gg DEPOSITES PRODUCED BY EXDOSMOTIC CURRENTS. 



212. There is no absolute diameter at which a pore will cease to permit a hydraulic cur- 

 rent to pass it, and the phenomena of endosmosis to commence. Size is but one of the 

 elements involved in producing this action, and deviation in respect of it may be often 

 compensated by variations in the other conditions ; its relation to pressure is not unimpor- 

 tant. Water has been forced through the interstices of gold, and melted tin through 

 the pores of solid copper. 



213. We have next to consider the different cases of decomposition, ostensibly 

 brought about by endosmosis. 



214. If litmus water be placed on one side of a piece of bladder, and alcohol on the 

 other, the water will forsake the colouring matter to pass through the bladder and unite 

 with the alcohol ! This experiment, which was originally cited by me as explanatory 

 of the fact, that colouring matter in the intestines could not give its peculiar tint to the 

 chyle, does away with one of the most important objections to the direct absorption 

 of medicaments by the lacteal system. In estimating its true value among the facts 

 now under consideration, we shall find that it is very far from supporting the hypothesis 

 that chemical decompositions can be brought about by endosmosis. There is no proof 

 that the colouring matter, though permanently suspended in the water, is chemically 

 united with it ; analogies would lead us to the very opposite opinion. All that can 

 be predicated of this experiment is, that it exhibits a refined kind of filtration, which, prob- 

 ably, may hereafter become of considerable importance in its applications in the arts ; as 

 in the separation of colouring matter from solutions, or the preparation of medicines, such 

 as the vegetable alkalies, which should be formed from colourless solutions. It is prob- 

 able that the non-solubility of litmus in alcohol is not without its influence in this matter. 



215. The results referred to in (186), (187), (188), may all be classed together. They 

 have been taken as proving that currents may set in determinate directions through a 

 membrane ; thus, it has been inferred from the experiment (187), that when a solution of 

 oxalic acid is on one side of a membrane, and lime-water on the other, the acid passes free- 

 ly through the pores, but a passage to the lime-water is denied. It has been thought that 

 an action of this kind was the result of organization, an important property possessed 

 by membranes only; hence it has been inferred that tissues aliened of the transit of 

 bodies in certain directions through them. 



216. A more careful investigation of the circumstances deprives this phenomenon 

 of all its mysterious importance. It is by no means confined to tissues or organized 

 matter (197). If we take a cupping-glass, the edge of which is truly ground, and, 

 having filled it with lime-water, place it upon a piece of clean plate glass, then, on 

 pouring a solution of oxalic acid on the plate, so that it may encircle the edge of the 

 cupping-glass, it will be perceived that, while the acid solution on the outside remains 

 clear and colourless, innumerable streams of oxalate of lime will pass from the bottom 

 of the glass, and rising in white clouds, render the solution turbid. Here, surely, we 

 cannot ascribe any organic function to the chink between the two pieces of glass; yet 

 the current apparently sets only in one way, and exhibits, to all intents and purposes, a 

 phenomenon identically the same with that referred to in (187). 



217. The instances here referred to are those which have hitherto attracted attention. 



