Acid solutions are the only fluids which have yet been found to occasion the endosmotic cur rent to flow towards water when separated from this fluid by an animal membrane. The whole of the acids, without exception, exhibit this phenomenon, which was long overlooked by me, from its having been confounded with another phenomenon, namely, the abolition of endosmosis. I have in fact shown, in a work already before the public,* that all fluids which act chemically on the membrane of the endos mometer, put an end, with greater or less cele rity, to the phenomenon of endosmosis,—it goes on for some time, but it never fails to cease at length. Sulphuric acid, above all the other acids, has the propeity of putting an end to endosmosis. This acid, poured into the en dosmometer, sinks by virtue of its simple gravity towards the lower water, filtering mechanically through the membrane placed between it and the water. If the position of the two fluids be reversed, the endosmometer being charged with water, and the sulphuric acid placed externally and on the lower level, the water still sinks to wards the acid, passing in its turn mecliani cally through the membranous septum of tho instrument, rendered incapable of effecting en dosmosis. From these experiments I was led at first to conclude that sulphuric acid was in active as regards endosmosis ; in other words, was incapable of exhibiting or producing this phenomenon. I have since found, however, that the sulphuric, like all the other acids, has the faculty of exerting endosmosis in the two opposite directions, but always during a very brief space of time only. Thus the tempera ture being + cent., sulphuric acid, of the density of 1.093, separated from water by a piece of bladder, the endosmotic current is directed from the water towards the acid, but the phenomenon lasts only for a short time ; the current soon ceases, and if the acid be on the higher level, it then begins to sink by sim ple mechanical filtration towards the water. At the same temperature of + 10° cent., the sulphuric acid attenuated to 1.054 being placed in the endosmometer, and the reservoir and a part of the tube being plunged in water, en dosmosis is established, but in this case the current is from the acid towards the water, so that the acid liquor sinks in the tube ; and that this sinking is due to endosmosis is demon strated by the fact of the acid continuing to sink in the tube of the endosmometer a consi derable way below the level of the external water, and not stopping short when the level is obtained, as it does when the descent is owing to simple mechanical filtration. In this expe riment, as in the one detailed immediately be fore it, the endosmosis towards the water is abolished, and then the column in the endos mometer begins to rise again slowly, until the level of the external and included fluids corre spond. We, therefore, see that at a tempera ture of 10° cent., sulphuric acid, of the density of 1.003, presents the current of endos mosis from the water towards the acid ; whilst the density being 1.054, the endosmosis is from the acid towards the water. Between these two opposite endosmotie currents there necessarily exists a mean when no phenomena of the kind occur. This mean, the tempera ture continuing 10°, I find to belong to sul phuric acid of the density of 1.07. The two fluids, divided by the animal membrane of the endosmotneter, penetrate one another athwart the septum reciprocally and in equal measure, so that the contents of the endosmometer re main for a certain time at the same height in the tube of the instrument ; subsequently the contained fluid begins to sink in consequence of the cessation of all endosmosis. These ex periments were necessarily undertaken when the temperature was moderate or low ; the phenomena detailed would not else have been appreciable ; for in a warm atmosphere the abolition of endosmosis by sulphuric acid is accomplished so rapidly, that it is with diffi culty the slight current established in the first instance can be observed.
Sulphurous acid, of the density 1.02, sepa rated from water by an animal membrane, only exhibits endosmosis towards the water ; this cndostnosis is pretty active at first ; but after the lapse of a brief interval the current ceases, just as it does with the sulphuric acid. These results I came to after a number of experi ments, the temperature being at one time + 5°, and at another ± 25° cent.
Formerly I regarded the hydrosulphurie acid as inactive in regard to endosmosis ; I assimi lated it, in this respect, with the sulphuric acid. The fact, however, is that, like the sulphuric acid, it. has the property of producing endos mosis. The acid I employed was of the den sity of 1.00628. With a piece of bladder be tween this acid and water, the endosmosis was constantly towards the water. This conclusion
was not influenced by variations of tempera ture between 4- 4° and + 25° cent. The ac tion was somewhat protracted, but the endos mosis never failed to cease after a certain time, as in the case of the sulphuric acid.
The nitric acid of considerable density exhi bits endosmosis towards the acid when sepa rated from water by a piece of animal mem brane. Thus, at a temperature of + cent. this acid (density 1.12 or higher) presents the current flowing towards the acid. Under the same circumstances, but of the density of 1.08, the endosmosis is towards the water. Of the density 1.09, the mean term between the two opposite endosmoses is obtained. At higher temperatures the nitric acid very speedily puts an end to the phenomena of endosmosis, espe cially when its density is not very high, so that it becomes difficult to perceive the very tran sient currents produced in the first instance.
The hydrochloric is the most potent of all the mineral acids in directing the current of endosmosis from the water towards the acid. Its density must be considerably reduced before it offers the direction of the current changed, or from the acid towards the water. At a tempe rature of + 22° cent, for instance, the hydro chloric acid has to be brought, by the addition of water, to a density no higher than 1.003, before it presents the endosmosis flowing to wards the water, from which, as understood, it is divided by a layer of animal membrane. Of greater density the endosmosis is towards the acid. When the temperature is lower than + 22°, the same acid, of greater density, ac quires the property of causing endosmosis to wards the water. Thus, with the centigrade thermometer at 10°, I found that hydro chloric acid of 1.017 density presented the mean term between the two opposite endosino ses. At the same temperature hydrochloric acid, of 1.02 density, presented endosmosis to wards the acid, and of 1.015 density, endos mosis towards the water. Under a higher temperature, however, and of the latter density (1.015), the endosmosis was towards the water, so that a depression of 12° cent. in tetn perature causes the mean term of the density of hydrochloric acid, which separates the two op posed endosmoses, to rise from that of about 1.003 to that of 1.027; that is to say, the quantity of acid added to the water must be increased almost six-fold to produce the same effects.
In the present state of our knowledge, we find it quite impossible to give any explanation of the remarkable phenomenon exhibited in the changes of direction of the endosmotie currents according to the degree of density of the acid and the temperature. The singularity of this phenomenon will appear the greater when the following observation is taken into the account. Hitherto it was always by a layer of animal membrane that I separated the acid from the water. Instead of the animal membrane I now tried the effect of one of vegetable origin. We have seen above that oxalic acid, whatever its density and under whatever temperature, when separated from water by an animal mem brane, always exhibited endosmosis from the acid towards the water. I filled a pod of the colutea arborcscens, which being opened at one end only and forming a little bag, was readily attached by means of a ligature to a glass tube, with a solution of oxalic acid, and having plunged it into rain-water, endosmosis was ma nifested by the ascent of the contained acid fluid in the tube ; that is to say, the current flowed from the water towards the acid. The lower part of the leek (allium porruni) is en veloped or sheathed by the tubular petioles of the leaves. By slitting these cylindrical tubes down one side, vegetable membranous webs, of sufficient breadth and strength to be tied upon the reservoir of an endosmometer, are readily obtained. An endosmometer, fitted with one of these vegetable membranes, having been filled with a solution of oxalic acid, and then plunged into rain-water, the included fluid rose gradually in the tube of the endosmometer, so t hat the endosmosis was from the water towards the acid, the reverse of that which takes place when the endosmometer is furnished with an animal membrane. The tartaric and citric acids of densities below 1.05, and at a tempe rature of + 25° cent, exhibit eudosmosis to wards the water with an animal membrane ; but with a vegetable membrane the case is altered ; the endosmosis being then directed from the water towards the acid. I have tried solutions of tartaric acid, decreasing gradually in density from 1.05 (11 tartaric acid to 100 solution) to a density so low as 1.0004, (1 tar taric acid, 1000 solution,) and always seen the endosmosis towards the acid. A gradual fall in the temperature from +25° to near zero did not affect the result.