( 565^) 



The tciii|K'i'aturo of the oxjieriiiu'iit.-il iiiltc was ri'ad willi an eve- 

 p,lass to \\illiiii .OJ on an "Einsclihissthcrnionietor", dixided ii'ito 

 ()°.l, willi a scaU' on niilkgla^s. From lime lo time this was 

 compared at dillei-ent temperatnres with a simihir thermometer tested 

 at the Iveiehsanstalt with llie air thermometer, wliile the variation 

 ol" the zero of the latter in the mean lime was aeeoniiled for and 

 also the temporary depression of the zero, wliieli, aflei- the thermo- 

 meter had for a long' time heen heated at 4(S° (\, amonnled to 



(i. licdnrtion of the o/tsrrrafions. In eases where, in contrast 

 to the deterniiiialion of the plailpoint and the [)oint of cojitact of 

 the mixtnres and the critical point of carbon dioxide, the greatest 

 possil)le constancy of the temperatnre was not absolntely necessary, 

 tlie temperatnre dnring the determination of an isothermal which 

 histed on an a>erage from o to 4 honrs was allowed to \ary a few 

 hnnth-edths of a degree. For the rednction to one temperatnre, 

 temperature coelïicients for the different volumes were derived from 

 the observations. 



In the two series, one at an always increasing, the other at an 

 always decreasing pressure, pressures ^vere measured, corresponding 

 to different volumes, which in the two series diifered but little. 

 Pressures were derived from them for the same volume and then 

 the mean was found. When it appeared that these pressures agreed 

 sutïiciently (comp. § 4) we have afterwards simjily taken for two 

 corresponding points the mean of the Nolunie and the pressure. 



Physics. — "■ I.sothcniKÜ.s of niictni-es of oj:i/(/eit diul cai-bon dlo,t'ul('.. 

 1 I . Isotkermah of pure carbon (lio.vlde hettoeeii 25° C. and 

 (-)()^ C. and Ix'tween 60 aiuJ 140 aftnosp/o'rrs.'' By W. H. Keesom. 

 Conununication N". 88 (4^'' |)art) from the Physical Laboratory 

 at Leiden, by Prof. H. Ka:\iekling]i Onnes. 



§ 1. Rea.'ion for tke inrcstn/ation of c<(rhon dku-idc. Although the 

 isothermals of carbon dioxide have been extensively investigated by 

 Amagat, I iiaxe yet determined a number of isothermals together 

 with its critical point. I was led to it by the following considerations: 

 ['^. it was desiral)le that 1 should be able to judge of the purity 

 of the caibon dioxide which 1 used for the [)reparation of the mix- 

 tures, and it seems thai this judgment may best be derived from the 

 increase of the va[>oiii- pressin-e with condensation at a stationary 

 temjieratni'c M ; 



1) Comp. Cuiiuii. .\". 7'.). I'loc Apiil \'.n)-2. 



37* 



