barus.] VISCOSITY OF GASES. 251 
volume of the platinum capillary. Indeed, variations of 100° within the 
spires of the helix at temperatures as high as 1 ,200° are not impossible. 
If, however, the interior of the helix be filled with some non-conducting 
substance like asbestus fiber, and the exterior surface be snugly sur- 
rounded by a little box of non-conducting substance, like mica, the de- 
gree of constant temperature is much improved. Better results are ob- 
tainable by surrounding the helix with alternate layers of good and bad 
conductor. But in its practical application this method is troublesome, 
and I have therefore preferred to measure temperatures at both the ex- 
terior and the interior surfaces of the helix. In the final experiments 
two thermo-electric junctions were in contact with the outside and one 
with the inside of the helix. 
Pressures were read ofi° in mercury columns by aid of the Grunow 
cathetometer already referred to. It is frequently necessary in this ap- 
paratus to open {he stop-cocks at particular pressures. To obtain these, 
preliminary experiments are made ( u Einsehiessen"), and the desired 
positions of the meniscus are indicated by adjustable fiducial marks. 
For the measurement of intervals of time an excellent chronometer 
of Brocking in Hamburg was available. 
METHODS OF COMPUTATION. 
The general equation. — The computations of the present memoir are 
based on the PoiseuilleMeyer transpiration formula, the special appli- 
cation of which to gaseous How is due to Meyer. 1 It is available in two 
forms. The first form is 
w= iw i"-^ 2 ^ m 
where u denotes the velocity of a particle at a radius, r, from the axis 
of the capillary tube, the diameter (bore) and length of which are 2R and 
L, respectively ; where P and p are the pressures at the two ends of the 
capillary tube, and where ?/ denotes the coefficient of internal friction 
(viscosity), C the coefficient of slip (Gleituug's coefficient). The second 
form is obtained by integration from equation (1). It contains a new 
variable, viz, Y l the volume of gas transpiring through any section of 
the tube where the pressure is pi during the time t. 
F^^^l+4- 
IGa/ p 
1) (-> 
If this equation is to be used for the absolute evaluation of ?/ it must 
P 2 — p 2 
be borne in mind that the dimensions of — - — are those of a pressure. 
Hence if P and p have been expressed in terms of the heights of col- 
umns of mercury, the factor 6g must be inserted in the right-hand mem- 
Meyer: Pogg. Ann.. *ol. 127, 1866, p. 269. 
(905) 
