CRYOGENIC LABORATORY AT LEIDEN— GUILLIEN Jg^ 



2.° 19 K., so that at that temperature the concavity changes sign. 

 Then Professors Keesom and Wolfke found that the variation of the 

 dielectric constant presented the same singularity as that of density. 

 Finally Professor Keesom and Clusius have shown that the specific 

 heat, c, increases in passing from very low temperatures to a maxi- 

 mum value of 3 at 2.° 19 K., then decreases rapidly so that the curve, 

 c, T, also shows a cusp. In considei-ation of the singularity in the 

 form of this curve, the temperature at which the infection occurs has 

 been designated by the Greek letter lambda. Clusius showed later 

 that the special temperature lambda depends on the pressure, and 

 that upon a pressure-temperature diagram the curve Tx, P\) 

 divides in two parts the region of liquid helium. 



The designations Hen and Hei have been given respectively to the 

 colder and the warmer of the two heliums. Miss Keesom has shown 

 that the specific heat discloses an abrupt discontinuity in an interval 

 of less than 1/1000 degree, and that the coefficients of expansion, of 

 compressibility, and of pressure also exhibit discontinuities. The 

 transposition of Hen into Hbi is in fact the most studied example of 

 what is termed a transformation of the second order. At the i^res- 

 ent time an investigation is in progress on the behavior of the heat 

 of fusion in passing from solid helium to the states Hcj and Hen. 



The study of the properties of substances gaseous at ordinary tem- 

 peratures has profited remarkably by the use of low temperatures. 

 In this way a partial separation of the isotopes 20 and 22 of neon has 

 been made on a nearly industrial scale by a fractional distillation of 

 liquid neon. Such a distillation occupies 4 persons during 4 

 times 24 hours, and after 14 fractional distillations there results 4 

 liters of neon of atomic weight 20.091, and 4 liters of neon of atomic 

 weight 21.157. By a similar process it is also possible to create dif- 

 ferences of the order of 1.5 percent between the densities of fractions 

 of hydrogen, owing to a partial separation of the heavy component. 



The structure of solids is studied at low temperatures by means of 

 X-rays. It has been possible to obtain for such study in monocrys- 

 tallic state substances which are gaseous at ordinary temperatures, 

 such as ethylene and nitrogen, and this facilitates the interpretation 

 of the results. 



In another field, at the behest of Einstein and after hmi Debije, 

 many measurements have been made of the specific heats in order to 

 check formulation of the quantum theories. These have led to im- 

 portant modifications of these theories. Recently it has been shown 

 that for some metals the free electrons carry an important part of 

 the specific heats at the low temperatures obtainable with helium. 

 This falls in with the theory of Sommerfeld. 



The specific heats are also actively studied for metals and alloys 

 in the supraconducting state. With tin, for example, there is found 



