238 HISTORY OF COLD AND THE ABSOLUTE ZERO. 



would vani-sh altogether, and all pure metals become perfect con- 

 ductors of electricity. This conclusion, however, has been rendered 

 very doubtful by subsequent observations made at still lower temper- 

 atures, which appear to point to an ultimate iinito resistance. Thus 

 the temperature at which copper was assumed to have no resistance 

 was minus 223-, but that metal has been cooled to minus 253^ without 

 getting rid of all resistance. The reduction in resistance of some of 

 the metals at the boiling point of hydrogen is very remarkable. Thus 

 copper has only 1 per cent, gold and platinum 3 per cent, and silver 

 4 per cent of the resistance they possessed at zero C. , but iron still 

 retains 12 per cent of its initial resistance. In the case of allo3^s and 

 impure metals cold brings about a much smaller decrease in resis- 

 tivity, and in the case of carbon and insulators like gutta-percha, glass, 

 ebonite, etc., their resistivity steadil}' increases. The enormous 

 increase in resistance of bismuth when transversely magnetized and 

 cooled was also discovered in the course of these experiments. The 

 study of dielectric constants at low temperatures has resulted in the 

 discovery of some interesting facts. A fundamental deduction from 

 Maxwell's theory is that the square of the ref racti\'c index of a Ijody 

 should be the same number as its dielectric constant. So far, however, 

 from this being the case generally, the exceptions are far more numer- 

 ous than the coincidences. It has been shown in the case of many 

 substances, such as ice and glass, that an increase in the frequency of 

 the alternating electromotive force results in a reduction of the dielec- 

 tric constant to a value more consistent with MaxwelFs law. B}^ 

 experiments upon many substances it is shown that even a moderate 

 increase of frequency brings the large dielectric constant to values 

 quite near to that required b}^ Maxwell's law. It was thus shown that 

 low temperature has the same effect as high frequency in annulling the 

 abnormal dielectric values. The exact m(>asurement of the dielectric 

 constant of liquid oxygen, as well as its magnetic permea])ility, com- 

 bined with the optical determination of the refrative index showed 

 that Tuiuid oxygen strictly obeys Maxwell's electro-optic law even at 

 very low (>lectric frequencies. In magnetic work the result of great- 

 est ^ahie is the proof that magnetic susceptibility varies inversely as 

 the absolute temperature. This shows that the magnetization of para- 

 magnetic bodies is an affair of orienttition of molecules, and it sug- 

 gests that at the absolute zero all the feebly paramagnetic bodies will 

 be strongly magnetic. The diamagnetism of l)isnuith was found to b(^ 

 increased at low temperatures. The magnetic moment of a steel mag- 

 net is temporarily increased by cooling in liquid air. but the increase 

 seems to have reached a limit, because on further cooling to the 

 temperature of liquid hydrogen hai-dly any further change was 

 observed. The study of the thermo-electric relations of the metals 

 at low temperatures resulttnl in a great extension of the well-known 

 Tait Thermo-Electric Diaoram. Tait found that the thermo-electric 



