392 
MR. W. CROOKES OK THE VISCOSITY 
on it or by tbe rotation of the bulb and tube on its axis by the movement of the 
arm p between the stops, is measured by a beam of light from the lamp s, reflected 
from the mirror g to the graduated scale t. 
640. The angle through which the apparatus is rotated is small. The distance of 
the scale from the mirror is 4 feet, and the amplitude of swing carries the line of light 
between 100 and 200 divisions of the scale. 100 divisions of the scale =62 millims., 
so that the deflection of the line of light is never more than about 10 degrees. As 
the scale forms the tangent of a circle a small correction is required for the scale¬ 
reading. Let S be the scale-reading; R the radius or distance of mirror (centre of 
motion) from the scale; S' what would be the scale-reading if it were curved in a 
circle of radius, Pi; then— 
S'=S 
S 5 
The ratio of S to R is, however, so small as to make j — and the succeeding terms 
insensible. The correction therefore is simply— 
JL 
3 
S 3 
E 2 ' 
Except in special cases this correction was not applied, for unless the amplitudes 
were very long the amount to be deducted was less than the average error of each 
observation. 
641. Air is introduced into the apparatus through a very fine glass tube drawn out 
to almost the fineness of a hair, and projecting from the connecting tubes at a. A 
momentary contact with a spirit flame seals up the point, and when air is to be 
admitted the tip of the tube is broken off, and the air slowly enters, passing through 
the drying-tubes, in, before it reaches the apparatus. 
Gases other than atmospheric air are introduced into the apparatus through the tubes 
u and v. The tubes u and v are only open when actually in use, and when the appa¬ 
ratus has been filled with a pure gas and exhausted a sufficient number of times, they 
are sealed off at u or at v, as the case may be, and when required are again attached 
for another gas. This prevents the apparatus when actually at work having any mer¬ 
cury joints, blown joints being used in all other cases. At m is a system of six 
tubes, for the double object of drying the gas and of keeping mercury vapour out of 
the torsion bulb. The tube nearest the pump contains pure sulphur which has been 
fused, then finely powdered, and introduced while warm into the tube. The next tube 
contains finely-divided copper reduced by hydrogen from the oxide, and the four 
remaining tubes contain phosphoric anhydride. The phosphoric anhydride must be 
tightly packed in the tubes, so as to force the air to pass through, and not merely 
over it, or the last traces of water will not be removed. The sulphur tube stops mer¬ 
cury vapour, and in the course of some months becomes darkened at the end nearest 
