MI;, .i. Mm; ON Till-: i:i:mvm <>K II;<>N H:<I\I .\ i .KM i;\lN. 45 



been allowed to rest for many weeks, and had been tented and found t<> I*- <|iiite 

 elastic up to a stress of 35 tons per square inch. The tension specimen was, however, 

 1 ><>iled for some time as a further precaution, and then the compression specimen was 

 cut from it, and the test illustrated by Curve No. 4 was performed. The modulus 

 given by this curve agrees very well with that obtained for the virgin material from 

 Curve No. 1. The marked discrepancy shown in this curve, No. 4, at the lowest 

 loads may evidently be discarded ; it was probably due to imperfect facing of the ends 

 of the specimen, or some such cause. 



Curve No. 4, further, very clearly shows that tensile overstrain which raises the 

 yield-point in tension lowers that in compression, or, it may be more definite to say, 

 lowers the load at which anv arbitrary amount of plastic contraction occurs. This is 

 in agreement with Professor MAI > IIINUER'S conclusion with regard to the elastic 

 limits. \i/... " that the elastic limit in tension cannot be raised without lowering the 

 limit in compression, and vice versd."* Professor BAUSCHINUER draws a further 

 conclusion from his experiments, namely, that when the elastic limits of a material 

 are varied by overstrain, the range of perfect elasticity seems to remain constant, so 

 that, if the elastic limit in tension l>e raised, then that in compression is lowered by 

 an equal amount. The author's experiments do not bear this out. They show that 

 such a proposition cannot be applied to the yield-points, for the yield-point in tension 

 of the material in the condition whose compression properties are illustrated by 

 Curve 4. Diagram XIII., was found to occur at a stress between 12 and 13 tons per 

 square inch, above the yield-point of the material in the primitive condition, and no 

 matter where the yield-points in Curves 1 and 4 be supposed to exist, the lowering, 

 which is the result of the tensile overstrain, cannot be greater than 4 or 5 tons of 

 stress. 



The characteristics of overstrained iron in respect of hysteresis and imperfect 

 elasticity may be considered as illustrating MAXWELL'S views on the ' Const it ution of 

 Bodies,' as set forth by him in the ' Encyclopaedia Britannica.' t In that article 

 all bodies are assumed to be composed of groups of molecules oscillating about more 

 i >r less stable configurations. If the oscillations are such as to cause all the groups to 

 be continually breaking up, then we have a viscous fluid. But if "groups of greater 

 stability are disseminated through the substance in such abundance as to build up a 

 solid frame work, the sxibstance will be a solid, which will not be permanently deformed, 

 except by a stress greater than a certain given stress." A solid, however, is not 

 assumed to be entirely composed of these stable groups of molecules, or say of 

 sensible particles, but to contain groups of less stability, and also groups which break 

 up of themselves. When a solid has been permanently deformed or overstrained 



* See UNWIX'S hook on ' Testing of Materials of Construction," p. 386, or BAI:SCHINGER, " Ueber die 

 Veriindciiing der Ehinticitategrenze iind die Festigkeit de Eisens und Stahls," ' Mittheihmgen aus dem 

 Me. h. Techn. Lalwratorium in Miinchen,' 1886. 



t Or Ha tin- 2nd volume of CI.KKK MAXWKI.I.'S 'Collected Paper*.' 



