84 Dr. J. Muir. On Changes in Elastic Properties [Aug. 11, 



was then removed. The contraction which occurred on the removal' 

 of the load was almost perfectly elastic. Had the load been imme- 

 diately replaced, the material would have shown perfect elasticity up 

 to the stress of 30,000 lbs,, but immediately the load was increased 

 beyond this amount larger yielding would have occurred, and a smooth 

 continuation of Curve B would have been obtained. The specimen 

 was, however, allowed to rest for about an hour before the load was 

 replaced and increased. This rest proved to have a comparatively 

 large effect, the material showing very perfect elasticity up to the 

 stress of 34,000 lbs. per square inch. At this stress a partial yield- 

 point was exhibited (represented by about 6 units of extension on the 

 diagram), and on increasing the load gradual extension was produced, 

 until ultimately fracture occurred at the high stress of 61,300 lbs. per 

 square inch. The ultimate extension is marked at the foot of the 

 diagram, and was less than that obtained with annealed material. 



It may be recorded that another specimen of" this Lowmoor iron was 

 quenched from 700° C.,but, before testing, this specimen was re-heated 

 to about 200° C. in order to see if any appreciable return to the elastic 

 condition illustrated by Curve A, Diagram 3, would be obtained. 

 The behaviour of the specimen was more nearly elastic for low loads 

 than is shown by Curve B, but all the main features of Curve B were 

 corroborated ; gradual departure from Hooke's law was obtained until 

 fracture occurred at 61,500 lbs. per square inch, the extension being 

 0"*54 on 4 inches omitting, or 0"*9 including, local extension. 



Before leaving the consideration of iron and steel, the effect pro- 

 duced by quenching iron, as illustrated by compression tests, may next 

 be considered. 



Diagram No. 4 shows by a comparison of two compression curves 

 the change in elastic properties produced by quenching mild steel from 

 a red heat. Specimen A, was a short annealed block of mild or 

 semi-mild steel; the diameter of the specimen was 1"'156 and its- 

 length 1|- inches. The compression instrument employed enabled the 

 contraction on a length of 1J inches to be measured to the owoiro °^ 

 an inch. Curve A shows that the annealed material was elastic up to- 

 the stress of 21 tons, but 23 tons per square inch had to be applied 

 before really large yielding occurred. A tension test of this material 

 showed a well-defined yield-point at 22J tons per square inch. 



Specimen B was exactly similar to Specimen A, but the material in 

 this case instead of being annealed was heated to redness and quenched 

 in cold water. Curve B shows the marked loss of electricity produced 

 by the quenching. The rounding of Curve B at the two points where 

 the load was removed is probably to be accounted for by experimental 

 errors of the nature of back-lash in the testing machine or compression 

 instrument. 



Diagram No. 5 illustrates the results obtained by experiments with 



