. iii. p. 251 ; or Phil. Mag. 1859, vol. xvii. 

 p. 89. 



