( 109 ) 
in order to prevent convection currents was bent entirely downwards 
outside the bath. The standard barometer has been mounted in an 
other room, and the connecting tube led thither under the floor; 
it was read by Mr. Boupin while I sémultaneously made a measu- 
rement of the difference in level of the mercury in the U-tube of 
the piezometer. It is this latter determination which most renders 
uncertain the measurement of pressure. For the diameter of the 
U-tube is only about 8 m.m., so that with the greatest height of the 
meniscus that occurs viz. 1.66 m.m. the depression becomes 0.69 m.m. 1) 
and a difference in diameter of 0.1 m.m. gives here already a difference 
of 0.03 m.m., while a difference in height of 0.1 m.m. gives a 
differeace in depression of 0.06 m.m. If moreover we take into 
account that the height of the mercury in the limbs of the U-tube 
is read through a water-layer of 6 e.m. thickness, it is obvious that 
in the measurement mentioned an error may enter, which greatly 
diminishes the accuracy of the measurement of pressure otherwise 
to be expected in the rormal volume; this may certainly explain 
the fact that the difference between the largest and the smallest 
values of the normal volume with tube IV amounts to even 3300 
which under otherwise similar circumstances leads us to expect an 
accidental difference of 0.22 m.m. in the measurement of pressure. 
Only from many measurements and repeated mountings — of tube 
IV 17 were made on 5 different days and every day the apparatus 
was mounted anew — we can learn the normal volume with sufficient 
accuracy, for the mean error then appeared to be Peek 
10.000 
The volume occupied by the gas during these measurements was 
measured in entirely the same way as described in the Proceedings 
of May 25% ’01 (Comm. N°. 70, 2rd continuation). 
for the determination of the normal volume of one of the piezometer tubes was made 
in four ways: 1st. with the connecting tube; 2"4. the connecting tube being disconnected 
from the waterbath; 314, the connecting tube being disconnected from thebarome ter; 
4th, without the connecting tube. These four kinds of observations agreed to 
within 
8000 ° 
1) In order to judge in how far the depressions occurring in my measurements agree 
with those given in MENDELEJEFF and GurKowsky’s table I have measured directly 
some depressions for the given width of tube. These gave values which sometimes 
differed mutually 0.05 m.m., and of which the mean was about 0.03 m.m. higher 
than in the table. To determine the influence of moisture, I lightly breathed into the 
tube; this greatly diminished the depression, while the height did not perceptibly 
decrease. 
