( 156 ) 



What the real meaning is of the often so highly differentiated 

 organs as many extrafloral nectaries are and of the secretion of sugar 

 which they present in most cases, can only be settled by new 

 investigations which however will have to bear not only on the 

 biology but also on the physiology of the plant. 



Physics. — ''Methods and apparatus used in the cryogenic labora- 

 tory at Leiden. X. Hoio to obtain baths of constant and 

 uniform temperature by means of liquid hydrogen." By Prof. 

 H. Kamerlingh Onnes. Communicalion N". 94/ from the Physical 

 Laboratory at Leiden. 



(Communicated in the meeting of 28 May, 1906). 



^ 1. Introduction. Communication N". 14 of Dec. '94 treated of 

 the results I had obtained after I had employed regenerators for 

 the cascade method, and especially discussed "the way how to obtain 

 a permanent bath of liquid oxygen to be used in measurements at 

 the tiien observed lowest temperatures. At the end of that paper I 

 expressed the hope to be able to construct a cycle of hydrogen 

 similar to that of oxygen. A mere continuation of the cascade method 

 would not do. By means of liquid oxygen or nitrogen, even when 

 they evaporate in vacuo, we practically cannot reach the critical 

 temperature of hydrogen; for the liquefaction of this gas we iiad 

 therefore to avail ourselves of cooling by adiabatic expansion. 



Tn Comm. N°. 23 of Jan. '96 1 made some remarks on what could 

 be derived from van der Waals' law of corresponding states for the 

 liquefaction of hydrogen following this method. I had found that an 

 apparatus to liquefy hydrogen beginning with — 210° C. might be 

 constructed almost after the same model as an apparatus that had 

 proved suitable for the liquefaction of oxygen beginning with ordinary 

 temperatures and without any farther frigorific agents. My efforts, 

 however, to obtain an apparatus for isentropic cooling by combining 

 to a regenerator the outlet- and inflow-tubes of a small expansion 

 motor, fed with compressed gas, had failed. Therefore I directed 

 my attention towards the then newly published (1896) application 

 of the Joule-Kelvin process (Linde's apparatus for liquefying air 

 and Dewar's jet of hydrogen to solidify oxygen). 



Though the process of Linde was the most promising, because he 

 had succeeded with his apparatus to obtain licpiid air statically, yet it 

 was evident that only the principle of this method could be foliow^ed. 



