272 MEASUREMENT OF HIGH TEMPERATURES. [hull. 54. 
inferences to be drawn. Turning to Table 90, on page 271 , the singularly 
high values there incorporated at once strike the eye. The clue to these 
large discrepancies is, however, at hand ; for 
_ 7r \ 1 + k) P 2 ~p 2 1 jk* 
V ~ 16 p V L 
and it is therefore clear that since — - = 4 - Tr even slight errors in R at 
Tf K 
once produce serious effect on 77. I have stated that because of the dif- 
ficulty encountered in constructing the capillary apparatus faultlessly, 
I did not wish to subject the tubes to any experiment which might tend; 
to injure them ; I stated also that in the discussions of the present part 
of the chapter absolute data were of inferior interest. Hence, without 
forgetting the occurrence of the uniformly large values of tj here found, 
the question may be waived, to be resumed in the next part in con- 
nection with other relevant data. The one property of ?j which has an 
important bearing on the present discussion is its degree of constancy 
as regards time. If the values of ?j in Table 90 be graphically con- 
structed as they vary with the data belonging to each, the curves re- 
sulting show that ?/o has a slight tendency to increase. Disregarding 
the data between March 3 and March 7, which refer to an earlier form 
of apparatus, it appears that in the hydrogen data between March 8 
and March 14 the mean increase of ;/ is somewhat less than 0.5 per 
cent, per day of use; in the results for air between March 15 and 
March 26 less than 0.2 per cent, per day of use ; in the final results for 
hydrogen between March 20 and 26 the results vary irregularly, and 
their mean ascent is zero. These data constitute an exceedingly favor- 
able verdict relative to the pyrometric application of the principle of 
viscosity. They show that for large mercury pressures like 125 cm and 
76 cm , respectively, at the two ends of the platinum capillary tube, no 
serious change of the radius of the tube need be apprehended at tem- 
peratures even as high as 1,400° (white heat). In this respect the pres- 
ent results are valuable. The slight tendency of ?j to increase just 
mentioned, is in accordance with an increase of the capillary radius re- 
sulting from the excess of internal pressure. This follows from Meyer's 
equation, but whether the observed result may not be equally wel 1 ex- 
plained as resulting from variations in the composition of the gases or 
from similar progressive causes can not be ascertained. It suffices for 
the present purposes that the time variations of 7/ have been found 
negligible, and that therefore the dimension of the platinum capillaries 
used in the above experiments have remained practically unchanged, 
when the mean excess of internal pressure at white heat was about £ 
of an atmosphere. 
The use of different gases (air, hydrogen) in these experiments was 
principally to vary the conditions of experiment and to detect the laws 
(926) 
