74 [July, 1844. 



Memoir on the scientific character and researches of the late 

 James Smithson, Esq. By Walter R. Johnson. Philadel- 

 phia, 1844. From the Author. 



Prof. Johnson made some remarks on the specimen of Plum- 

 bago mica presented this evening, and on the comparative 

 characteristics of other varieties of Plumbago from various 

 localities, and as an artificial product, all of which were ex- 

 hibited. 



The specimens of plumbago from Saybrook occur in felspar, as- 

 sociated with, and passing directly into dark brown mica. Indeed 

 so intimate is the union of these two minerals, that in several in- 

 stances the folia are mica on one edge, and plumbago on the other. 

 It is in some cases difficult to determine where the micaceous cha- 

 racter begins and where the plumbaginous ends. The mass has, 

 with the exception of possessing a crystalline form, much the aspect 

 of some samples of furnace cinder from anthracite iron works which 

 have fallen under Prof. J.'s notice, and which exhibited large folia- 

 ted portions of keesh or artificial plumbago, passing into a greyish 

 shining, silicious mass of nearly the same colour as the brown mica 

 which accompanies the plumbago of Saybrook. 



The only mineral with which plumbago is liable to be confounded 

 by the casual observer, is sulphuret of Molybdenum — this mineral 

 is likewise represented by authors to occur in or near Saybrook. 

 A specimen of the sulphuret of Molybdenum which Prof. Johnson 

 had tried, possessed a specific gravity of 4.6676. A specimen of 

 fibrous plumbago presented at the last meeting, (probably from Cey- 

 lon,) had a specific gravity of 2.200; a fragment of the plumbago 

 from Saybrook 2.353 ; a specimen of plumbago from Russia 2.227; 

 and a mass of plumbaginous deposite from the interior of a gas re- 

 tort, derived probably from the decomposition of bicarburetted hy- 

 drogen, had a specific gravity of 1.957. The relation between the 

 several specimens of plumbago and their contrast with the sulphuret 

 of molybdenum, are sufficiently indicative of the true nature of the 

 specimen this evening presented. The peculiar silvery colour, and 

 the greenish streak of the sulphuret were also referred to and ex- 

 hibited in contrast with the darker grey colour, and the similarly 



