164 PROPERTIES OF IRON AND STEEL CONSTRUCTIONS. 



gen compounds which it brings down from the atmosphere. According to 

 the experiments made four years ago bj Vogel at Munich (briefly referred 

 to in the Journal for May, 1873), freshly fallen snow furnished water con- 

 taining .003 gramme of ammonia to the litre ; and snow that had lain twenty- 

 fours on a field which had been manured the previous autumn was found, 

 when melted, to contain .012 gramme to the litre. The whole amount 

 brought down by rain and snow must constitute no insignificant fraction 

 of the nitrogenous food of plants. — Boston Journal of Chemistry. 



TECHNOLOGY. 



PROPERTIES OF IRON AND STEEL CONSTRUCTIONS,* 



EXCESS OP ELASTIC LIMIT. 



The limit of elasticity is generally defined as that stress per square unit 

 beyond which permanent changes of form occur, while under less stresses 

 the body returns to its former condition. Reference is made, not to sudden 

 changes in stress and shocks, but to gradually increasing strains. But the 

 definition is theoretically worthless, for a limit so definite is not probable, 

 and much less is it proven. On the contrary, Hodgkinson and Clark have 

 observed that there are permanent ohanges of form under very small loads. 

 At present we must be content with defining this limit with Fairbairn, as 

 that stress below which the changes in form are approximately proportional 

 to the forces, while above this they increase much more rapidly. The 

 words "approximately" and "much" are not so indeterminate as might be 

 supposed, for in the experiments of Bauschinger, the passage beyond the 

 limit of elasticity could be determined very precisely; as for example in 

 tension; "for with the same increase of load a disproportionately great 

 elongation occurred at once, the maximum of which was in every case 

 reached after some time." This sudden elongation mast be credited to per- 

 manent changes of form; further elongations until near the breaking limit 

 remain proportional to the stresses, and the modulus of elasticity is always 

 found to be independent of the latter. In the first definition the changes 

 of form which are permanent from Bauschinger's point ©f view are neg- 

 lected. All experiments, up the present time, have shown that when the 

 elastic limit is passed, the tensile resistance is considerably increased, whil-e 

 ductility and tenacity diminish ; the metal becoming brittle, and having 

 little power of resistance to shock. In experiments at the Woolwich Arse- 

 nal, an iron rod, four times ruptured by pull, gave the successive values of 



-From Weyrauch's Work on the Strength and Calculation of Dimensioos of Iron and Steel Constmc- 

 tions. D. Van JTostrand, New York. 



