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Physics. — "Isotherms of diatomic gases and their binary mixtures. 

 VI. Isotherms of hydrogen between — 104° C. and — 217° C." 

 (Continued). By Prof. H. Kamerlingh Onnes und C. Braak. 

 Communication N". 99 a from the Physical Laboratory at Leiden. 



(Communicated in the Meeting of June 29, 1907). 



§ 14. Survey of the determinations. 



The determinations mentioned in this Communication constitute 

 one whole with those of Communication N°. 97 a . They may partly 

 serve to control the earlier determinations at — 104° and — 136°, 

 which, being the oldest observations, are not quite so reliable as the 

 others. For the determinations of isotherms at lower temperatures 

 they are a valuable supplement for the smaller densities from 70 to 

 100 times the normal one. With the exception of the isotherm of 

 — 217° the determinations communicated now may also be considered 

 as a whole in themselves. To complete this set of determinations a 

 part of the isotherm mentioned for the density at about 170 times 

 the normal one is still wanting. We hope soon to publish the 

 additional determinations referring to this. To the standard-tempera- 

 tures at which we determined the isotherms, we have still added 

 — 164° C. From the data mentioned in Coram. N°. 97 n may be 

 derived (see § 13 of the communication mentioned) that the point 

 where the inclination of the pv-ci\vxe for exceedingly small densities 

 becomes zero, lies at about this temperature. The purpose of the 

 determinations at — 164° is to determine this point, which we shall 

 call the Boyle-point, more definitely. 



The determinations, with the exception of that at — 140", were 

 made at temperatures which differed little from the standard-tempe- 

 ratures of Comm. N°. 97". They may be reduced to these standard- 

 temperatures by a simple correction (See Comm. 97" § 6). This 

 reduction has not yet been carried out for the isotherms mentioned 

 below. In Table XIX the temperatures are given at which the 

 measurements were made. They were determined and calculated in 

 exactly the same way as those of Comm. N°. 97"; just as to these 

 latter temperatures the correction of Table XVIII Comm. N°. 97 4 is 

 still to be applied to them. 



We may still remark about the measurement of the pressure 

 (cf. § 31, that for the lowest pressures a direct connection with the 

 open manometer was required, because the closed auxiliary mano- 

 meter cannot be used below 20 atins. 



§ 15. Remarks about the manometers and the piezometers. 

 When the determinations were finished, the auxiliary manometer 



