EEICTION BETWEEN WATEE AND AIE. 
595 
motion : one has only to put in the measuring tube with its open end upward ; the 
soap-lamin® cannot now be produced directly, but a large cylinder must be put first of 
all into the solution of soap, and afterwards the lamina must be transferred from this 
cylinder to the measuring tube. This procedure can also be of use sometimes when 
the measuring tube is directed downward. 
It was found by such experiments that the weight of the lamina is of no appreciable 
influence, as both positions of the measuring tube gave the same numbers for the 
volume of the aspirated air under similar circumstances. The downward position of 
the measuring tube was therefore constantly adopted in the following experiments, this 
position being the more convenient. 
The radius of the measuring tube, however, is of great importance, and it will be seen 
by using tubes of different radii that the larger tubes give a greater volume. But this 
augmentation becomes continually smaller if we proceed with enlarging the measuring 
tube, and from a certain radius the volume increases no more. At this point we can 
state that the soap-lamina is without influence on the quantity of aspirated air. 
We can make similar observations also on the water-manometer mentioned before. 
It does not indicate any difference of pressure when during the aspiration the measuring 
tube remains open ; but if we close it by a soap-lamina, the manometer shows a dimi- 
nution of pressure that becomes the more insensible as the radius of the tube increases. 
The radius at which the influence of the section of the measuring tube vanishes 
depends certainly on the quantity of the aspirated air. A radius of 2 - 4 centimetres 
seems to be quite sufficient for my experiments. 
The motion of a soap-lamina is of course much slower in larger tubes than in small 
ones, and can therefore be measured with greater accuracy. The employment of larger 
measuring tubes is more advantageous on this account also. 
The following Table gives the radii of all the measuring tubes that were made use of : — 
Meas. tube. 
I. . 
II. . 
III. . 
IV. . 
y. . 
yi. . 
Eadius. 
1- 096 centim. 
4-00 centims. 
2- 42 
1-57 centim. 
1-13 
0-85 
These tubes may also be employed by couples, placing the second measuring tube in 
lieu of the manometer. A few experiments which I made in this way will be found 
among the following measurements. 
The first part of these measurements was executed in July 1875 with the aspirating 
tubes 1-4, the length of which was gradually diminished to find the influence of this 
dimension. The volume of the air was at the time measured by tube I., the radius of 
which has not the necessary value, as was found by later experiments. But this circum- 
