1881.] On the Viscosity of Gases at High Exhaustions. 449 



Viscosity of Air. 



The mean of a very large number of closely concordant results gives 

 as the log decrement for air for the special apparatus employed, at a 

 pressure of 760 millims. of mercury and a temperature of 15° C, the 

 number 0*1124. According to Maxwell, the viscosity should remain 

 constant until the rarefaction becomes so great that we are no longer 

 at liberty to consider the mean free path of the molecules as practically 

 insignificant in comparison with the dimensions of the vessel. 



The author's observations show that this theoretical result of Max- 

 well's is at least approximately and may be accurately true in air up to 

 such exhaustions as are above referred to ; and that at higher ex- 

 haustions the viscosity falls off, as it might be expected to do accord- 

 ing to theory. 



The results are embodied in a table and diagrams. 



The first half of the table gives the viscosity of air, in so far as it is 

 represented by the log decrement, at pressures intermediate between 

 760 millims. and 0*76 millim. (1,000 millionths of an atmosphere). In 

 order to avoid the inconvenience of frequent reference to small frac- 

 tions of a millimetre, the millionth of an atmosphere* (=M) is now 

 taken as the unit instead of the millimetre. The second half of the 

 table is therefore given in millionths, going up to an exhaustion of 

 0*02 millionth of an atmosphere. 



Starting from the log decrement 0*1124 at - 760 millim., the vis- 

 cosity diminishes very regularly but at a somewhat decreasing rate. 

 Between 50 millims. and 3 millims. the direction is almost vertical, and 

 a great change in the uniformity of the viscosity curve commences at 

 a pressure of about 3 millims. At this point the previous approxima- 

 tion to, or coincidence with, Maxwell's law begins to fail, and further 

 pumping considerably reduces the log decrement. 



From 1,000 M the diminution of viscosity is very slight until the 

 exhaustion reaches about 250 M ; after that it gets less with increasing 

 rapidity, and falls away quickly after 35 M is reached. 



The curves of increasing mean free path and diminishing viscosity 

 closely agree. This agreement is more than a mere coincidence, and 

 is likely to throw much light on the cause of viscosity of gases. 



In the table is also given the measurements of the repulsion exerted 

 on the blackened end of the mica plate by a candle-flame placed 500 

 millims. off. The repulsion due to radiation commences just at about 

 the same degree of exhaustion where the viscosity begins to decline 

 rapidly, and it principally comes in at the exhaustions above 1,000 M. 



The close agreement between the loss of viscosity and the increased 

 action of radiation is very striking up to the 35 millionth, when the 

 repulsion curve turns round and falls away as rapidly as the viscosity. 

 * 1 M = 0-00076 millim. ; 1315789 M = l millim. 



