BETWEEN THE YISCOSITT OF LIQUIDS AND THEIR CHEMICAL NATURE. 433 
The vapour density was ascertained by means of the modified Hofmann apparatus 
already described by one of us (Thorpe, ‘ Chem. Soc. Trans.,’ 1880), which allows of 
the introduction of all the corrections necessary to obtain normal values. The formula 
used in the reduction of the observations is 
D = 760W (1 + aT)/-0,8957V [H - h (1 - ^T)], 
where 
D = required vapour density, 
W = weight of substance taken, 
a = ’003665, 
T = temperature of vapour-jacket, 
V = observed volume, corrected for meniscus and volume of bottle 
X (1 + •000025T), 
TI = barometric height corrected for scale error and reduced to 0°, 
h = height of mercury in experimental tube corrected for scale error at the 
temperature T of the vapour-jacket, 
/3 = -00001808. 
It will be seen that the agreement between the observed and theoretical values is, 
as a rule, very satisfactory. 
Deduction of the Working Formula. 
When a stream of liquid flows through a tube of uniform section, so far as 
experiments have been able to show, the liquid molecules appear either to move 
in straight lines parallel to the axis of the tube, i.e., the motion is linear; or the 
stream breaks up into eddies, i.e., the motion is turbulent. 
With a given liquid flowing through a given tube, linear movement corresponds to 
the lowest velocities, turbulent movement to the highest, and for intermediate values 
of the velocity, the two kinds of movement may occiu’ alternately. 
In order to deduce the value of the viscosity coefficient from the time of flow, it is 
essential that the movement be linear. 
If a horizontal tube of indefinite length and of radius R be traversed liy a constant 
current of liquid, pressure falls in passing along the tube in the direction of the 
movement, and if measurements of pressure be made at any two sections of the tube 
distant I from one another, the difference of pressure observed, if the movement is 
linear, may be attributed to two causes :— 
1. To friction of the liquid against the walls of the tube (external friction). 
2. To the viscosity of the liquid (internal friction). 
Experiment seems to show, however, that (I) is inoperative, because it is found 
that if the tube be wetted by the liquid, the loss of pressure is independent of the 
nature of the material of which the tube is made. It is, therefore, assumed that the 
layer of liquid in immediate contact with the wall of the tube is stationary, and the 
MDCCCXCIV.-A. 3 K 
