462 



the stop-eock A'„ puts B^ into cominnnication witli tlie manometer 

 yl/jj. To begin witli, when (he pressure is first admitted, (lie stop- 

 cocks Kt K^„ /C^j, /T,,, 7^2 J, /v„, K,„ K^i — /^3, are all open and 

 the pressure is raised to about 60 atmospheres, when the mercury 

 in i1A,„ will rise ver^- nearly to Ihe top, whereas the mercurj' surfaces 

 in ihe open gauge will remain where they are. A55 is ihen closed 

 and tiie pressure is further raised, whereby the merciu'y in Ihe 

 manomeler-lubes goes up in tlie usual way and lliiis indicates the 

 excess of the pressure abnxe the pressure of aliout 60 atmospiieres, 

 which is read on i/,,,. In this manner the tubes B^ etc. are put in 

 series behind M ^^ fis indicated in section 1. Further details of the 

 arrangement will be sufticientiy clear from an inspection of the 

 Plate witliout any furtlier description. 



If it is desiralile to be able to use the two parts of the open 

 gauge simultaneously, viz. the first five tubes as o])en gauge up to 

 20 almos|»heres and the ue.vt ten as differential manometer from 

 60 to 100, or also to connect them up into a single open gauge 

 from — (50 atmospheres, this is easily attained by means of a side 

 connection (0 the pressure-cylinder with T-piece and two stop-cocks 

 at the branching-point, as was actually the case in our experiments. 



With the above arrangement of the manometer it was impossible 

 to go beyond 100 atmos|il)ei-es. In order to continue the measure- 

 ment in a similar way, the open gauge of 20 atmospheres remaining 

 available, it will be necessary to have a new index-manometer on 

 which 100 atmospheres may be read to replace M ^„, with Ihe 

 addition of five suitable tubes to be joined up as a difi'erential mano- 

 meter for the difference between 100 and 120 atmospheres. 



3. The normal I'oluina. As mentioned above, the reading-tube 

 of the manometer had been carefully calibrated. The comparison 

 with the open standard-gauge could therefore serve at the same 

 time as a determination of the isothermal of hydrogen at 20° C. 



It was even po.ssible to determine accurately the normal volume 

 before and after the compression, because the vessel of the mano- 

 meter (of the pattern of Comm. 50) is provided at its lower end 

 with a snuiU 6-tube, also calibrated and containing the mercury 

 which closes the tube, when it is not immersed in the mercury of 

 the pressure-cylinder. 



At the same time in our experiments this was not done. In a 

 first determination of the isothermal of hydrogen from 60 to 100 

 atmospheres we thought ourselves justified in using an indirect 

 determination of the normal volume, obtained by calculation from 



