200 Dr. M. Smoluchowski de Smolan on 



With diminishing pressure it decreases very rapidly (as 

 found already by Kundt and Warburg, see above) — whence the 

 sloping-down of the curves A — and from a certain limit we 

 have only pure conduction of heat, just as in solids ; this is in- 

 dicated by the horizontal part of the curves A and B, since the 

 coefficient of conductivity for heat is independent of the gas 

 pressure, just as well as the coefficient of viscosity. This fact- 

 is not so very surprising, it was foretold by Maxwell before 

 even any measurements of it had been made, as the conduc- 

 tivity depends on the product of the number of molecules 

 with the mean length of their free paths, which are varying 

 with pressure in an inverse way *. 



The final bending down of the curves, shown on a larger 

 scale in part of B and in C, is exactly the phenomenon here 

 discussed, which I attribute to the discontinuity of tempera- 

 ture. This theory explains why this effect is more conspicuous 

 and begins at higher pressures in the small vessel than in 

 the large one, exactly as in my experiments, and its largeness 

 in hydrogen is accounted for by the great value of (yp) found 

 for this gas. 



I have tried even to calculate the values of 7 from the 

 curves for air and hydrogen in the small bulb, which had the 

 cylindrical form required for the application of formula (5), 

 and I have found the product (yp) (of course also y/X) to be as 

 nearly constant as can be expected, considering the inac- 

 curacy of such a method. 



Taking for example the curves for air with the ordinates 



1 

 I 



and abscissae 



p: -0 3 62 -0 3 44 -0 3 35 *0 4 644 -0 4 362 -0 4 192 



we get the values of yp : 



•0 4 149 146 159 157 154 164, 



whence the mean value for 7 = 0*0000155 cm. — , 



P 



and similarly for hydrogen 7 = 0*0000724 cm. . 



These values, though somewhat smaller, are of the same 

 order of magnitude as those found in my experiments ; the 



* The curves for the small bulb are not quite horizontal, but show a 

 minimum at intermediate pressures, which does not seem to have been 

 noticed by other observers Wore ; what its cause may be, it is difficult to 

 say, it may be due to a more complicated effect of the currents. 



. 45-5* 43-1 42-3 41*3 31-7 27-7 23*5 16*6 



