ADDKESS. 36 



difficult as to bridge the gap of 150 degrees that separates liquid chlorine 

 and liquid air. By the use of a new liquid gas exceeding hydrogen in 

 volatility to the same extent as hydrogen does nitrogen, the investigator 

 might get to within five degrees of the zero ; but even a second hypothe- 

 tical substance, again exceeding the first one in volatility to an equal 

 extent, would not suffice to bring him quite to the point of his ambition. 

 That the zero will ever be reached by man is extremely improbable. A 

 thermometer introduced into i-egions outside the uttermost confines 

 ■of the earth's atmosphere might approach the absolute zero, provided 

 that its parts were highly transparent to all kinds of radiation, other- 

 wise it would be affected by the radiation of the sun, and would there- 

 fore become heated. But supposing all difficulties to be overcome, 

 and the experimenter to be able to reach within a few degrees of the 

 ^ero, it is by no means certain that he would find the near approach 

 of the death of matter sometimes pictured. Any forecast of the phe- 

 nomena that would be seen must be based on the assumption that 

 there is continuity between the processes studied at attainable tem- 

 peratures and those which take place at still lower ones. Is such an 

 assumption justified ? It is true that many changes in the properties of 

 substances have been found to vary steadily with the degree of cold to 

 which they are exposed. But it would be rash to take for granted that 

 the changes which have been traced in explored regions continue to the 

 same extent and in the same direction in those which are as yet unex- 

 plored. Of such a breakdown low-temperature research has already 

 yielded a direct proof at least in one case. A series of experiments with 

 pure metals showed that their electrical resistance gradually decreases as 

 they are cooled to lower and lower temperatures, in such ratio that it 

 appeared probable that at the zero of absolute temperature they would 

 have no resistance at all and would become perfect conductors of elec- 

 tricity. This was the inference that seemed justifiable by observations 

 taken at depths of cold which can be obtained by means of liquid air and 

 less powerful refrigerants. But with the advent of the more powerful 

 refrigerant liquid hydrogen it became necessary to revise that conclusion. 

 A discrepancy was first observed when a platinum resistance thermometer 

 was used to ascertain the temperature of that liquid boiling under atmo- 

 spheric and reduced pressure. All known liquids, when forced to evaporate 

 quickly by being placed in the exhausted receiver of an air-pump, undergo 

 a reduction in temperature, but when hydrogen was treated in this way 

 it appeared to be an exception. The resistance thermometer showed no 

 such reduction as was expected, and it became a question whether it was 

 the hydrogen or the thermometer that was behaving abnormally. Ulti- 

 mately, by the adoption of other thermometrical appliances, the tempera- 

 ture of the hydrogen was proved to be lowered by exhaustion as theory 

 indicated. Hence it was the platinum thermometer which had broken 

 down ; in other words, the electrical resistance of the metal employed in 

 its construction was not, at temperatures about minus 250° C, decreased 



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