260 EIGHTH RKPORT 1838. 



wholly converted into plumbago, and whicli heated to such an 

 extent in a quarter of an hour after exposure as to evaporate the 

 water contained in its pores. He adds, "W-e know not pre- 

 cisely what passes under these circumstances." — {Traitd de 

 Chim. vol. iii.) Dr. M^CuUoch states, that plumbago thus 

 formed always possesses the property of spontaneous heating. 

 This, however, from the writer's own observation, is certainly 

 erroneous. — {Edin. Phil. Jour. No. 14.) 



Hatchet examined a specimen of plumbago which remained 

 long immersed in sea water at Plymouth : he found it contained 

 a little chloride of iron, and that it was composed of 



Oxide of iron . . 0*81 

 Plumbasro . . . 0-16 



0-97 



Dr. M*Culloch made several experiments upon the artificial 

 formation of this phunbago by the action of diluted acids ; he 

 found it bore in quantity no definite relation to the species of 

 cast iron from which it was obtained. 



Pig iron produced more than that cast into guns or shot ; of 

 the latter the blackest varieties, as might be expected, produced 

 the most. 



This author mentions a case of its production from the action 

 of London porter on iron, and also of the recovering of some of 

 the iron guns of the Armada off the coast of Mull, which be- 

 came so hot on being weighed, that they could not be touched. 

 He found that the produce of plumbago from the blackest cast 

 iron, dissolved in dilute acetic acid, equalled the bulk of the 

 iron, and was not pulverulent, but coherent, so as to be cut 

 with a knife. 



In some cases the plumbago heated, and in some it did not ; 

 in the latter he presumes oxygenation to have taken place du- 

 ring solution. 



From his experiments Dr. M'CuUoch drew the rather singu- 

 lar conclusion, that the plumbago was the oxide of a peculiar 

 metal, the oxygenation of which produced the heating. 



Dr. Thompson, in commenting upon this paper, observes, 

 that M'CuUoch appears ignorant of the existence of silicon in 

 cast iron,,an,d of Daniell's experiments upon the subject. 



22. Mr. Daniell has given some interesting experiments on 

 this subject in a paper, on the structure of iron developed by 

 solution, in the Journal of the Royal Institution. In this, after 

 describing the formation of this plumbago by the action of di- 

 lute acids, and its properties, he gives an analysis of the sub* 



