HISTORY OF COLD AND THE ABSOLUTE ZERO. 225 



zero, it is ])y no ineiins certain tiiat he would find tlie near apj)rotieh 

 of the death of matter sometimes pictured. An}- forecast of the phe- 

 nomena that would 1)0 seen nuist be l)ased on the assumption that 

 there is continuity l)etween the processes studied at attainable temper- 

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

 assumption justitied? It is true that many chanoes in the properties 

 of substances have been found to vary steadil\' with the degree of cold 

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

 that the changes wdiich have been traced in explored regions continue 

 to the same extent and in the same direction in those which are as yet 

 unexplored. Of such a In-eakdown low-temperature I'csearch has 

 already yielded a direct proof at least in one case. A series of experi- 

 ments with pure metals showed that their electrical resistance gradu- 

 ally decreases as they are cooled to lower and lower temperatures, in 

 such ratio that it appeared probal)le that at the zero of absolute tem- 

 perature they w^ould have no resistance at all and would become perfect 

 conductors of electricity. This was the inference that seemed justifi- 

 able ])y o])servations taken at depths of cold which can ])e obtained I)}' 

 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 tirst observed 

 when a platinum resistance thermometer was used to ascertain the 

 temperature of that liquid ])oiling under atmospheric and reduced 

 pressure. All known liquids, when forced to evaporate quicklv by 

 being placed in the exhausted receiver of an air pump, undergo a 

 reduction in temperature, but when hydrogen was treated in this wa}' 

 it appeared to l)e an exception. The resistance thermometer showed 

 no such reduction as was expected, and it became a question whether 

 it was the h^'drogen or the thermometer that was behaving abnormallj'. 

 ritimately. ])y the adoption of other thermometrical appliances, the 

 temperature of the hydrogen was proved to be lowered bv 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 I)}' cold in the same proportion as at temperatures 

 about minus 200 . This being the case, there is no longer an}^ reason 

 to suppose that at the absolute zero platinum would become a perfect 

 conductor of electricity; and in view of the similarity between the 

 behavior of platinum and that of other pure metals in respect of tem- 

 perature and conductivity, the presumption is that the same is true of 

 them also. At any rate, the knowledge that in the case of at least one 

 property of matter we have succeeded in attaining a depth of cold 

 sufficient to bring about unexpected change in the law expressing the 

 variation of that propert}" with temperature, is sufficient to show the 



6M 1902 — ^15 



