PROFESSOR CHRISTISON ON THE ACTION OF WATER UPON LEAD. 273 



As connected closely with the subject of the preceding observations, I beg to 

 append a few remarks on the nature of the compound of lead which is formed in 

 the course of the action of water on the metal. 



Where salts are present, whose acids are capable of forming insoluble com- 

 pounds with oxide of lead, the deposit which gradually forms on the lead consists 

 partly of these compounds. But where a substance is produced which floats 

 loosely in the water, as in the action of distilled water ; or where the water, from 

 being clear when fresh drawn from a pipe, deposits a white precipitate on exposure 

 to the atmosphere as in the instance of the two spring- waters mentioned above 

 a compound of a different kind is produced. That which is formed in distilled 

 water is the only variety produced readily in sufficient quantity for examination. 



GUYTON MORVEAU thought this substance was the hydrated oxide of lead. 

 From my experiments, published in 1829, 1 was led to infer that it is carbonate 

 of lead, for it effervesces strongly while dissolving in diluted acids ; and it appeared 

 to me to be the neutral carbonate (PbO + C0 2 ), partly because no other carbonate 

 of lead was known at that time to chemists, and partly because, when deprived 

 of hygrometric water by a temperature of 212, its loss of weight, on being subse- 

 quently heated to redness, corresponded closely with the theory of its elements 

 being united in the proportion of a single equivalent of acid and oxide. In 1834, 

 however, Captain YOHKE, who made some interesting experiments on this subject, 

 without being, for some time, aware of those previously conducted by me, 

 thought, in the first instance, that the substance formed in distilled water was the 

 hydrated oxide : but he afterwards found reason for considering that it contained 

 both carbonate and hydrated oxide of lead, although in proportions variable and 

 not definite. 



It appears improbable, that a substance which puts on invariably and entirely 

 a crystalliform appearance, as this compound does when formed in distilled water, 

 should be a mere mixture, of indefinite composition. Accordingly, I find that it 

 is for the most part, and under certain conditions invariably, a regular definite 

 compound of the carbonate and hydrate of the oxide of lead. 



If lead be immersed in distilled water deprived of its gases by ebullition, and 

 exposed to atmospheric air which has been freed of carbonic acid by solution of 

 potash, the water soon becomes turbid and the lead tarnished, and there is slowly 

 formed on the lead a crust of transparent microscopic crystals, presenting trian- 

 gular and quadrangular facettes, and on the bottom of the vessel a whitish powder 

 with a shade of leaden blue, and not crystalline. Both the crystals and powder 

 are soluble, without effervescence, in nitric acid, and convertible into a yellow 

 powder, with disengagement of much moisture, when they are heated to redness 



VOL. xv. PART ii. 4 E 



