328 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



the respirable air of the calx of this metal in the glass had been attracted, and 

 reduction effected. This reduction might be produced by 3 substances here pre- 

 sent, namely, phosphorus, inflammable air, and charcoal; but I impute it to 

 the charcoal; 1st, because I obtained no particles of lead by passing phosphorus 

 through a tube filled with powdered white glass, heated to the degree of ren- 

 dering the tube soft, though on cooling I found the internal surface of the tube 

 was turned black, which colour could not be removed by rubbing, or by acids. 

 This appearance I cannot explain. !2dly. This reduction is effected when there 

 is no water present, at least when no inflammable air is extricated. 3dly. The 

 greatest quantity of regulus of lead was obtained in those parts of the alkaline 

 matter which contained the smallest quantity of charcoal, and therefore I con- 

 ceive the charcoal, formed in those parts, had united to the air of the calx after 

 the phosphorus had been driven through the alkali; so that the carbonic acid thus 

 composed could not be decompounded, but was combined with the alkali, which 

 was always redundant. In calculating the proportion of carbonic acid decom- 

 posed, it will be necessary to consider the reduction which here takes place. 



If the air-apparatus be not affixed to the tube, containing a charge of phos- 

 phorus and alkali, charcoal and respirable air will be formed; but the phosphorus 

 will take fire at the open end of the tube, and burn with splendour, as in dephlo- 

 gisticated air. Porcelain, or well glazed Wedgwood tubes, answer in these expe- 

 riments better than glass ones, the insides of which are apt to melt; but un- 

 glazed vessels allow the phosphorus to pass through their pores, though part of 

 the carbonic acid may be decompounded. The heat applied must be greater than 

 the white glass now made can endure without melting; for I passed phosphorus 

 through a tube containing mild fossil alkali, heated so that it appeared red-hot 

 in the dark, and no charcoal was formed, though the inside of the tube was 

 blackened. 



Experiments with phosphorus applied to mild vegetable alkali, calcareous earth, 

 baryles, magnesia alba, and clay. — Similar experiments to the preceding, made 

 with mild alkali of tartar, instead of fossil alkali, afforded apparently as much 

 charcoal, and which was easily obtained; but as the phenomena were similar, 

 and as I have not ascertained with any tolerable precision the proportion of the 

 carbonic acid decompounded, and of the products, it is unnecessary to give any 

 further account of them. 



By the like experiments, I endeavoured to decompound the carbonic acid in 

 calcareous, barytic, magnesian, and argillaceous earths. The matter remaining 

 in the tubes, after exposure to heat, was blackish, and grey, seemingly from 

 charcoal being formed, though in much smaller quantity than in the preceding 

 experiments with fixed alkalis. For the reasons above given I omit the particulars 

 of these experiments on earths. 



