involved in the Construction of Artillery. 163 



verse or longitudinal strain as the same sized bar, of the same metal, cast 

 alone (i.e. in an isolated mould), and under the same "head" of metal. This 

 circumstance, no doubt due to the conjoint influence, ofseveral of the molecular 

 conditions that have been under discussion, appears to be in part due to the 

 extreme slowness with which the small bar cools in close proximity to the large 

 mass of which it forms an appendage. 



49. It was most probably owing to this cause that Mr. Hodgkinson found the 

 specific gravity of the thin webs, of the cruciform section of castings, adopted 

 by him for experiments on the extension of cast-iron, under tractile forces, to 

 be less than that of the parts of the same bar having greater hidk. The fact seems 

 to conflict with the general one ascertained by me, that the specific gravity of 

 castings is less in proportion to some function of their increased volume, as 

 shown in Table i., following. 



The law, however, enunciated in this Table applies to the average specific 

 gravity of separate masses of similar form, but different volume, and cast each in 

 a separate mould, and in the same way, and is, therefore, not affected by Mr. 

 Hodgkinson's case. 



50. Slow cooling developes a coarse, uneven grain,with large but thoroughly 

 irregular and confused crystallization. Cast-iron with such a grain is never 

 strong or cohesive, though perhaps soft and extensible. The more rapidly a 

 casting once consolidated can be cooled, without introducing injurious effects, 

 the finer, closer, and more even will be its grain on fracture, and with any given 

 metal the greater will be its strength. The rate of cooling cannot be accele- 

 rated beyond a moderate limit/. If this limit be exceeded, as by casting in a cold, 

 thick, highly conducting metallic mould, the iron is " chilled," its chemical, or, at 

 least, its mixed constitution changed, and the uncombined graphite is exuded, the 

 combined carbon only remaining in the white chilled metal. It cannot be so fast 

 as to endanger unequal contraction, nor must it be so fast in large castings, such 

 as guns requiring to be " fed" from a " feeding head," with fresh portions of hot 

 fluid metal during consolidation, to fill up the internal cavities or porosity due 

 to contraction and crystallization, as already explained, that this feeding cannot 

 be accomplished. The prevalent notion, however, that the soundest and 

 strongest castings are obtained by letting them cool slowly in the moulds, is 

 founded on a radical error. 



