424 Professor Sir James Dewar [Jan. 16, 



The Action of Low Temperatures on Metals 

 AND Alloys. 



In a former discourse it was shown that all the chief metals and 

 alloys acquired a greatly increased cohesive attraction at low 

 temperatures as measured by the breaking stress. The increase in 

 the breaking stress may reach from 30 to 50, or even 100 per cent. 

 It was further shown that in some metals, before rupture took 

 place at the temperature of liquid air, no diminution of the extension 

 under stress had taken place as compared with similar tests made at 

 the ordinary temperature. This led to testing the flow of a metal 

 into wire about the temperature of liquid air. The only metal that 

 could be examined in this way was lead. At the ordinary tempera- 

 ture in the apparatus used lead flowed into wire under a pressure of 

 7 J tons, but at — 170° C. it was necessary to apply the pressure of 

 67J tons, or nine times the pressure, to cause any flow. In the same 

 manner solder flowed into wire at the ordinary temperature when 

 35 tons was applied, but at the temperature of — 170° C. the ap- 

 plication of 125 tons pressure caused no motion of the alloy 

 through the aperture. This is the greatest pressure that any of the 

 dies used in the experiments would stand without explosive rupture. 



Cooled Rubber Films. 



One of the most interesting illustrations of the increased strength 

 and elasticity of a body at the lowest temperatures is to take the case 

 of a very thin transparent film of indiarubber. The film is stretched 

 over one end of a short glass tube about the diameter of a good wide 

 test tube, the other end being contracted and sealed on to a long, 

 narrow tube that after being bent twice at right angles, has still one 

 limb more than 30 in. long. The film end of the test tube can now be 

 immersed in liquid air, while the end of the long tube is placed in a 

 vessel containing mercury, in order to observe the diminution of 

 pressure in inches of mercury. When the whole test tube part 

 covered by the film is cooled in liquid air, a diminution of from 

 9 to 10 in. of mercury may be observed. Under such conditions the 

 film is perfectly tight, provided it has been tied on to the glass after 

 a little coating of melted rubber has been applied to the surface. 

 No liquid oxygen seems able to diffuse through the film, which is 

 indeed remarkable considering the rapidity with which gaseous 

 oxygen is known to pass. But the most remarkable fact of all is that 

 the liquid air surrounding the film may be replaced by a vessel con- 

 taining liquid hydrogen, which instantly solidities all the air in the 

 film-enclosed space, giving almost a perfect vacuum, as proved by the 

 mercury column rising to the height of the barometer at the time, and 

 yet the film stands the pressure when cooled to — 252° C, and further 

 resists the passage of hydrogen by molecular diffusion. In the cooled 



