J^gQ ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



there also that discussions of the work in progress take place. There, 

 too, Professor Keesom has given, under the auspices of L'Institut In- 

 ternational du Froid, a course in thermodynamics for the research 

 men at the laboratory. They also have access to the library of the 

 laboratory, and also to that of the Bosscha Institute for Theoretical 

 Physics, where they find all the books and periodicals which they are 

 apt to require. This Institute, indeed, is under the direction of 

 Professor Kramers. 



(e) Instruction in physics. — On the first floor of the laboratory is 

 given instruction in physics for students of the University of Leiden. 

 The courses are given by Professors Keesom, De Haas, Crommelin, 

 and Kramers, and Mrs. De Haas. They prepare for examinations 

 corresponding to a certificate of licentiate. 



Large halls, equipped in the most modern fashion, serve for illus- 

 trative experiments conditcted by assistants. These assistants, 13 

 in number, may at the same time undertake researches in the labora- 

 tory leading to the doctor's degree of the University of Leiden. 



INVESTIGATIONS ACCOMPLISHED AT THE LABORATORY 



It is not possible to cite here all the researches made at the Cryo- 

 genic Laboratory of Leiden, renamed in 1929 the "Kamerlingh 

 Onnes Laboratory." I may only indicate some of the most important 

 of them, or those relating to problems of special interest. 



In its early years the laboratory was not devoted solely to low- 

 temperature researches. It was there that in 1896 Professor Zeeman 

 discovered the magneto-optical phenomenon which bears his name. 

 Lorentz, then a professor at the University of Leiden, found the* ex- 

 planation of the phenomenon, and therein lay the possibility of de- 

 termining the ratio of the electric charge of the electron to its mass. 



On July 10, 1908, Kamerlingh Onnes for the first time obtained 

 helium in the liquid state. Helium gas, compressed to 100 atmos- 

 pheres, then cooled to —259° C. by liquid hydrogen, was cooled by 

 expansion and became liquid. Since then the low-temperature scale 

 has been extended by the use of helium from 4.°2 to l.°2 K. in ordi- 

 nary practice, and exceptionally to 0.°71 K. These temperatures are 

 yielded by operations with liquid helium. At 0.°7l K. helium is still 

 a liquid, and requires to be compressed to 30 atmospheres to solidify. 

 This was accomplished in 1926 for the first time by Professor Keesom. 



The properties of helium have been extensively studied at Leiden. 

 Without speaking of the very precise determinations of isotherms 

 used to define temperatures, one may recall many researches relating 

 to helium. One unexpected discovery is that there are two kinds 

 of helium. First Kamerlingh Onnes and Boks discovered that 

 the curve of density and temperature has a point of inflection at 



