402 Professor Dewar [Jan. 19, 



— 182° with the ratio of the rigidity modulus between the same 

 limits of temperature, has resulted in finding that both constants are 

 increased in the same proportion. From this it would follow that 

 the resistance to compression of the substance at — 182° C. must 

 be increased in a similar ratio. The comparative behaviour of 

 strong steel spirals at 15° C. and - 182° as to their elongation on the 

 repeated addition of the same load was a subject examined on several 

 occasions. The most careful comparison of such spirals, however, 

 revealed no measurable differences in their elongation between the 

 ordinary temperature and that of boiling oxygen. This may be 

 due to the want of sufficient sensibility in the testing machine when 

 applied to such delicate experiments. In the meantime it is reason- 

 able to conclude that the rigidity modulus of very hard steel is not 

 much changed by cooling it to — 182° C. If balls of iron, tin, lead 

 or ivory are cooled to —182° C. and dropped from a fixed height on 

 a massive iron anvil the elastic rebound is markedly increased in 

 all cases. The flat distortion surface produced on the lead sphere 

 after impact is only one-third the diameter of the circular surface 

 produced at the ordinary temperature when the lead ball falls from 

 the same height. 



The examination of the magnetic condition of matter at low 

 temperatures is a subject of great interest and offers a wide field for 

 investigation. In a former lecture the magnetic properties of liquid 

 oxygen and air were discussed. Owing to the experimental diffi- 

 culties, accurate quantitative measurements of the permeability have 

 not yet been successful. Faraday was the first experimenter who 

 examined the magnetic condition of matter at the lowest temperature 

 that could be commanded in his time, viz. about — 110° C. He did not 

 succeed in making any substance which was non-magnetic at ordinary 

 temperatures assume the magnetic state at the lowest temperature of 

 the solid carbonic acid ether bath in vacuo. Later experimenters 

 have directed their attention more especially to the action of high 

 temperatures on magnetism, and the work of Professors Hopkinson 

 and Ewing in this field of research is well known. Professor Trow- 

 bridge examined the effect of a temperature of — 80° C. on a permanent 

 magnet, and came to the conclusion that the magnetic moment was 

 diminished by about 50 per cent. Professor Ewing found that an 

 increase of temperature of 150° C. above 10°, caused a reduction of 

 the magnetic moment of a bar magnet by about 40 per cent., and 

 that the magnet on cooling recovered its original state. This result 

 would lead us to expect that if the same law is followed below the 

 melting point of ice as Ewing found above it, then a bar magnet 

 cooled to — 182° C. ought to gain in magnetic moment something like 

 30 to 50 per cent. The experiment of Professor Trowbridge is, how- 

 ever, apparently opposed to such an inference. It appears, however, 

 that Professor Trowbridge cooled a magnet that had not reached a 

 constant state (that is to say, one that on heating would not have 

 completely recovered its magnetisation on cooling), because after the 

 magnet had been cooled to — 80° on regaining the ordinary tempera- 



