90 Scientific Intelligence. 



too near the limit of detection by this method to make the result 

 very certain. T. R. IMerton has now developed a new method in 

 which a ring system of interference fringes is photographed on a 

 plate so that they may be measured by a micrometer. In order 

 to take account of variations in the diameter of the rings which 

 would result from changes of temperature in the interferometer, 

 or a shift in the position of the source of the light due to wander- 

 ing of the arc which was used, the lead under study was alloyed 

 with cadmium. In this way a comparison system of fringes was 

 recorded which would indicate any variation in the difference of 

 path between different plates whether due to temperature changes 

 or alteration of the arc, and serve to correct the other lines. In 

 carrying out the observations, if two specimens of lead emitted 

 different wave lengths it could be noted what fraction of the dis- 

 tance between fringes any fringe was displaced. The sensitive- 

 ness of the apparatus may be judged from the alteration of the 

 wave length which was necessary to shift the pattern by a whole 

 fringe. This difference of wave length amounted to 0.084A in 

 the lead line A ^ 4058A and 0.146A in the thallium line X =^ 

 5350A. Three specimens of lead were examined: (1) a pure 

 sample of ordinar}^ lead; (2) lead extracted from Joachimsthal 

 pitchblende residues; (3) lead extracted from Ceylon thorite. 

 The author 's conclusions may be thus stated for the line A ^ 

 4058A : 



A (lead from pitchblende) — A (ordinary lead) = 



.0050A ± .0007A 

 A (ordinary lead)— A (thorite lead)= .0022A ± .0008A 



which indicates that these substances are arranged in the order 

 of their atomic weights. Further experiments were made on 

 thallium using the line A = 5350A, which indicated that 



A (ordinary thallium) — A (thallium from pitchblende) = 



.0055A ± .OOIOA. 



The result for thallium does not command quite the weight of 

 those for lead for it was not possible to isolate the pitchblende 

 specimen, but it does seem to indicate that the thallium of pitch- 

 blende is an isotope of greater atomic weight than ordinary thal- 

 lium.— Proc. Roy. Soc. London, 96, 388, 1920. f. e. b. 



8. Magnetic Susceptibilities of Low Order. — In carrying out 

 the new magnetic survey of the British Isles some instrument was 

 required capable of measuring the susceptibilities of various rock 

 specimens. This need was met by the apparatus devised by 

 Ernest Wilson, in which the pull exerted by the magnetic field 

 of a specially designed electromagnet on the specimen was bal- 

 anced against the torsion of a phosphor bronze strip. 



After a careful determination of the instrumental constant the 

 author measured the susceptibilities of a considerable variety of 

 rocks either worked into the form of a rod or after they had been 



