180 ESSAY XIII. 



SECTION IV. 



Mr. J. G. Gahn, in roasting plumbago upon a test, ex- 

 perienced a loss of ninety parts in a hundred of plumbago, 

 without any visible smoke ; and Mr. P. J. Hjelm, upon 

 repeating the same experiment, obtained the same result. 

 The remainder is nothing but ochre of iron. One should be 

 easily tempted to believe that whatever has been volatilised 

 during the ustulation was nothing but phlogiston ! For 

 (1) there is not any sulphureous smell perceived from pure 

 plumbago, and the calcination requires the free access of air ; 

 and (2) from its detonation with nitre, etc. But then the 

 phlogiston would, according to this supposition, constitute 

 the greatest part of the weight of the plumbago ; but it is 

 not probable that such a small quantity of iron should fix 

 such a large quantity of phlogiston ; which would then be 

 present in a much larger quantity in plumbago than even in 

 charcoal: for five parts of nitre are sufficient to decompose 

 one part of charcoal ; whereas ten parts of nitre are required 

 to produce the same effect upon one part of plumbago. This 

 induced me therefore to examine the vapours which arise 

 from it in such large quantity during detonation. 



SECTION V. 



One part of levigated and sifted plumbago was mixed 

 with ten parts of powdered nitre, and a little of this mixture 

 was put at intervals into a red hot tubulated retort, to which 

 a large glass receiver was adapted. At last the receiver was 

 found full of nitrous air, and covered with a white crust, 

 which was easily soluble in water, and proved, on an exact 

 examination, to be nothing else than nitre. Thence it is 



