SOME CONTEMPORARY AOVANCES IN PHYSICS- V 629 



kept ciiUI so lung. This corresponds to a rL-sislance lowi-r than 

 ;i.lO ' ohms; the resistance of the coil at room-teniperatiire was 

 734 ohms. Few discoveries in physics can have been so exciting ns 

 this one, and further news from Leyden is awaited witii keen antici- 

 pation. I'ntii the present liquefied helium has been made nowhere 

 else, but from now on the process will be carried on at Toronto also. 

 I'ressure affects the resistance of a metal much less than tempera- 

 ture; that is to say, doubling the hydrostatic pressure upon a metal 

 makes no perceptilile ditTerence with its resistance if the initial pres- 

 sure is one atmosphere or less, and usually alters it only by a few per 

 cent, if the intial pressure amounts to thousands of atmosphere. The 

 art of applying enormous pressures under controllable conditions has 

 been developed furthest by Bridgman in the Physical I.aboratory 

 of Har\ard L'niversit>-, which through his work holds the same unique 

 rank in high-jiressure investigations as Kamerlingh Onnes' laboratory 

 at Leyden in low-temperature research. The highest pressure which 

 Bridgman has applied to metals during resistance-measurements 

 exceeds 12,000 kg cm"-, which amounts practically to twelve thousand 

 atmospheres. No one has ever applied temperatures twelve thousand 

 times as great as room-temperature, nor even four thousand times 

 as great as the lowest accessible temperature; but when the pressure 

 is altered in this enormous ratio the resistance changes only by a few 

 per cent. The volume likewise changes by onl\' a small fraction, 

 which rather suggests that it is the change in closeness of packing 

 of atoms rather than the creation of intense internal stresses which is 

 responsible for the change in conductivity: however, there is no close 

 correlation between relative change in volume and relative change 

 in resistance; sometimes the two are of opposite signs. Usually, but 

 not always, the conductivity increases with the pressure; as if squeez- 

 ing the atoms together facilitated the flow of electricity across the 

 metal. The rule applies to thirty-five elements, distributed as 

 follows in the PericKlic Table: in the first column, 11 \a, 19 K, 29 Cu, 

 47 Ag, 79 Au; second column, 12 Mg. 30 Zn, 48 Cd, 80 Hg; third, 

 13AI, 31 r.a, 49 In. 81 Tl; fourth, 0(\ 22 Ti, 40 Zr, .50 Sn, 82 Pb; 

 fifth. 1.5 P. 33 As, 73 Ta; sixth, 42 Mo, .52 Te, 74 \V, 92 U; seventh, .53 I ; 

 eighth, 20 Fe, 27 Co, 28 .\i, 45 Rh, 4(5 Pd, 77 Ir, 78 Pt; rare earths, .57 

 La, GO Ntl. -Several of the non-metallic elements are found in the list. 

 The exceptions are the five curiously assorted metals 3 lithium, 20 

 calcium, 38 strontium, ol antimony, 83 bismuth — five elements 

 distributed over three columns of the Peri(xlic Table, each of which 

 contains several other elements which conform to the rule. One 

 modification of 55 caesium belongs under the rule, another among 



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