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glass tubes to an air reservoir at 0.0° C. and a barometer, the 
whole forming a perfectly closed system at constant pressure. The 
barometer was placed on a screw stand of the type of Comm. n°. 60 
and was read by the cathetometer used to read the mercury menisci 
in the U-tube of the piezometer. 
The normal volume was ordinarily determined at least twice 
both before and after the measurements of compressed volume. 
2. When the normal volume has been determined the piezometer 
tube is removed to the pressure cylinder A (Plate I, Comm. n°. 69) 
previously filled with mercury, it is then die warmed so 
that the air shall be completely expelled from the outer leg of 
the U-tube. When this operation is finished the small piezometer 
reservoir is brought into the bath neeessary for the particular con- 
stant temperature required for the isother m ; the construction of this 
bath and the constancy of the temperature De be considered with 
the various isotherms. 
The graduated tube J, is kept at a constant temperature by a 
stream of water through the water bath 5s from a similar thermostat 
to the one above mentioned. Although this must be at a distance of 
10 M. the connecting tube is so well wrapped with wool that the 
temperature at b4 is constant to 0.02 deg C. It was set at either 
15.6° C. or 20.0° C. and determined to the above accuracy. 
Finally we must consider the temperature of the steel capillary 
which sometimes is 130 cm. long and owing to its exposed position 
usually shows considerable differences of temperature although it was 
carefully wrapped in wool and enclosed in paper. In many cases the 
difference at different parts reaches 1 deg. C. As however the 
influence of this uncertainty depends upon the relative volumes of 
the capillary and the piezometer, this question will be further con- 
sidered with the measurements. 
§ 8. The calculation of the measurements. 1. The normal volume. 
For this calculation we divide the volume of gas in the piezometer 
tube into 4 parts (Comm. N°. 69) at the four temperatures ¢’), t's, tus t;. 
The constants are as follows: 
V', the volume of the small piezometer reservoir A, with the glass 
capillary Ja 
‚V's of the steel capillary from fj to a mark on the top of the 
glass tube 0; 
V', from this mark to the zero point of b, 
V', from this zero point to the zero point of the U-tube bz 
