(29) 
Physics. — Communication N°. 50 from the Physical Laboratory 
at Leiden by Dr. H. KAMERLINGH Onnes: “Standard Gas- 
manometers.”’ (Preeision-piezometers with variable volume 
for gases.) 
S 1. Purpose. Further progress in the knowledge of the laws 
from which we derive the equation of condition for the gaseous 
and the liquid state, depends for a great deal on accurate determina- 
tions with perfectly pure gases and their mixtures in proportions 
exactly known. In these determinations the principal thing is to 
measure the pressure and volume of a precisely known quantity of 
gas at constant temperatures. The standard open manometer !) of 
reduced height formerly described offers us a means of attaining 
great accuracy in measuring the pressure. In the following pages 
I intend to describe piezometers in which (at temperatures above 
the freezing-point of mercury) the volume of a gas, shut off by 
mercury and compressed to different pressures, in proportion to the 
volume which it would occupy at 0° and 760 m.m. (the normal 
volume) can also be read with great accuracy. These apparatus 
enable us to determine the isothermal lines for gases to within 
5000? at temyeratures (above the limit mentioned) which can be 
kept sufficiently constant to allow measurements with the standard 
open manometer of reduced height. 
If the piezometer-tubes are filled with a standard gas, of which 
the equation of condition for ordinary temperatures has been deter- 
mined by means of the open manometer, or if their indications have 
been compared immediately with the open manometer indepently 
from a determination of constants, they can also conversely be used 
to replace the open manometer when the measurements have to be 
made in a shorter time than is required for reading the standard 
open manometer of reduced height at high pressures. 
This method has been followed by me in an investigation taken up 
a long time ago on the isothermal lines of hydrogen (now to be ex- 
tended of course to the since discovered argon and helium) together with 
those of different mixtures of gases at low temperatures (for the obtaining 
and keeping constant of which the-cryogenic laboratory was devised *) ). 
1) Comm. from the Physical Laboratory at Leiden NO, 44, 
*) Comm. Phys. Lab. Leiden N°, 14; the means for the accurate measuring of 
the low temperatures are treated of in Zitt. Versl. 30 Mei and 27 Juni 1896, Comm, 
Phys, Lab, Leiden N°, 27, 
