180 



SCIENCE 



[N. S. Vol. XXVIII. No. 710 



ation," by C. G. Abbott and F. E. Fowle, Jr. 

 Notes : " Activity in Magnetic Work " ; " Per- 

 sonalia." Abstracts and Eeviews: W. van 

 Bemmelen on " Registration of Earth-currents 

 at Batavia," by L. Steiner; Cirera et Barcells 

 on " Activite solaire et les perturbations mag- 

 netiques," by J. A. Fleming; Meyermann on 

 " Korrektion der Reduktionsconstanten eines 

 magnetiscben Tbeodoliten," by J. A. Fleming. 

 List of Recent Publications. 



TEE LIQUEFACTION OF HELIUM 

 Information communicated by Sir James 

 Dewar to the London Times from Professor 

 Kamerlingb Onnes, of Leiden, shows that 

 helium is a liquid having a boiling point of 

 4.3 degrees absolute, which is not solid when 

 exhausted to a pressure of ten millimeters of 

 mercury, at which pressure the temperature 

 must have been reduced to within three de- 

 grees of the absolute zero — i. e., about one 

 fourth of the temperature of hydrogen in cor- 

 responding conditions, as that again is about 

 one fourth of the corresponding nitrogen 

 temperature. If we could obtain another 

 similar drop by the discovery of a gas still 

 more volatile than helium we should have a 

 liquid boiling about one degree above the 

 absolute zero. The Times also gives a few 

 notes upon the steps by which the liquefaction 

 of helium has been reached. In 1895, by the 

 application of the method of sudden expan- 

 sion from high compression, Olscevski, start- 

 ing from the temperature of exhausted air, 

 failed to get any appearance of liquefaction. 

 In 1901, Dewar, in the Bakerian lecture, de- 

 scribed his repetition of that experiment, using 

 liquid hydrogen under exhaustion instead of 

 liquid air, again without obtaining any trace 

 of condensation. Reasoning from the analogy 

 of his experiments on the liquefaction of 

 hydrogen, he showed that by regenerative cool- 

 ing starting from the temperature of liquid 

 hydrogen, we might expect to liquefy a gas 

 whose boiling point might be as low as four 

 or five degrees absolute. In his presidential 

 address to the British Association in the fol- 

 lowing year he gave reasons for placing the 

 boiling point of helium at that figure, showing 



at the same time how great are the experi- 

 mental difficulties of getting within five de- 

 grees of absolute zero. In 1905 Olscevski re- 

 peated Dewar's experiment of 1901, using 

 higher pressures, and reached the conclusion 

 that the boiling point of helium must be below 

 two degrees absolute, and that after all the 

 gas might be permanent. The same experi- 

 ment was repeated early in 1908 by Professor 

 Onnes with a much larger quantity of helium 

 than had previously been available, and he at 

 first thought he had obtained solid helium, but 

 found that the appearance was due to im- 

 purity in the gas. Dewar again repeated the 

 experiment by circulating helium in a re- 

 generative apparatus, but though he got cool- 

 ing, he was baffled by the inadequacy of his 

 supply of helium to maintain the cooling 

 process sufficiently long to reach liquefaction. 

 At last, by the experiment of July 10, Pro- 

 fessor Onnes has definitely settled the matter. 

 As new and richer sources of helium have been 

 discovered, and its separation has been enorm- 

 ously facilitated by Dewar's charcoal method, 

 it is possible that helium may become suffi- 

 ciently abundant in cryological laboratories 

 to be used as liquid hydrogen is now used in 

 physical research. 



SPECIAL ARTICLES 

 ELECTROMAGNETIC MASS 



The variations of meaning attached to 

 d'Alembert's principle, that depend upon what 

 we may call the genesis of the terms involved 

 in its expression, has been insisted upoii in a 

 previous article.' We find a similar double 

 chance open for instructive interpretation in 

 many other equations of theoretical physics, 

 among which we now select that important 

 result in hydrodynamics which may be re- 

 garded as furnishing the original suggestion 

 of " electromagnetic mass." For a solid of 

 mass m, moving in the line X through an ideal 

 liquid free from boundary conditions, the fa- 

 miliar power equation is 



Xu^d/dt(imu' + imiu'). (1) 



Here X denotes the aggregate of force ex- 

 ternal to the system, consisting of m and the 

 •SciEKCE, Vol. XXVII., p. 154. 



