CHAPTER V. 
THE PYROMETRIC USE OF THE PRINCIPLE OF VISCOSITY. 
INTRODUCTION. 
Remarks, — It has been said that a method for making metallic capil- 
lary tubes was described hypothetically by Begnault in his celebrated 
memoir. 1 So far as I know, however, the first platinum tubes made for 
actual high-temperature use are those described in the present volume. 
The dimensions of the capillaries used in the air-thermometer work 
have already been given 5 similar tubes of silver and of copper were 
also in hand. It seemed expedient therefore in view of the excellent 
quality which these tubes eventually came to possess, to put them to 
more general use than originally contemplated. Indeed, the attempt 
to obtain absolute thermal measurements in the region of high temper- 
atures, from the transpiration data obtainable by passing gases through 
red-hot capillary tubes of platinum, presented itself as an important 
final step in the present investigation. The kinetic theory of gases has 
not, as yet, given any satisfactory clue for the prediction of the thermal 
relations of gaseous viscosity. It is nevertheless probable, from the 
nature of a gas, that an experimental law, which might be found to hold 
between 0° and 1,200°, could be safely assumed to hold for a much 
larger interval of temperature. In other words, judicious extrapolation 
is much more nearly permissible in the case of thermal results applying 
to gases than it possibly can be in the case of results which apply to 
liquids or to solids. Again, since the rate at which transpiration takes 
place varies inversely as the absolute temperature of the gas, as well as 
inversely as its viscosity, it is clear that the construction of a trans- 
piration pyrometer will be practicable, even if the thermal variations 
of viscosity should prove unfavorable for such a purpose. 
Apart from practical applications, however, physical science can not 
but profit by any attempt at Ifigh-temperature measurement, rationally 
based on some other phenomenon than the thermal expansion of a gas. 
This is proven, for instance, by the pains which V. Meyer, Troost, Ber- 
thelot, and others have taken to ascertain whether the coefficient of 
thermal expansion in all its high-temperature applications could be as- 
sumed to be rigorously constant. Even if the present method should 
fail of further purpose than the co-ordination of data in a field of high 
temperature, wnere absolute results are either isolated or wanting, its 
' Cf. pages 167, 169. 
(893) 239 
