274 PROFESSOR CHRISTISON ON THE ACTION OF WATER UPON LEAD. 



after having been dried at 212. This is evidently a hydrated oxide of lead. The 

 powder becomes carbonated when left exposed to the air, even in the dry state. 



When the action of the water takes place in the air, the product is more 

 abundant, and seems to consist entirely of a mass of white, pearly, microscopic 

 crystals. These acquire a pale gray tint, when removed from the water and al- 

 lowed to dry spontaneously. When examined in water with a powerful compound 

 microscope, they present the appearance of a congeries of thin tables. The pri- 

 mitive form of the table is probably the equilateral triangle, the crystals being 

 therefore thin sections of a regular tetrahedre. Some crystals present the trian- 

 gular form (1.), but with the angles always slightly truncated ; others, by exces- 

 sive truncation, have become hexagons (2) ; others present slender radiating lines, 

 dividing the hexagon into six constituent equilateral triangles (3) ; and others of 

 the latter construction assume the appearance of rosettes, probably by erosion of 

 the angles of the hexagon (4). 



When 28.62 grains had been dried at 180, they lost only 0.01, on being 

 heated to 250 ; and no further change took place till the temperature rose to 

 350, at which point moisture began to be slowly discharged. A low red heat ex- 

 pelled much carbonic acid and a considerable quantity of water. 



An apparatus was constructed for transmitting the disengaged gas through 

 fragments of chloride of calcium to absorb the moisture, as well as for collecting 

 the dry gas over mercury. When the whole gas and water had been expelled by 

 heat, air, previously deprived of carbonic acid gas and moisture by means of 

 caustic potash, was passed through the tubes composing the apparatus, for the 

 purpose of driving into the gas-jar the carbonic acid left in the tubes. The volume 

 of carbonic acid was then ascertained by absorption with solution of potash, and 

 properly corrected to the temperature of 60 and the barometric pressure of 30 

 inches. The results obtained with 24.20 grains of the substance formed in large 

 quantity by the continuous action of distilled water for twenty months, were 



Oxide of lead, .... 21.035 grains. 



Carbonic acid (5.588 cubic in.), . . 2.641 . . . 



Water, ..... 0.535 ... 



24.211 grains. 



These numbers correspond nearly with the theory, 3PbO+2C0 2 + Aq; that 

 is, a compouud of three equivalents of oxide of lead, two of carbonic acid,- and one 

 of water, or rather, a compound of two equivalents of carbonate of lead in union 



