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Physics. — ''Isotherms of monatomic substances and their binary 

 mixtures. II. Isothe.rirs of helium at — 253° C. and — 259° C", 

 by Prof. H. Kamerlingh Onnes. Communication N". 102^ from 

 the Physical Laboratory at Leiden. 



^ 1. Survey of the determinations. The measurements were made 

 in the same way as those of Coram. W. 102« (Dec. '07). The whole 

 of the piezometer had a four times larger content, viz. about 2 liters, 

 the piezometer reservoir on the other hand was more than four times 

 smaller, it was, namely, somewhat more than 2 cm^ Accordingly 

 the densities to which the measurements refer, are considerably larger, 

 and lie between 591 and 794 times the normal one. The temperatures 

 at which the determinations were made, are measured on the hydrogen 

 thermometer of Comm. N°. 95^. 



^ = — 252°.84C. and ^ = — 258°.94 C. 



from which by extrapolation by means of table XXV of Comm. 

 N». 101^ (Dec. '07) see § 3 of Comm. W. 102^ follows for the tem- 

 peratures below 0° C. measured on the absolute scale 



6 = — 252^84 -f 0M2 = — 252°.72 

 and <9 = — 258 .94 + 0M2 = — 258°.82 



The determination of the mean temperature of the gas in the capil- 

 lary stem of the piezometei' reservoir, with regard to the part that 

 extends above the bath in the cryostate, required here greater accuracy 

 than before, because compared with the quantity of the gas in the 

 smaller reservoir that in the stem was of more importance. With a 

 view to the determination of this mean temperature a cylindric reser- 

 voir of the same height as the capillary was placed by the side of 

 and on a level with the capillary, which reservoir was filled with 

 helium, and provided with an appliance to read the pressure in it ^). 

 By means of this pressure it is easy to derive with the required 

 accuracy what mean density for the gas in the capillary of the 

 piezometer must be taken. At 0° the pressure in this auxiliary apparatus 

 'Avas 118.3 cm. of mercury. With the measurement at — 253° C. it 

 varied between 33.1 and 5J.1 cm., at — 259° C. between 31.8 and 

 48.1 cm. 



1) A similar contrivance has been applied by different observers in the deter- 

 mination of the mean temperature of the capillary of a gas thermometer (Travers, 

 Senter and Jacquerod, Ph. Tr. Royal Soc. London Ser. A, vol, 200 p. 143 (1902)). 



