PROPERTIES OF STRUCTURAL METALS. 37 



dividing line can not be drawn even in theory, much less in 

 practice; when, wherever it falls, it must divide, not inter- 

 mediate, but finished products, used in enormous quantities, 

 and blending into one another by insensible gradations, and 

 when every shade of these variations is the subject of rigorous 

 engineering specifications." 



It is customary and necessary, in ordering steel, to give a 

 certain margin in filling specifications, and it will be evident, 

 no matter how close this margin is, that if a line could be drawn, 

 it would not infrequently happen that he who ordered ingot 

 iron would receive steel, and he who ordered steel would re- 

 ceive ingot iron. 



Many different tests have been proposed at various times 

 for determining the mechanical properties of steels, but al- 

 though some of them are of value in special cases, the one 

 method of investigation which has become well nigh universal 

 is to break by a tensile stress and measure the ultimate 

 strength, the elastic limit, the elongation, and the reduction of 

 area. Strictly speaking, none of these properties has any direct 

 connection with hardness, and it is also true that in special in- 

 stances, as with very high carbons, hardening may reduce the 

 tensile strength by the creation of abnormal internal strains ; 

 but in all ordinary steels it is certain that hardening is accom- 

 panied by an increase of strength, by an exaltation of the elas- 

 tic limit, and a degree in ductility. 



" The fact that common soft steel is materially strengthened 

 by chilling has been widely recognized for many years, but the 

 extent of the alteration in physical properties in the softest 

 and purest metals is not generally understood/' 



The table on page 1 7 shows the results of a series of tests 

 made by Mr. H. H. Campbell. 



Again from ' Manufacture and Properties of Structural 

 Steel " : 



"The classification by hardening is a dead issue in our 



