APPARENT DECOMPOSITION OF METALLIC SALTS BY MEMBRANES. gg 



itself a compound. The same observation applies, to a certain extent, to the result 

 (189), which is an effect analogous to that produced by a few fibres of greasy cotton 

 when dipped into a mixture of oil and water, as in a common lamp : effects which are 

 totally distinct from chemical decomposition. 



J-24. The experiment referred to in (190) is apparently the most important of these 

 cases of tissue action. ' A tube of suitable dimensions, having one of its extremities 

 closed with a piece of bladder, a b.Jig. 25, and filled to a certain height, e e, with pure 

 water, into which a few iron nails were dropped, was immersed in a solution of sulphate 

 of copper. In the course of a few days a deposite of metallic copper was found on the 

 surface of the membrane towards c, but none on the inner side, or upon the iron nails. 

 A number of other metallic salts, such as acetate of lead, nitrate of silver, &c., afford 

 similar results." 



225. Certainly this is a remarkable phenomenon, if the conditions under which it 

 occurs are accurately detailed. The operation and laws of chemical affinity would 

 lead us to ascribe the decomposition of sulphate of copper to the action of the iron ; 

 but then it would appear that those same laws require the metallic precipitate to be 

 deposited on the disturbing metal ; here, however, it is found that the deposite really 

 occurs on the membranous tissue. 



226. On placing on one side of a membrane pure water, and on the other sulphate 

 of copper, or anv other salt reputed to be capable of decomposition under the circum- 

 stances given, no chemical action whatever occurred, in many days, at all analogous to 

 the phenomena ; but the water passed out of the tube into the copper solution with 

 considerable rapidity. 



227. On repeating the experiment, as given in (224), in the course of six days the 

 results were as follows : A great disturbance of hydrostatic level had occurred, there 

 being an accumulation in the tube containing the iron nails ; the under side of the blad- 

 der was coated to a considerable thickness with bright metallic copper, and a small 

 portion, in a pulverulent form, was found on the inner edge, and here and there patches 

 were discovered coating the surface of the iron nails, and minute veins of copper pass- 

 ing through the bladder. In many repetitions of this, the.same results were uniformly 

 observed. 



228. These numerous experiments showed that in all cases there was a perfect 

 metallic communication from the iron nails, by means of veins of copper, through 

 the bladder, with the cupreous mass on the under side. A piece of zinc, z, was there- 

 fore suspended in the tube, a a, fig. 26, in pure water, at a distance of three quarters 

 of an inch above the bladder, and the arrangement exposed to a solution of acetate of 

 lead. For several hours no chemical action whatever occurred, the water exosinosing 

 towards the metallic solution, and the level in the tube, of course, falling. At the 

 same time, a small portion of solution of acetate of lead passed into the pure water, 

 as was shown, after the lapse of twenty hours, by the formation of some thin filaments 

 of lead on the lower edge of the zinc ; these kept increasing in length, and finally reach- 

 ed the bladder ; soon after, they appeared to have made their way through the pores 

 of that structure, and then the usual deposite occurred on the under side of the mein- 



