A HISTORY OF SCIENCE 



what has just been said of pure metals, becoming great- 

 er and greater as the carbon is cooled. If an hypothesis 

 were invented to cover this case there would still re- 

 main a puzzle in the fact that alloys of metals do not 

 act at all like the pure metals themselves, the electrical 

 resistance of such alloys being, for the most part, un- 

 affected by changed temperature. On the whole, 

 then, the facts of electrical conduction at low tem- 

 peratures are quite beyond the reach of present ex- 

 planation. They must await a fuller knowledge of 

 molecular conditions in general than is at present 

 available a knowledge to which the low-temperature 

 work itself seems one of the surest channels. 



Even further beyond the reach of present explana- 

 tion are the facts as to magnetic conditions at low tem- 

 peratures. Even as to the facts themselves different 

 experimenters have differed somewhat, but the final 

 conclusion of Professor Dewar is that, after a period 

 of fluctuation, the power of a magnet repeatedly sub- 

 jected to a liquid - air bath becomes permanently in- 

 creased. Various substances not markedly magnetic 

 at ordinary temperatures become so when cooled. 

 Among these, as Professor Dewar discovered, is liquid 

 oxygen itself. Thus if a portion of liquid air be further 

 cooled until it assumes a semi-solid condition, the oxy- 

 gen may be drawn from the mass by a magnet, leaving 

 a pure nitrogen jelly. These facts are curious enough, 

 and full of suggestion, but like all other questions hav- 

 ing to do with magnetism, they hold for the present 

 generation the double fascination of insoluble mystery. 

 To be sure, one may readily enough suggest that if 

 magnetism be really a whirl in the ether, this whirl is 



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