CRYOGENIC LABORATORY AT LEIDEN— GUILLIEN ^33 



temperature of 0.°0044 K. This low temperature was obtained in a 

 volume of 60 cubic centimeters. The rate of warming was indeed 

 very slow — only 0.°002 in 30 minutes. 



Heretofore no studies have been made of the properties of bodies 

 at these very low temperatures. Thus far experimentation has suc- 

 ceeded only in controlling definitely the temperatures corresponding 

 to the magnetic susceptibility and in measuring their values with 

 precision. The determination of k for the salt is made with extreme 

 accuracy. 



Being thus highly organized and specialized, the cryogenic labora- 

 tory of Leiden has attracted numerous foreign scientists. Mme. 

 Curie proved that the radioactivity of radium is unchanged down 

 to the temperature of liquid hydrogen. Boudin studied the co- 

 efficient of expansion of hydrogen. Mathias, in collaboration with 

 Dr. Crommelin, measured the rectilinear diameters for many gases, 

 of which the last explored are carbon monoxide and krypton. 



At Leiden, also, Pierre Weiss, in a collaboration with Kamer- 

 lingh Onnes, studied magnetization to saturation of ferromagnetic 

 substances, and this led to the discovery of the magneton. Jean 

 Becquerel studied the absorption spectra of the rare earths, their 

 decomposition under a magnetic field, the dissymmetry of intensity 

 of the spectral components, and thereby discovered paramagnetic 

 rotary polarization. He showed that it is possible to observe mag- 

 netism (in particular with the rare earth elements) by optical meth- 

 ods much more accurately than the magnetic susceptibility may be 

 determined in the ordinary manner. He also demonstrated the im- 

 portant role played by the internal electric field in crystals at low 

 temperatures. One should mention also the experiments made here 

 by Paul Becquerel upon the effect of very low temperatures upon 

 seeds and bulbs of flowers which afterward were normally developed. 



I have attempted to describe the present activities of the cryogenic 

 laboratory of Leiden, whose former successes are but stepping stones 

 to advance in new researches. The reader may obtain a fuller ac- 

 count of its history in the description already given by Mathias. 

 I do not wish to close this article without acknowledging how cor- 

 dially foreigners, and especially the French, have been received at 

 the Kamerlingh Onnes Laboratory, and not only well-known scien- 

 tists but also young investigators. The professors guide them and 

 help them to master their difficulties. They work in an atmos- 

 phere of good comradeship which makes their researches still more 

 pleasant. 



BIBLIOGRAPHY 



The works of the Cryogenic Laboratory of Leiden are published 

 in Communications from the Physical Laboratory of the University 



