OP AIR AND OTHER GASES. 25 



ordinary pressure. In November 1863 I made a series of experiments with an 

 arrangement of three brass disks placed on a vertical axis exactly as in 

 M. Meyer's experiments, except that I had then no air-tight apparatus, and 

 the disks were protected from currents of air by a wooden box only. 



I attempted to determine the viscosity of air by means of the observed 

 mutual action between the disks at various distances. I obtained the values 

 of this mutual action for distances under 2 inches, but I found that the results 

 were so much involved with the unknown motion of the air near the edge of 

 the disks, that I could place no dependence on the results unless I had a 

 complete mathematical theory of the motion near the edge. 



In M. Meyer's experiments the time of vibration is shorter than in most 

 of mine. This will diminish the effect of the edge in comparison with the total 

 effect, but in rarefied air both the mutual action and the effect of the edge 

 are much increased. In his calculations, however, the effect of the three edges 

 of the disks is supposed to be the same, whether they are in contact or sepa- 

 rated. This, I think, will account for the large value which he has obtained 

 for the viscosity, and for the fact that with the brass disks which vibrate in 

 14 seconds, he finds the apparent viscosity diminish as the pressure diminishes, 

 while with the glass disks which vibrate in 8 seconds it first increases and then 

 diminishes. 



M. Meyer concludes that the viscosity varies much less than the pressure, 

 and that it increases slightly with increase of temperature. He finds the value 

 of p, in metrical units (centimetre-gramme-second) at various temperatures, 



Temperature. Viscosity. 



8-3 C. '000333 



21'-5 C. '000323 



34-4 C. -000366 



In my experiments, in which fixed disks are interposed between the moving 

 ones, the calculation is not involved in so great difficulties ; and the value of /u, 

 is deduced directly from the observations, whereas the experiments of M. Meyer 

 give only the value of -J^p, from which \L must be determined. For these 

 reasons I prefer the results deduced from experiments with fixed disks inter- 

 posed between the moving ones. 



M. Meyer has also given a mathematical theory of the internal friction of 

 gases, founded on the dynamical theory of gases. I shall not say anything of 

 this part of his paper, as I wish to confine myself to the results of experiment. 



VOL. II. 4 



