low temperatures, by which Onnes was enabled to maintain and control the temperature ranges from -23° to -90° (methyl chloride), from -105° to -165° (ethylene), and -183° to -217° (oxygen). This goal may be said to have been attained about 1894. The second stage was characterized by the introduction of liquid hydrogen and the production of temperatures below -217°. The abnormal behavior of hydrogen gas when it is allowed to expand under reduced pressures made it impossible to liquefy it at higher temperatures; and the condensation of this gas was first achieved by Dewar, of London, on May 10, 1898. This added a new range of available low temperatures from -253° to -250° in which Dewar made a number of highly remarkable observations, including the solidification of hydrogen. But Onnes very promptly appropriated this new range for his research work, and constructed novel and very efficient apparatus for the production and utilization of the new refrigerant.
The Netherlands government, realizing the importance of the work, now granted considerable appropriations for the extension and equipment of the laboratory, and with its completion a new era of constantly increasing low temperature research began. New methods and instruments for the exact measurement of temperatures below the boiling-point of liquid hydrogen were devised, and the behavior of mixtures of hydrogen and helium was systematically investigated. Finally, the apparently incoercible gas, helium, was reduced to the liquid state. This crowning triumph of low temperature research was achieved on July 10, 1908. This achievement aroused universal interest in the work of Onnes and doubtless prompted the award to him, in 1913, of the Nobel Prize in Physics.
During the past few years Onnes has made some most remarkable discoveries with reference to the electrical resistance of certain metals at temperatures only a few degrees above the absolute zero of temperature. The resistance of metals ordinarily varies approximately with the absolute temperature, but at temperatures only a few degrees above the absolute zero it suddenly becomes so small that it can hardly be measured. For mercury this "critical temperature" is 4.2° absolute; for lead it is 6.1°, and for tin 3.8°. Below these temperatures the resistance is practically nil, and Onnes terms this the "supraconductive" state. In this state the metals no longer obey Ohm's law—there is neither a potential drop nor a production of heat.
SCIENTIFIC ITEMS
We record with regret the deaths of Joseph Johnston Hardy, professor of mathematics and astronomy at Lafayette College; of Dr. Samuel Baldwin Ward, since 1884 dean of the Albany Medical College and professor of the theory and practise of medicine, and of James Blaine Miller, of the Coast and Geodetic Survey, a passenger on the Lusitania.
The Barnard gold medal awarded every fifth year by Columbia University, on the recommendation of the National Academy of Sciences, "to that person who, within the five years next preceding, made such discovery in physical or astronomical science, or such novel application of science to purposes beneficial to the human race, as may be deemed by the National Academy of Sciences most worthy of the honor," will be given this year to William H. Bragg, D.Sc, F.R.S., Cavendish professor of physics in the University of Leeds, and to his son, W. L. Bragg, of the University of Cambridge, for their researches in molecular physics and in the particular field of radio-activity. The previous awards of the Barnard medal have been made as follows: 1895—Lord Rayleigh and Professor William Ramsay; 1900—Professor Wilhelm Conrad von Röntgen; 1905—Professor Henri Becquerel; 1910—Professor Ernest Rutherford.
