76 VISCOSITY OF GASES 181 



simplest assumption would be that it does not depend upon 

 temperature ; but, as we have seen in 71, other possibilities 

 are not excluded. From this, then, we cannot take it as 

 certain, but only as a probable consequence of the theory, that 

 the viscosity of gases increases with the temperature. Gases 

 would therefore in this respect behave oppositely to liquids, 

 whose viscosity is the less at the higher temperatures. 



77. Observations on the Friction of Gases at 

 Different Pressures 



The remarkable laws of the viscosity of gases deduced 

 by Maxwell in 1860 from the kinetic theory of these 

 media challenge experimental proof, not only on account of 

 their apparently innate improbability, but also especially 

 because an experimental proof of the laws of viscosity might 

 in general give at the same time a decision as to the truth 

 and admissibility of the kinetic theory. For if we do not 

 verify by experiment the laws that are consequences of the 

 theory, the theory which requires them must be rejected as 

 erroneous. The importance of this question prompted both 

 Maxwell and myself almost simultaneously to carry out 

 experimental investigations, which were published in the next 

 following years, and were founded on exactly similar methods. 



Of the methods employed up to that time for the deter- 

 mination of the viscosity of liquids, that invented by 

 Coulomb 1 presented itself first of all as the most suitable, 

 because with it the same pressure is exerted everywhere 

 throughout the gas investigated. If a circular disc is 

 suspended horizontally by a wire fastened to its centre, it 

 may, by means of the torsion of the wire, be put into oscilla- 

 tion in its own plane about that centre. If the disc is in 

 a fluid, the amplitude of the oscillations will gradually 

 decrease by reason of the friction which is exerted on each 

 other by the layers of the fluid that are set in motion, and, 

 indeed, the decrease of successive amplitudes follows the law 

 of a geometrical progression. If we measure the amount of 

 decrease, and determine therefore the so-called logarithmic 

 decrement of that progression, we can from the observed 



1 M&m de Vlnst. National, an IX, iii. p. 246. 



