400 Professor Dewar [Jan. 19, 



to make it support an ounce weight. The same spiral, cooled to 

 — 182° C, will support a couple of pounds, and will vibrate like a steel 

 spring so long as it is cool. In the same way, a bell or tuning fork of 

 fusible metal gives a distinct metallic ring at —180°. If two tuning- 

 forks are taken of identical pitch, and one cooled to —182°, then 

 on simultaneouly striking them beats are very distinctly heard. The 

 simplest plan of getting some idea of the change in the cohesive 

 force at low temperatures, is to ascertain the tenacity or breaking 

 stress of the metals and alloys under such conditions, and to comparo 

 such results with similar experiments made at the ordinary tempera- 

 ture with the same metallic samples, using the same apparatus. In 

 this way the comparative values are reliable. The only difficulty 

 is the large quantity of liquid air or oxygen required to cool the 

 steel supports of the wires, which have to be broken. Seeing that 

 wires less than T \j inch in diameter are unreliable, good strong rigid 

 steel supports are needed, and as these have to be cooled each time a 

 wire is broken, the experiments involve large quantities (gallons) of 

 liquid air and oxygen. Further, as not less than three, and in many 

 cases six experiments must be made with each sample of wire, and 

 the stress in each case can only be applied slowly, work of this kind 

 extends over long periods of time, and this means increased waste of 

 liquid gases. Fig. 6 shows the general plan of the part of the 

 testing machine which supports the wires which have to be broken. 

 In the figure, A is the steel rod which is connected to the multiplying 

 levers, the stress being gradually increased as usual by running in 

 water into a vessel hung from the long end of the lever ; C is the 

 wire to be tested, B is an arrangement which measures roughly the 

 extension of the wire, and D is a large silvered vacuum vessel holding 

 the liquid oxygen. This latter vessel must be large, in order to avoid 

 any part of the supports of the wire coming into contact with the 

 sides, otherwise the shock of the wire on breaking shatters the 

 vacuum bulb. The rupture must be made while the wire is immersed 

 in the liquid oxygen, and the whole of the supports thoroughly cooled 

 down. The wires must be caught in long V-shaped grooves made in 

 the steel supports in order to avoid slipping, and change in the cross 

 section of the wire. As a rule, the wires used were y 1 ^ inch in 

 diameter and 2 inches long. The following table gives the mean 

 results of a large number of experiments : — 



Table I. — Breaking Stress in Pounds op Metallic Wires 

 0-098 Inches Diameter. 



An inspection of this table proves that all the common metals and 

 alloys increase in tenacity at low temperatures : thus iron has 

 doubled its breaking stress, and the other metals and alloys arc all 

 increased from a third to a half the normal amount. This increase 



