( 80) 



obliged to return to oxygen. Although in this case the volume of 

 gas that must be displaced in order to allow the use of the cryostat 

 described in the previous paper, becomes very large, still it may 

 be controlled at — 217^ C. by a Burckhardt-Weiss vacuumpump, 

 arranged as described in Comm. N". 83 V, which can displace 

 360 W an hour. Fortunately we could set an additional pump working 

 to this end. Not only that in this way the Burckhardt-Weiss pump 

 of the methylchloride circulation (Comm. N". 87, March '04) remains 

 free, but now there is also little reason to use nitrogen for tempe- 

 ratures between — 195° and — 210° C. With the same cryostat 

 with oxygen we can produce temperatures ranging from — 180° to 

 — 21 7° C. The arrangement of the cryostat also admits of keeping 

 the temperature constant to within 0°.01 a 0°.02 C. 



On the annexed plate iigs. 2, 3 represent a cryostat which is 

 used for measurements at these low temperatures and which differs 

 only in a few minor details from that of the last communication 

 (N°. 94c May '05) to which I refer for the rest (the same letters 

 denote the same parts). In this cryostat the wires with ^vhich the 

 valved stirrer is moved up and down do not pass over two pulleys 

 /j, so that they run parallel with each other outside the apparatus, 

 but they leave the apparatus over a single pulley xj divei'ging from 

 each other (see fig. 2). Wearing and friction are lessened by this. 

 In the india-rubber tubes surrounding these wires, at distances of 

 8 mm. small spirals, 3 mm. high and 3 mm. in diameter, are intro- 

 duced, which prevent the compression of the tubes when the pressure 

 in the cryostat is reduced. Tube T^^ is taken wider and had there- 

 fore (in order that we still might use the existing apparatus) to be 

 flattened at the end where it passes into ^V^s'. 



Fig. 1 represents the determination of the isothermals of hydrogen 

 by means of the piezometers of Comms. N". 69, 78, 84 § 19, 

 (general letter P) in this cryostat. The temperature is measured 

 (cf. Comm. N". 83 III) with a thermoelement (Comm. N". 89), and 

 regulated according to the indications of the resistance thermometer 

 li (fig. 4 shows this in bottom view). As in the model given in 

 Comm. N°. 93, Oct. '04, (the small letters added to the general letter 

 it ha\e the same meaning as in Comm. N". 93 VIII, § 2), the 

 resistance thermometer consists of naked platinum wires wound upon 

 two glass cylinders and of one protecting cylinder. The improvements 

 which are spoken of already in the pre\ious paper, consist in using 

 instead of the mica sheets /„, Zj, i^, 4 (the form of the supporting 

 ridges being modified) the glass tubes /„', //> h'> 4' (hence in the 

 figure R;^' etc.), so that short circuiting between the different parta 



