270 EIGHTH REPORT — 1838. 



of sea salt and sub-carbonate of soda greatly promotes its 

 oxidation. Unless, then, it were impervious to the fluid agent, 

 it could never prevent the oxidation, however uftiform. 



43. It cannot have escaped the notice of any one who has had 

 an opportunity of observing castings, with what rapidity the 

 Avater of a fresh-fallen shower of rain, which is highly charged 

 with oxygen, attacks fresh-made castings of iron ; and this,, 

 according to my observation, more rapidly in "dry sand" or 

 "loam" castings than those made in damp or "green sand," 

 contrary to the opinion of the French engineers : which I at- 

 tribute to the circumstance, that in "loam" or "dry sand" 

 moulds, moisture not being present, but little hydrogen is 

 generated by the fluid metal to burn off the " facing" of char- 

 coal, which remains "parseme" on the surface of the casting, 

 producing innumerable voltaic couples in contact Avith water ; 

 while, in the case of "green sand " castings, most of the char- 

 coal facing is removed in a gaseous form from the casting before 

 it leaves the sand. 



44. "Chilled" cast iron, or that whose substance, to a greater 

 or less depth, has suffered an alteration of cr3'stalline arrange- 

 ment by having been cast in a cold iron mould, is unquestionably 

 that which suffers least change, in a given time, in water 

 charged with air, whether fresh or salt ; and this from two di- 

 stinct causes : first, from its greatly increased density and hard- 

 ness ; and, secondly, from the fact that a very large portion of 

 luicombined carbon is pressed or squeezed out by the expansion 

 of the crystals of iron at the moment of consolidation. 



45. I have presented to the Chemical Section two specimens 

 of chilled cast iron, in which, by a little management, this phe- 

 nomenon has been rendered very apparent. On these the sus- 

 pended or uncombined carbon is seen exuded in the form of a 

 metalline dew, and adherent to the surface in drops of various 

 sizes. 



These specimens are interesting in another point of view, as 

 affording decisive instances of the expansion of iron in con- 

 solidating. 



I have also in my possession a piece of an unusually dense and 

 white "chilled" iron, of large dimensions, whose entire substance 

 is filled with interspersed octohedral crystals of apparently 

 pure carbon. They are nearly all of equal size, the principal 

 axis of the crystal being about one twentieth of an inch in 

 length. They are hard enough to scratch quartz, and are ex- 

 ceedingly obvious and striking from their dark colour, compared 

 with the iron in which they are imbedded, the grain of which 

 also is brilliaiit and highly crystalline. 



