ACTION OP WATER ON IRON. 261 



Stance, and a theory of the cause of its heating on exposure 

 to air. 



Hydrochloric and sulphuric acids both produced it. Nitric 

 acid produced it, but in a state incapable of beating in air. It 

 did not lose this property by long exposure in a solution of a 

 salt of iron, or in water. It absorbed oxygen from the air with 

 evolution of heat. In pure oxygen or chlorine it became much 

 hotter, absorbing either : the residue, after absorption of oxy- 

 gen, was found to contain silex; and Mr. Daniell considers 

 that the plumbaginous cDmpound consists of carburet of iron 

 and silicon, and that, by absorption of oxygen, these became 

 protoxides without separation from the carbon. 



The experiments of Berzelius and Stromeyer, however, ad- 

 duced by Mr. Daniell in support of this view, appear rather to 

 militate against its truth ; and however it may be a " vera 

 causa " that the presence of silicon may occasionally produce 

 the spontaneous heating of this plumbago, the result of my 

 own experiments prove that it can be produced from many 

 specimens of cast iron which do not contain a particle of si- 

 licon. 



23. Dr. William Henry has given, in Thomson's Annals for 

 January 1815, an interesting account of his examination of this 

 substance produced from cast iron in a coal-pit shaft near New- 

 castle-on-Tyne. The cast iron was part of a pipe used to con- 

 vey the water, and evolved gases from a bed of quick sand ; its 

 external characters were the same as those previously described. 

 The specific gravity of the specimen was from 2-008 to 2-155. 

 He states its composition "as iron, plumbago, and the other 

 impurities usually present in cast iron ; " his examination, how- 

 ever, was cursory and rather imperfect. The water from the 

 shaft contained 64 grains in a wine-pint of chlorides of sodium, 

 calcium, and magnesium, and of the sulphate and carbonate of 

 lime. He ascribes the i-emoval of the metal to decomposition 

 of the chlorides, and instances their capability of removing the 

 iron from ink. He also adds a case of conversion of cast iron 

 into plumbago by the action of steam and powdered charcoal 

 on it. 



24. Dr. Thomson gives in his Annals for 1817 a case of like 

 change, produced, with un\isual rapidity, by the action of sour 

 paste, or weavers' "dressing" to cast-iron rollers. The change 

 was so rapid as to oblige the substitution of wood for iron. It 

 is not stated whether the rollers were heated by steam or 

 otherwise, or were at the atmospheric temperature. In the 

 Annals for 1825, a very interesting case is -given in a letter 



VOL. vxi. 1838. s 



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