Chemistry and Physics. 313 



In accordance with general usage the author has included the 

 elements zirconium and thorium in his treatment, although they 

 appear to fall outside the limits of the rare earth group proper. 

 He has also included titanium chiefly on account of its general 

 occurrence in the rare earth minerals and its relation in the 

 periodic system to zirconium, cerium and thorium. The book 

 is divided into three parts, the first of which deals with 

 the occurrence of the rare earths, and is largely devoted to 

 the minerals in which these earths occur. The second part, com- 

 prising about 250 pages, discusses the chemistry of the group, 

 while the third, part gives the technology of the elements and 

 includes an interesting account of their applications in the light- 

 ing industry. h. l. w. 



5. Ampere Molecular Currents. — It has been known for a long 

 time that the electrical resistance of metals decreases in an approxi- 

 mately linear manner as the temperature is lowered, so that, if 

 the straight line law continues to hold down to the absolute zero, 

 the resistance should practically vanish in the vicinity of —273° C. 

 That this inference is correct has been shown by the experimental 

 investigations of H. Kamerltngh Onnes of Leyden. The obser- 

 vations made with a closed coil of lead, wire are very striking and 

 instructive, and a brief account of them will now be given. 



The coil consisted of 1000 turns of wire, 1/70 mm. in diameter, 

 wound on a brass bobbin. The resistance of the coil at ordinary 

 room temperature was 734 ohms, and it was calculated that the 

 induced current would last for only 1/70,000 sec. after removal of 

 the electromotive foree. At 1*8° K., however, the "relaxation 

 time," deduced from earlier determinations of the resistance, 

 should be a matter of days. Theoretical considerations also led 

 to the conclusion that the limiting value to which the current 

 might be raised before the ordinary resistance would suddenly 

 make its appearance was 0*8 ampere at 1*8° K. The coil, sur- 

 rounded by a Dewar flask, was first placed between the poles of a 

 large, excited electromagnet and then liquid helium was poured 

 into the flask. After the coil had cooled down to about the 

 boiling-point of helium at very low pressure (1*8° K.) the cir- 

 cuit of the electromagnet was broken in order to induce an elec- 

 tromotive force in the lead coil. The unexcited magnet was next 

 removed and the presence in the lead coil of a current of about 

 0*6 ampere was shown by means of a suitable magnetometer sys- 

 tem. No decrease in the magnetic moment caused by the induced, 

 current could be detected in the course of an hour, notwithstand- 

 ing the fact that the temperature had risen to that of the boiling- 

 point of helium under ordinary atmospheric pressure, namely, 

 4*26° K. When the coil was removed from the bath the current 

 ceased as soon as the temperature exceeded 6° K., which is the 

 "vanishing point" of the electrical resistance of lead. 



As a check on the preceding results the following experiments 

 were performed. The observations were repeated with the plane 

 of the lead coil parallel to the magnetic field, in order to test the 



