Cast IroTif Steel, and Malleable Iron. 427 



attraction, should be able to destroy the molecular cohesion 

 of another body before its own molecular attraction of equal 

 force is overcome. 



The colour of the different sorts of iron seems to depend for 

 the most part on the closer, looser, or the more or less regu- 

 lar arrangement of their molecules, as the molecules of all the 

 different descriptions of iron when viewed by means of the 

 microscope present an equally bright appearance. 



Nevertheless we must be cautious in drawing our conclu- 

 sions from appearances under high magnifying powers. Mi- 

 croscopes seem to have a power of penetrating space as well 

 as Herschel's telescopes; at least, the same piece of razor, the 

 fresh fracture of which I had exposed to a dull red heat, had 

 considerably lost the bright colour of its fracture ; neverthe- 

 less, viewed through the microscope, the molecules appeared 

 to retain their accustomed brightness, and could not be distin- 

 guished from those of a fresh fractured piece when placed by 

 the side of it. Further, upon exposing the same piece to a 

 higher degree of temperature, till the fracture was covered 

 with a skin of oxydul impenetrable to the naked eye, yet 

 viewed through the microscope the whole granulated texture 

 was distinctly visible, and only a slight nebulous veil appear- 

 ed to cover a little of its original brightness. 



The more or less compact arrangement of the molecules is 

 expressed by the specific gravity of the iron, but in ascertain- 

 ing the specific gravity of them, it must be always borne in 

 mind, that cast iron in large lumps has, from the surface to 

 the centre, not only a varying degree of density, but very often 

 also a different chemical composition. I ascertain, therefore, 

 the specific gravity on a large scale by weighing a fragment 

 of an entire section of a cast-iron pig, or by taking out of the 

 liquid iron a small but deep ladleful of metal, the specific 

 gravity of which was ascertained after slowly cooling. 



The specific gravity of thirty-five different sorts of iron 

 which I analysed with all possible care are the following, re- 

 duced to 61° F., and 30 inches height of barometer. 



Specific Gravity for 61° F., and 30 inches height of Barometer. 

 Boilings (puddlings), slag adhering during working to the stirring 



tools... 3-773 



Boilings, slag towards the end of the puddling process 4*519 



Slag forming the bottom of the puddling furnaces 4'6109 



Iron. 



1. Gray cast iron from Vienne (France) 6*898 



2. English bar iron heated in a smith's forge 6*949 



3. Grayish iron from Yorkshire 6*954 



4. Lumps of iron (?) which remained on the bottom of crucible 



in which iron with lamp-black had been melted (crystallized) 6*998 



