70 APPARENT DECOMPOSITION OF METALLIC SALTS BY MEMBRANES. 



brane : simultaneously, the hydraulic current changed, and the level of the fluid in the 

 tube began to rise. 



229. A semicircular piece of very thin sheet-iron was placed on the inside of the 

 bladder; it caused, in a very short time, a copious precipitate of the copper on that 

 part of the bladder on which it lay, the remainder being free from metallic deposite. A 

 very well-marked change of hydrostatic level occurred. On leaning the tube, the thin 

 piece of sheet-iron was found to adhere to the bladder ; and, on tearing away the 

 cupreous deposite, the little veins, communicating through the porous texture, were vis- 

 ible without a lens. 



230. Lastly, when the relative position of the arrangement was reversed, the pieces 

 of iron being placed below the membrane, and the copper solution above, so that the 

 little filaments of metallic matter could not reach the tissue on account of their weight, 

 but fell down ; or when they were destroyed by mechanical means as fast as they 

 formed, no deposite ever occurred on the bladder. 



231. The experiments (228) and (230) especially, and the other experiments incident- 

 ally, prove that a perfect metallic communication must extend from the decomposing 

 metal to the under side of the membrane, through the very substance of that structure ; 

 and this observation enables us to give the true theory of the process. Let a a a a (fig. 

 27) be a tube, divided in some portion of its length by a membranous partition, b b, on one 

 side of which, A, pure water is placed, containing the decomposing metal, c, and on the 

 other side B, the metallic solution intended to be decomposed. Owing to the obstruc- 

 tion caused by the membranous diaphragm, the diffusion of the water into the solution, 

 and of the solution into the water, will be retarded ; in the course of time, however, it 

 will take place. Then, as soon as any of the metallic salt has reached the lower part 

 of the decomposing metal, reduction ensues in virtue of the common play of chemical 

 affinity, the first portion of the metal thus eliminated adhering to the surface of the de- 

 composing metal, and forming with it an active voltaic couple ; the decomposing metal 

 being thereby rendered positive, and the eliminated metal negative. Of course, the re- 

 duction of the dissolved metal continues, the aggregate of filaments extending itself down 

 by its weight towards the diaphragm; and as the solution becomes stronger and stronger 

 the nearer the filaments approach the bladder, so the metallic deposite becomes more 

 abundant. No obstruction is experienced in passing through the pores of the diaphragm, 

 because the metallic solution is still present there, and is reduced. But the moment any 

 one of the metallic veins has penetrated to the under side of the membrane, the process 

 goes on with tenfold activity. The metallic deposite, when it first started from the de- 

 composing metal, was, perhaps, merely a capillary thread, for the solution through w hich 

 it was passing was so weak that a greater quantity could not be presented ; as it 

 neared the upper surface of the diaphragm, it became of stouter substance, because the 

 solution, being stronger, the supply was more abundant ; and now, having passed the 

 barrier and reached the strong metallic solution, the deposite is at once at a maximum. 

 Hence we account for the circumstance that the reduction of the metal chiefly takes 

 place on the under side of the membrane. 



232. It may here be asked, How is the acid in the metallic solution conveyed from a 



