( 206 ) 



heats as well), and which are essential for an accurate (leterniina- 

 tion (already once unsuccessfully attempted) of the critical tempera- 

 ture of helium \). 



A short description of this cryostat, therefore, is desirable. 



§ 2. Description of the apparatus. The chief difference between 

 this and the apparatus described in the van BEMMEi.EN-Jubilee volume 

 lies in the separation of the helium liquetier from the cryostat chamber 

 by a valve, and in the siphoning over of the liquid helium through 

 a cooled siphon. For the rest, the apparatus on the one side resem ble.s 

 the a[)[)aratus described in the van Bemmki-en xolume, and on the 

 other the liquefier with enlarged reception chamber described in 

 Comm. N". 119 Proc. March 1911. Reference may be made to 

 Comm. N". 119 for a descri])tion of all that is common to the 

 earlier and the later apparatus; the same letters are used to indicate 

 identical parts, while, where one of the partb has been moditied, the 

 letters are distinguished by accents. As in Comm. N". 119 reference 

 must be made to the earlier Communication for a detailed description 

 of the parts connnon to the heUum licjuelier there described and 

 that given in Comm. N". 108, while the plates given in Comm.'s 

 N". 108 and N°. 119 should be consulted along with the figure 

 given with the present paper. -) 



ij For a criticism of the thermal properties of helium an accurate knowledge 

 of this temperature is essential. In this connection we may remark that circum- 

 stances other than those brought forward in § ü of Comm. N'. 119 (Proc. March 

 1911) may influence the deviation of helium from ordinary normal substances in 

 the sense opposite to that in which associated substances deviate from them. An 

 increase in the elongation of the vibrators would bring the attracting particles 

 closer to the surface of the atoms, and this could lead to an increased association 

 at higher temperatures. But, in particular, a peculiarity in the equation of state, 

 ascribed to the change of a and b, may just as well be brought about by the 

 non-appearance of changes occurring hi ordinary normal substances (and perhaps 

 loo m substances such as argon and oxygen wliose critical temperature is not 

 yet loo low) which had not been allowed for in the examination of the changes 

 undergone by their a and b. For it is the comparison of helium and ordinary 

 normal substances, that is regarded. An increase in the value of b for helium 

 would, therefore, correspond with the total or partial absence of a (hiuinution of 

 b (compressibility) of normal substances, and similarly an increase of a with tem- 

 perature for helium would correspond with the absence of aii increase in a at 

 temperatures down to those usually reached with ordinary normal substances. 



(_Cf. too the note on the compressibility of argon atoms in Comm. No. 121^ by 

 Kamerlinsh Onnes and Crommelin, These Proc. p. 16:2). 



~) In tlie construction of this cryostat as well as of the apparatus placed in it 

 during the various experiments 1 owe much to the skill and ingenuity of Messrs. 

 Fuji and Kesselring, instrument maker and glass blower respectively at the 

 Cryogenic Laboratory. 



