1884.] Stress and Strain on the Properties of Matter. 389 



value as at first. A tendency to a similar state of things is seen with 

 the other metals, bat in none is the phenomenon so pronounced as in 

 platimim-iridium . 



The value of "Young's modulus" was determined for platinum- 

 iridium, both by the method of static extension, and by that of longi- 

 tudinal vibrations, and the results obtained were in fair agreement. 

 " Young's modulus" is with the alloy considerably greater than with 

 'the two components, so much so as quite to equal if not to excel the 

 value of the same modulus for piano-steel. The " simple rigidity " 

 got by the method of torsional vibrations is also considerably greater 

 with the alloy than with its components. 



Unstretched platinum-silver was found to be thermo-electrically 

 positive to the temporarily stretched metal, and forms a very striking 

 exception to the rule which holds good for most metals, namely, that 

 those metals which are increased to a comparatively large extent by 

 traction in specific resistance are those which are rendered most 

 thermo-electrically negative by traction. 



A further examination was made of the " critical points " of metals, 

 i.e., points at which there is a sudden increase of the ratio of the per- 

 manent extension or permanent change of resistance produced by 

 loading, and the load when the latter is increased by small and equal 

 amounts at a time. This examination ended in the discovery of a 

 fourth " critical point," occurring with a load exactly half of that at 

 the previously so-called first " critical point ;" further, it was ascer- 

 tained that the four " critical points " are. in the remarkably simple 

 ratio's 1:2:3:4. Table I gives the loads at the four " critical points," 

 and their relation to " Young's modulus " for all the metals which 

 have been as yet examined. 



The above table shows not only that the four " critical points " are 

 in the above-mentioned simple ratios, but also that with all metals the 

 value of "Young's modulus " bears a constant ratio to the loads at 

 each of the " critical points." 



In Table II are given most of those results of the present inquiry 

 which can be expressed by numbers : 



