2 FLUIDITY AND PLASTICITY
urements of the rates of flow of liquids through capillary tubes,
which are still perhaps unsurpassed. They lead directly to the
laws of viscous resistance and they will be described in detail
in a later chapter. The theoretical basis for .these laws and a
definition of viscosity were supplied by the labors of Hagen
(1854), G. Wiedemann (1856), Hagenbach (1860), Helmholtz
(1860), Maxwell (1860). Since the velocity of flow through the
capillary may be considerable, a correction is generally necessary
for this kinetic energy, which is transformed into heat. Hagen-
I bach was the first to attempt to make this correction but
Neumann (1858) and Jacobson (1860) were the first to put the
I correction into satisfactory form. Thus both the method of
; measurement and the formula used in calculation of absolute vis-
I cosities were practically the same by 1860 that they are today.
'• Unfortunately, these important researches have not been suffi-
! ciently well-known, hence their results have been repeatedly
i rediscovered, and there is an evident confusion in the minds of
I many as to the conditions necessary for exact measurement.
| 1 ! The so-called "transpiration77 or Poiseuille method was not the
only one which was worked out during this period of perfecting
\ the methods of measurement. The pendulum method was
| | j developed by Moritz (1847), Stokes (1849), 0. E. Meyer (1860),
: Helmholtz (1860) and Maxwell (I860). The well-known method
; of the falling sphere was worked out by Stokes (1849).
j During the period to which we have just referred, Graham
v (1846-1862) had been doing his important work on gases, but
: the development of the kinetic theory gave a great impetus to
the study of the viscosity of gases; and at the hands of Maxwell,
0. E. Meyer and others, viscosity in turn gave the most striking
confirmation to the kinetic theory. The work on the viscosity
; of gases has continued on until the present, being done almost
\ exclusively by physicists.
I To chemists, on the other hand, impressed by the relations
i. . between physical properties and chemical composition, so forcibly^
; brought to their attention by the work of Kopp, the viscosity of
I liquids has been an interesting subject of study. To this group
: belong the researches of Graham (1861), Rellstab (1868), Guerout
\ (1875), Pribram and Handl (1878), Gartenmeister (1890),
Thorpe and Rodger (1893) and many others.