32

FLUIDITY AND PLASTICITY

III

whereas if there were no slip and therefore X = 0 we would have
V = ^ 6.25 X 10-6
GT/ji
Thus it would appear that the rate of flow through a polished
metal tube should be nearly 20 times more rapid than through a
tube in which there is no slip. Since Poiseuille's experiments
prove that the viscosity is constant for tubes of very different
radius when calculated without regard to slipping, there can be no
slipping when water flows through glass tubes. This conclusion
is admitted by Helmholtz.
Jacobson (1860) criticised Helmholtz's use of Girard's experi-
ments in that he failed to apply any correction to the pressure.
Jacobson himself experimented with copper tubes as well as glass
tubes but found no evidence of slipping.
Warburg (1870) investigated the flow of mercury in glass
tubes. He found that Poiseuille's law of pressures and his law of
diameters were verified, which proved that slipping did not occur.
B^nard as reported by Brillouin (1907) page 152, has repeated
the work of Warburg using greater care, and he finds that X
cannot have a value greater than 0.00001.
Whetham (1890) caused water to flow through a glass tube
before and after being silvered, proper corrections being made for
changes in temperature and in the radius of the tube, due to
the silver layer. Different thicknesses of silver as well as
different pressures were used, but the difference in the times of
flow between the silvered and unsilvered tubes were all within
the limit of experimental error. Copper tubes were also used and
the results in all cases were in agreement with Poiseuille's
observations. Cleaning the tubes with acids and alkalies, polish-
ing with emery powder, coating with a film of oil and amalgamat-
ing with mercury were all without effect in producing a deviation
which could be detected. Whetham repeated an experiment of
Piotrowski with an oscillating glass flask, plain and silvered.
Care was taken to make correction for temperature and to
prevent changes in the bifilar suspension which seems to have
been neglected by Piotrowski. Whetham found the ratio of the
friction of water on glass to the friction of water on silver to be
1.0022, which may be taken as unity within the limits of experi-
mental error. Couette (1888-1890) attacked the problem