( 445 ) 



7. Some^ further details C(>ncofiiiii,L;' (ho app.'iratiis may liiially be given. 



The iiiouiitiiig and the philcs of the 5 in. in. élahtn are \)y Jobin. 

 Tlie inner surfaees of I he pl-ilos arc accurately Hat. The outer sur- 

 faces nee<l only ordinary llalncss, they are inclined at an angle of 1' 

 to the inner ones. 'I'lie plates of the élaloii are vertical, and the 

 whole apparatus is capable of the necessary adjuslnients iji a/,iniut, 

 while also a hori'/ontal sliding motion parallel to the plates of the 

 étalon was pi'ox ided for. 



An image of the \aciiiiin tube \\as focussed upon the étalon by 

 means of an achromatic lens of 12 cm. focus, the enlargement being 

 four times. All ()|»lical pieces were mounted upon double 7-pieces 

 and therefore rigidly connected. 



The ligures clearly indicate that for the investigation of ihe magnetic 

 sci)aration of the yellow merciu-y lines, ii would be of no value 

 to use an étalon of greater opti^^al thickness of the plate of air. On 

 the contrary the ellective width of Ihe yellow mercury lines wheji 

 under magnetic inlluence is rather large, so that the limits of the 

 method in this case are being rapidly apj»roached. 



Physics. -- " fs()thi'}-)))s o/'iiionatomic (/ascs ami fht'/'i' Jiumri/ mivlmu's. 

 1. hotherms of heHiun between +100'' C. (uid — 2L7° C'." 

 Communication N°. 102« from the Physical Laboratory at 

 Leiden. By Trof. H. Kamerllngh Onnes. 



§ J. On account of the important role, which van der Waals' 

 theory plays in man}^ chapters of thei-modynamics, experimental data 

 concerning the eipiation of state of a substance are of the greater 

 value as the interaction of the molecules of this substance conforms 

 the better to the hypotlieses from which van der Waals started. 

 The knowledge of the equation of state of the monatomic gases, 

 whose molecules we must consider as the simplest for the present, 

 is of the greatest im|)orlance from this point of view. 



In Comm. N". 69 (April 1901) on the isotherms of diatomic 

 gases and their binary mixtures it was already obserxed that the 

 investigation of the net of isotherms of argon and of helium promised 

 still more important results than the completion of the net of iso- 

 therms of the gases formerly called permanent, particularly of 

 hydrogen, at low temperatures, on which subject my attention had 

 been chielly tixed since (he establishment of the cryogen laboratoiy 

 (cf. Comm. N". 14, Dec. '94). Ihit the difüculty of obtaining argon and 

 helinm in so pnre a state and in such quantities as are required for 



