ADAYSON observed that the Paper was wanting in this particular, namely, that it took no cognizance of what was alloyed with the iron. The only difference in all the irons in the world, arose from the alloys with which the metallic iron was combined. This much he could say, in proportion that the alloying element was increased, so was the infirmity of such metals a t colour heat augmented. Blue heat certainly was not an exposition of the condition ; as it was manifested in several colours, and the higher alloyed metal would break up at black heat. In a Paper which he had read at Paris, '' On the Rlechanical and other properties of Iron and Mild Steel,"' he had given in an Appendix the chemical compositions, which materially assisted in underatanding the infirmity of alloyed metals. The greatest practical importance he attached to colour heat was relative to railway axles, in which the heat caused the grease to boil off. The temperature then was dangerous ; and such axles subjected to concussions were liable to be broken. The By-shafts of marine engines could only be protected in case they became overheated, by stopping the engines, and allowing the shafts to cool. Cooling them suddenly in cold water was sure to set up local cracks, and to end in complete rupture. Yr. Andrevs. Mr. T. ANDREWS observed that the interest of the Paper would have been enhanced had the Author given a complete chemical analysis of the various steels and wrought-iron employed, and also ascertained the exact temperature conditions of the metals at. the time of observation in each experiment. As regarded wroughtiron, his o m practical experience generally coincided with the Author's experimental observations ; neither iron nor steel ought to be worked a t a blue, or black heat, or considerable injury might ensue. It appeared from a Paper "On the Modification of tensile strength and ductility which Iron and Steel undergo, when heated to between certain temperatures," read by Mr. E.
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