276 MEASUREMENT OF HIGH TEMPERATURES [bull. 54. 
flexion discrepancy in question, or at least for the difference of behavior 
of hydrogen and air. 
A final remark relative to the surface disintegration (Zerstaubung) 
of red hot platinum, as observed by Nahrwold 1 , Berliner 2 , Kayser 3 , and 
others, must be made here. Unfortunately my work was too far ad- 
vanced when this phenomenon 4 was being discussed to permit me to 
make special investigations with reference to it; nor do I now see how 
allowance for the phenomenon is to be made. I do not believe that 
the error thus left unaccounted for is of a serious kind. For instance, 
in Nahrwold's 5 last paper it is shown that metallic particles fly off from 
red hot platinum much less easily in hydrogen than in air. 6 In case of 
both gases, however, the thermal relations of viscosity are subject to 
the same law. Hence the discrepancy due to surface disintegration is 
probably nil. 1889. 
Sliding coefficient.— To return to the formula selected on page 273, it 
appears from the remarks there made that the full form in which it 
applies to transpiration work must be 
rj" 
_ Vo 
1+4 iv > 1+P0" 
*+4: 
where B is the zero radius and /3 the mean coefficient of expansion of 
the platinum capillary through which transpiration takes place. This 
formula, follows at once for the law accepted above (rf J =r/ {l-\-ad)$)i 
and from the fact that C> which is Helmholtz's Gleitungs coefficient, has 
been proved by Meyer, Kundt, Warburg, and others to vary proportion 
ally to the mean free path. It is usual to take 
B 
as a quantity negligible in comparison with 1. If this be permissible, 
then will 
Co v / T+^ 7/ 
4 
Bo 1+06" 
within the limits of the present interval of temperatures also be negli- 
gible. For the coefficient 
y/l + aO" 
l+fid" 
1 Nahrwold: Wied. Ann., vol. 31, 1887, p. 473; vol. 35, p. 120, 1888. 
2 Berliner: Wied. Ann., vol. 33, 1888, p. 289. 
3 Kayser: Wied. Ann., vol. 34, 1888, p. 607. 
4 Prof. Cleveland Abbe kindly called my attention to it. 
5 Nahrwold: Wied. Ann., vol. 35, 1888, p. 120. 
«Cf. Elster u. Geitel: Wied. Ann., vol. 31, 1887, p. 109. 
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