Pus.] VISCOSITY OF GASES. 257 
Vn 
which easily results since ^~^=76.V ,B>n& the effect of (l+<*0) on the 
^orrecti ve member of formula 5 may be neglected, when 6 is small. 
Finally, Finust be corrected for the amount of air left in the adit tube 
g h, after the cock is closed. To reduce this dead space to the smallest 
value it is desirable to fill up the tube g h partially with glass rod. The 
residual space is easily measured, as follows: Let v be this residual 
volume. Then, if in the time % which elapses between opening and 
closing the cock k, the volume V escapes into the measuring tube, the 
volume ultimately found there is a= V+v. Again, if during the same 
time t the cock had been opened and closed n times (delays of opening 
and closing supposed to be allowed for) the volume found in the 
measuring tube is b= V+nv, Hence 
b— a=z(n—l)v 
whence v may be found. 
6 is the temperature of the cold ends V and V" of the capillary as well 
is the temperature at which the wet air in the burette is measured. It 
I determined by submerging a thermometer in the trough MM. 
6" is the temperature of the hot part I" of the platinum tube. Re- 
narks concerning its measurement will be found with each table since 
iris variable is the difficult one to evaluate correctly. The measure- 
ment of the actual value of 6" is directly dependent on the degree of 
jonstant temperature in the helix. 
i£, finally, denotes the radius of the capillary tubes. It is this quantity 
svhicli, in case of fine opaque capillary tubes, it is exceedingly difficult to 
leteriniue. In some of the experiments below I subsequently filled the 
tubes with mercury in ways there to be indicated, and weighed the 
thread. But in the experiment, Tables 81 to 80, I did not wish to en- 
tail ger the platinum tubes by employing this method. Nor did I think 
X safe to apply volumetric methods like those described on pp. 195, 214. 
The only procedure left therefore consists in weighing (mass m) 
known lengths, L, of the capillary, and measuring the external diam- 
eter E cx by screw calipers. From the known density 3 of platinum the 
internal radius B may be calculated at once in its square value as 
7?2_ r> 2__ m 
Cn fortunately this is a crude method at best, and the problem is even 
more seriously difficult, because formula 6 calls for the fourth power of E. 
Flence the absolute values of ?/' and rf in the table are distorted ; but 
isince this distortion is uniform for all data, and since it does not therefore 
affect the relative values, the errors introduced by incorrect R do not 
interfere with the chief purposes of the present investigation. 
I The attempt to obtain absolute values from metallic capillary' tubes 
Bull. 54—17 (911) 
