ADDRESS. 27 



order to connect the railway systems on the two sides, and that the 

 Frith of Forth is about to be spanned by a bridge exceeding in grandeur 

 anything as yet attempted by the engineer. The roadway of this bridge 

 will stand 150 feet above high- water mark, and its two principal 

 spans will measure a third of a statute mile each. Messrs. Fowler and 

 Baker, the engineers to whom this great work has been entrusted, could 

 hardly accomplish their task without having recourse to steel for their 

 material of construction, nor need the steel used be of the extra mild 

 quality particularly applicable for naval structures to withstand collision, 

 for, when such extreme toughness is not required, steel of very homo- 

 geneous quality can be produced, bearing a tensile strain fully double 

 that of iron. 



The tensile strength of steel, as is well known, is the result of an ad- 

 mixture of carbon with the iron, varying between -j^th and 2 per cent., and 

 the nature of this combination of carbon with iron is a matter of great 

 interest both from a theoretical and practical point of view. It could 

 not be a chemical compound which would necessitate a definite propor- 

 tion, nor could a mere dissolution of the one in the other exercise such 

 remarkable influence upon the strength and hardness of the resulting 

 metal. A recent investigation by Mr. Abel has thrown considerable 

 light upon this question. A definite carbide of iron is formed, it appears, 

 soluble at high temperatures in iron, but separating upon cooling the 

 steel gradually, and influencing only to a moderate degree the physical 

 properties of the metal as a whole. In cooling rapidly there is no time 

 for the carbide to separate from the iron, and the metal is thus rendered 

 both hard and brittle. Cooling the metal gradually under the influence 

 of great compressive force, appears to have a similar effect to rapid 

 cooling in preventing the separation of the carbide from the metal, 

 with this difference, that the effect is more equal throughout the mass, 

 and that more uniform temper is likely to result. 



When the British Association met at Southampton on a former 

 occasion, Schonbein announced to the world his discovery of gun-cotton. 

 This discovery has led the way to many valuable researches on explosives 

 generally, in which Mr. Abel has taken a leading part. Recent investiga- 

 tions by him, in connection with Captain Noble, upon the explosive action 

 of gun-cotton and gunpowder confined in a strong chamber, (which have 

 not yet been published), deserve particular attention. They show that 

 while by the method of investigation pursued about twenty years ago by 

 Karolye (of exploding gunpowder in very small charges in shells con- 

 fined within a large shell partially exhausted of air), the composition of 

 the gaseous products was found to be complicated and liable to variation, 

 the chemical metamorphosis which gun-cotton sustains, when exploded 

 under conditions such as obtain in its practical application, is simple and 

 very uniform. Among other interesting points noticed in this direction 

 was the fact that, as in the case of gunpowder, the proportion of 

 carbonic acid increases, while that of carbonic oxide diminishes with 



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