332 Formation of Miner ah by Electro-Chemical Action. 



hedrons having great transparency, very definite form, and from 

 several millimetres to one centimetre in length. 



2. Chloride of lead, in needles and cubes, slightly yellowish, of 

 very perfect form. .qo'% ,^ h ..labo,^, io ej^aod'^;. 



3. Sulphate of lead, in curieif*rm octahedrons, inuch' modified^ 

 precisely resembling in form the crystallised sulphate of lead of 

 Anglesea. i^^^-'^}- 



4. Chlorosulphate in needles. y;*} ed:^ 



5. Basic chloride, in microscopic crystals, disseminated here and 

 there throughout the whole product. 



6. Sulphuret of copper, black, without any appearance of crystal- 

 lisation. 



The whole of these substances covering the piece of galena gave it 

 the appearance of a specimen from a mineral vein. 



In some of the vessels there were formed only chloride and chlo- 

 rosulphate of lead, in others chloride and sulphate, which depended 

 no doubt upon the proportions of the sulphate of copper and of 

 chloride of sodium, and the density of the solutions. A Voltaic 

 couple, formed of a piece of galena, surrounded by a plantinum wire, 

 placed in a saturated solution of common salt and sulphate of copper, 

 diluted with three volumes of water, give rise to the formation of a 

 considerable quantity of crystallised chloride of lead in cubes, with- 

 out any other product ; they were similarly deposited, though a little 

 larger, upon a fragment of malachite which was placed in the solu- 

 tion. 



There is no evidence in opposition to the opinion that these re- 

 actions take place in nature. In fact, the pluvial waters which 

 reach the mineral masses and veins, formed of metallic combinations, 

 become charged with chloride of sodium and sulphate of copper, 

 arising from the decomposition of the cupreous pyrites ; the resulting 

 solutions, once in contact with the galena, react upon it.sjf^akly. and 

 give rise to the various compounds described above. ^^ ^\ , 



Two other compounds have been obtained, Pb O, CO^ and Oa 0, 

 CO^, by the following processes : — Into a saturated solution of carbo- 

 nate of soda and carbonate of copper was introduced a plate of lead, 

 4 centimes by 2, surrounded by a platinum wire, the whole placed 

 in a glass vessel imperfectly closed, and left to spontaneous action for 

 seven years. The lead gradually oxidised at the expense of the at- 

 mosphere ; the oxide formed, slightly soluble in water, reacted upon 

 the carbonate of copper, whence resulted hydrated oxide of copper and 

 carbonate of lead (Pb O, CO^). This was in very small crystals, 

 covering the plate of lead, and their form appeared the same as the 

 natural carbonate. The carbonate of lime was obtained by effecting 

 the decomposition of the sulphate of that base, a salt slightly soluble 

 in water, and naturally abundant, by a solution of bicarbonate of 

 soda, a compound found in several mineral waters. A plate of Mont- 

 martre gypsum was introduced into the solution (saturated or not) 



