72

FLUIDITY AND PLASTICITY

is dried by passing over sulfuric acid in a smaller bottle *#' .
whence tubes lead to the three-way stop cocks Z and Z' ;t»
thence to the two limbs of the viseometer. The pressure
measured on the water manometer D. The bath is stirred !
means of a motor connected with the mechanism shown a! ^
Since the fluidity of a substance like water changes from 1 tn
per cent with a change of 1° in the temperature, it is necesna- *
that the temperature be controlled to a few hundredths of *

degree. Since they were work is ^
(A, over a wide range of temperalur

Thorpe and Rodger controlled f!
temperature by hand.

A word may be added here 1
regard to stop-watches. The c<*».
mon form of stop-watch in whi»
the whole mechanism starts or st<«j *"
simultaneously with the time recur
may not give consistent results, cv* *
though it appears to neither g»r
nor lose during a long period of tin*'
This is the fault of the mechanic*.
The watches whose movements e««*
tinue, whether the time is bcif<**r
recorded or not, seem to be fr*'* r
from this defect.

The Measurement.—In preparing substances for measuremn/ *
as well as in cleaning and drying the instrument, many invent *
gators have strongly emphasized the importance of avoiding tf»«-
presence of dust particles. Both Poiseuille and Thorpe UK*\
Rodger took elaborate precaution in this regard. Figure #*»
shows the apparatus used by the latter for distilling pure liquid-*
It has the advantage of allowing a good determination of f \*>
boiling-point to be made while the liquid is being fractionated
To avoid contamination by dust and moisture in filling the vi*
eometer, Thorpe and Rodger used a special apparatus, Fig. ii?
The liquid was placed in the bottle H and forced over into tb*
right limb of the viseometer M by means of the pressure of *-*
mercury head A. The viseometer was held in a frame an* I
supported on the vertical rod by means of the setscrew X

FIG. 26.—Apparatus of
Thorpe and Rodger for obtain-
ing dust-free liquid.