TRUE THEORY OF IT. 7] 



distance to the decomposing metal, so as to unite with it ? It was to encounter this 

 query, and to answer it, that I have been induced to examine so much in detail the 

 whole of this process. There is no physical difference between the deposites here 

 spoken of, and those known formerly to chemists under the name of Arbor Saturni and 

 Arbor Dianae ; for the bladder or other membrane being freely penetrated, impresses 

 no sort of action or change on the process in any wise. The same difficulty here met 

 with occurs in all such cases of metallic precipitations. For example, in making the 

 lead tree, by the action of a piece of zinc on some saturnine solution, the particles 

 of lead are often evolved at a distance of six or eight inches from the decomposing 

 metal. How, it may be asked, is the acid transported in this case? More than a cen- 

 tury ago, LA CONDAMINE endeavoured to account for this, but the state of chemical 

 knowledge at that time did not enable him to assign the true reason, and he was forced 

 to infer that currents actually traversed the solution, making their way to the decom- 

 posing metal. All decompositions brought about by the agency of the voltaic battery 

 are cases in point. The elements of water are separately evolved when the electrodes 

 are many inches apart, and the decomposition of the very salts here under considera- 

 tion occurs in the same way. It is now generally admitted that these decompositions 

 do not occur to solitary particles at a time ; but a chain of them, extending from one 

 electrode to the other, undergoes simultaneous decompositions and recompositions, a 

 process which eventuates in the elimination of the separate ingredients at opposite ex- 

 tremities of the chain. This explanation applies to the case here considered. 



233. In support of the explanation here given, the phenomenon of change of hydro- 

 static level affords a very powerful evidence. It was stated (228) that, previous to the 

 establishment of a voltaic current from one side of the bladder to the other, the hydrau- 

 lic current set from the water towards the solution, and, therefore, the level in the tube 

 kept falling ; but so soon as the voltaic current passed through the barrier, the hydrau- 

 lic current changed, and the level in the tube rose rapidly. Similar changes of level 

 have been noticed, when voltaic currents pass through membranes, by Mr. Porrett and 

 other chemists ; they appear to be certain indexes of that molecular interchange (not 

 current) which eventuates in polar decomposition. 



234. I therefore reject entirely this experiment, as having anything whatever to do 

 with the phenomena of tissue action, or as affording an example of the powerful de- 

 composing agency of membranes. It has been considered in detail, inasmuch as it is 

 as necessary, in studying the elements of a science which, like physiology, is yet in its 

 infancy, to discard whatever is irrelevant or erroneous, as to point out what is pertinent 

 and true. 



235. After a very full examination of the question before us, it is not to be concealed 

 that the decision to which we are constrained to come is entirely unfavourable to the 

 opinion commonly held as respects endosmosis. To rank it as a peculiar power is un- 

 questionably erroneous ; to regard it as a manifestation of organization, or the attribute 

 of organized structures, is equally so. Ever since the leading phenomena were made 

 known by Porrett, Fischer, Gustave Magnus, and Dutrochet, they have been extensively 

 applied in the elucidation of kindred actions in the vital frame ; the causes of a variety 



