But let us now inquire in what way hetero geneousness of quality in two fluids, separated by a membranous partition, occasions the phe nomenon of endosmosis. Upon this point opinions arc greatly divided. M. Poisson and Mr. Power have each, in his own way, given an analytical explanation of die phenomenon, and ascribed it to the action of the capillary canals of the porous septum interposed be tween the two fluids. In this explanation the phenomenon of the current of exosmosis is set aside, or regarded as occurring merely acciden tally. Now this is entirely opposed to the fact, —we have constantly evidence of the simulta neous existence of the two opposite and une qual currents of endosmosis and exosmosis.
Endosmosis by others has been held to be simply the eflect of the viscidity of one of the fluids divided by a porous septum. This visci dity prevents the upper fluid from permeating the interposed septum, whilst the inferior fluid, little or not at all viscid, filters readily through the septum and mingles with the upper fluid, whose volume it consequently increases. This opinion, published by a matt of distinction, de serves to be seriously investigates).
N't'hen an equal weight of gum arable and of sugar is dissolved in two equal weights of water, the viscidity of the different solutions is by no means the same, the solution of the gum is ob viously more viscid than that of the sugar. Now if these two solutions be divided by a piece of bladder, the current of endosmosis will be found to flow from the solution of the gum towards that of the sugar ; in other words, from the more viscid to the less viscid fluid ; in this instance, consequently, we see the more viscid fluid permeating the membrane with greater facility or in greater quantity than the less viscid fluid. More than this, the same phenomenon takes place if the quantity of the gum be made double that of the sugar. I have, for instance, tried a solution of two parts of gum arabic in thirty-two parts of water, (den sity 1.023,) and a solution of one part of sugar in the same quantity of the menstruum, (den sity 1.014,) divided by a piece of bladder, and found that the endosmotic current was still directed from the solution of the gum towards that of the sugar. These facts suffice to prove that the endosinotic current does not always flow from the less towards the more viscid fluid. It is not, therefore, the inequality of
vicosity in these two fluids which is, in this instance, the cause of their unequal permeation across the porous lamina which separates them.
In order to place these facts beyond a doubt, the comparative viscidity of the gum-water and the sugar-water whit!' were made use of in the experiments of which I have been speaking, required to be accurately measured. Such a comparative estimate of the viscidity of fluids may be obtained by observing the time which an equal quantity of each of them, at the same temperature. takes to run through a glass capil lary tube. In this way I tried, 1st, pure water; 2d, a solution of one part of sugar in thirty-two parts'of water ; 3d, a solution of one part of gum-arabic in thirty-two parts of water; 4th, and lastly, a solution of two parts of gum in thirty two of water. With a temperature of -1-70 cent. I found that fifteen centilitres of pure water passed through a capillary tube of glass in one hundred and fifty-seven seconds; that fifteen centilitres of the solution of one part of sugar in thirty-two of water passed through the same tube in one hundred and fifty-nine seconds and a half; that fifteen centilitres of the solution of one part of gum in thirty-two of water passed through in two hundred and sixty-two seconds and one-third ; and that the same quantity of the solution of two parts of gum in thirty-two of water required three hundred and twenty-six seconds to pass through.
From these experiments it apeems that the viscidity of the solution of sugar, in the propor tion of one to thirty-two of water, (density 1.014,) is very little above that of pure water ; that the viscidity of the solution of gum-arabic, in the proportion of one to thirty-two of water, is much greater than that of the sugared water just mentioned ; and finally, that the viscidity of the gum-water, containing two parts of gum to thirty-two of water, (density 1.023,) is twice as viscid as the solution of sugar employed.
It seems that nothing more is wanting to these proofs of the fact that endosmosis does not depend on the mere viscidity of fluids. Nevertheless I shall cite another proof of this truth. The very singular fact I am about to mention will also prove that the septa employed exert a special influence on the direction in which endosmosis takes place.