85 VISCOSITY OF GASES 217 



and this formula seemed to suit the case of atmospheric 

 air when n was taken equal to f . Since the molecular speed 

 alters with the temperature proportionally to \/(l + a ^)> 

 the molecular free path must then be proportional to 



(1 + 



According to the observations of C. Barus 1 , who in- 

 vestigated the flow through capillary tubes within a very 

 wide range of temperature, n = f both for air and for 

 hydrogen ; with this value the formula holds from to 

 1,300. For both gases, therefore, the free path would in- 

 crease proportionally to 



(1 + <*)*. 



Experiments with other gases showed, on the contrary, 

 that this value for n cannot hold in general. Puluj 2 

 obtained by the oscillation method the value n = 0*92 for 

 carbonic acid, and von Obermayer 3 observed with capil- 

 lary tubes values for N0 2 , C0 2 , ethylene and ethyl chloride, 

 which were all nearly equal to 1. Eilhard Wiedemann 4 

 found for these gases that the value for n is variable with 

 the temperature, and is the smaller the higher the tempera- 

 ture. S. W. Ho 1m an 5 arrived at the same result, and 

 he therefore expressed his results by the usual series of 

 powers. 



0. Schumann 6 chose a formula with a double factor 

 of the form 



to represent his observations ; the square root here expresses 

 the dependence of the molecular speed ft on the ternpera- 



1 Bull, of the U. S. Geological Survey, No. 54, Washington 1889 ; Amer. 

 Journ. of Science, 1888 [3] xxxv. p. 407 ; Wied. Ann. 1889, xxxvi. p. 358. 



2 Wiener Sitzungsber. 1876, Ixxiii. Abth. 2, p. 589. 



3 Wiener akad. Sitzungsanzeiger, 1876, No. 8 ; Carl's Repert. 1876, xii. 

 p. 465. 



4 Arch. d. Sc. Phys. et Nat. 1876, Ivi. p. 273. Breitenbach, Wied. Ann. 

 1899, Ixvii. p. 816. 



3 Proc. Amer. Acad. Boston. 1877, xii. p. 41 ; 1885, xxi. p. 1 ; Phil. Mag. [5] 

 iii. p. 81 ; xxi. p. 199. 



6 ' Ueber die Eeibung von Gasen u. Dampfen u.s.w.' Tubinger Habilita- 

 tionsschrift ; Wied. Ann. 1884, xxiii. p. 353. 



