200 
MR. J. ZELENY ON THE VELOCITY OF THE IONS 
outer one in the apparatus, we obtain for the value of the critical velocity for air 
about 55 centims. per second, and for hydrogen about 390 centims. per second. It is 
evident that in the apparatus used where there are two concentric cylinders, the 
maximum velocity consistent with a uniform motion must he considerably larger 
than if the gas were flowing through the outer cylinder alone. Nevertheless the 
largest value of the velocity used in any experiment was 25 centims. per second for 
air and 44 centims. per second for hydrogen. As these values are well within the 
limits given above for a cylindrical tube whose radius is equal to that of the outer 
one here used, the conditions for a stable motion are fulfilled. The entrance of 
the gas through a funnel-shaped aperture and its subsequent passage for a con¬ 
siderable distance through a uniform section allowed the motion to come to a per¬ 
manent state before it reached the place where the observations were taken. 
An experiment which was tried showed that by blowing a stream of air down a 
large glass tube and with a velocity greater than that used in these experiments, 
the gas assumed a motion parallel to the axis after it had traversed but a short 
length of the tube, as was made visible by the presence in the air of irregularly 
distributed ammonium chloride particles. 
2. The volume of the gas emitted per second by the gasometer varied a little for 
different elevations of the gasometer, but there was a considerable range where it 
was quite constant, and this range only was used in making experiments, the rate of 
descent being determined in addition during each observation. Guide wheels pre¬ 
vented the tilting of the gasometer during its descent, and the readings on the 
attached scale could therefore be relied upon. The pressure of the gas was 
determined by a manometer attached to the gasometer, and the pressure in the 
apparatus was similarly obtained. The volume of the gas emitted by the gasometer 
per second was then reduced to the pressure in the apparatus, and dividing by 
the flow area in the tubes, the required value of U in equation (9) was obtained. 
3. In order to understand more clearly the manner in which the values of 
A and X of equation (9) were determined, let us consider the following case. In 
fig. 3, CC' represents a longitudinal section of the outer cylinder. DB is the 
