Adopting the second experiments as the most trustworthy, we have for 1 per cent. of chloride of mercury an osmose of 121 ms., and for the same, associated with half its weight of chloride of sodium, 60 ms.

The osmose of chloride of mercury in albumen was very trifling, being only 5 and 9 ms.; chloride of mercury diffused in sensible quantity, however, through both the albumen and membrane.

Protonitrate of mercury gave, in double membrane, an osmose of 232, 346, and 350 ms.; in albumen much less, namely 47, 63, and 61 ms. Pernitrate of mercury gave, in double membrane, 425 and 476 ms. for the 1 per cent. solution, and 296 ms. for the one-tenth per cent. solution, results which indicate osmotic power of the highest intensity.

The membrane preserved a considerable action after the last experiments, although macerated in water for a night, and imparted thereafter to a salt nearly neutral to osmose (nitrate of silver), a rise of 222 and 166 ms.

In albumen, pernitrate of mercury again was low, giving 32 and 54 ms. for 1 per cent. of the salt, and 34 and 46 ms. for the onetenth per cent. solution.

Silver. It is interesting to observe how this metal separates itself from mercury and the magnesian elements, and takes its place with the alkaline metals in the property of osmose, as in other chemical characters. Nitrate of silver appeared to possess a moderate positive osmose, like a salt of potash or soda. For the sake of comparison, the silver salt was followed by nitrate of soda in the experiments below. TABLE XXIX.-Solutions in Osmometer G of double membrane for five hours.

The experiments of the table indicate an average osmose of 35 ms. for 1 per cent. of nitrate of silver, and of 4.5 ms. for the same proportion of nitrate of soda. A considerable diffusate of silver appeared in all the experiments with the salt of that metal.

Gold and Platinum.-The chlorides of these metals have already been shown to possess a decided negative osmose, and in that respect to rank with acids.

In concluding this paper I may place together a series of numerical results which exhibit the osmose of substances of all classes. Some of these numbers have not been previously reported.

Osmose in membrane of 1 per cent. solutions expressed in millimeter

It will be observed that acid and alkaline salts are found at opposite ends of the series, or, while the acids possess negative osmose, the alkaline salts exhibit positive osmose in the highest degree. The remark will suggest itself, that in osmose water always appears to pass to the alkaline side of the membrane; as water also follows hydrogen and the alkali in the electrical endosmose.

The chemical action must be different on the substance of the membrane at its inner and outer surfaces to induce osmose; and according to the hypothetical view, which' accords best with the phenomena, the action on the two sides is not unequal in degree only, but also different in kind. It appears as an alkaline action on the albuminous substance of the membrane at the inner surface, and as an acid action on the same substance at the outer surface. The most general empirical conclusion that can be drawn is, that the water always accumulates on the alkaline or basic side of the membrane.

The analogy does not fail even when the osmometer is charged with an acid solution and the osmose is negative. The stream is then outwards to the water, which is a basic body compared with the acid within the membrane.

The high positive osmose of the salts of the alumina type is exceedingly remarkable. The property is common to salts of alumina, sesquioxide of iron, sesquioxide of chromium, and the corresponding oxide of uranium. Now the property in these salts is small where the salt is stable, as in the sulphates, but becomes great where the affinity between the acid and base is comparatively weak, as in the chlorides, nitrates, and acetates of these bases, salts which can be shown to be largely decomposed in the experiment by the action of diffusion. Here then, as with the preceding class of osmotic bodies, the osmose of the water is towards the basic side of the membrane.

But the most curious circumstance, with reference to this empirical

generalisation, is observed in the magnesian class of salts. The barytic subdivision of this class, including all the soluble salts of baryta, strontia, and lime, appear to be entirely unosmotic, or they oscillate between a small positive and small negative osmose. Such salts are neutral in their reaction, and further, have no disposition whatever to form subsalts. The salts of the earth, magnesia itself, offer the same characters. But in the salts of certain other oxides of the magnesian group an intensely osmotic character is developed, particularly in the salts of copper, protoxide of lead and protoxide of tin, with the exception of the soluble sulphates of these bases. Now those named are the members of the magnesian class most apt to break up into free acid and a basic salt. Like the aluminous salts, therefore, they are capable of investing the inner surface of the membrane with basicity, the necessary condition of positive osmose. Nitrate of uranium does not require to form a subsalt, as it is already constitutionally basic. The osmotic peculiarity of metaphosphoric acid, formerly referred to, also harmonises with the same view.

Neutral monobasic salts of the alkaline metals, such as the chlorides of potassium and sodium, and the nitrates of potash, soda, and silver, which possess a strict and unalterable neutrality, appear to have little or no true osmotic action. The salts named, together with the neutral magnesian sulphates and certain neutral organic substances, such as alcohol and sugar, give occasion, it is true, to an increase in the fluid of the osmometer, but only to the moderate extent which the exchange of diffusion-volumes might be supposed to produce. The comparative diffusibility of all these substances is well known, with the exception, unfortunately, of that of water itself, which I could only deduce by an indirect method in my previous inquiries respecting liquid diffusion. As salts generally appeared to diffuse in water four times more rapidly than they did in alcohol, the diffusibility of water was then assumed as probably four times greater than that of alcohol, and consequently five or six times greater than that of sugar or sulphate of magnesia. Diffusion is thus made to account for the substances last -named being replaced in the osmometer by five or six times their weight of water. This "diffusion-osmose" appears to follow in its amount the proportion of salt in solution, with a certain degree of regularity. The "chemical osmose" of substances, on the other hand, is found of high intensity with small quantities of the substance, such as 1 per cent. or even 0.1 per cent., and to augment very slowly with increased proportions of the substance in solution.

A small proportion of common salt accompanying carbonate of potash has been seen to possess a singular influence in diminishing the positive osmose of the last-named alkaline salt; while a mixture of small proportions of common salt and hydrochloric acid exhibits, with the membrane in certain conditions, an intense positive osmose which neither of these substances possesses individually.

The bibasic salts of potash, again, such as the sulphate and oxalate,

although strictly neutral in reaction, begin to exhibit a positive osmotic power, in consequence, it may be supposed, of their resolvability into an acid salt and free alkaline base.

The sulphate of potash, when strictly neutral, has in different membranes a variable but always moderate positive osmose, an osmose which the slightest trace of a strong acid may cause to disappear entirely, or even convert into a small negative osmose.

On the other hand, a minute addition of an alkaline carbonate to the sulphate of potash appears to give that salt a positive osmose of a high order. It was seen that the mixed salts produce much more osmose than the sum of the osmose of the two salts used apart from each other.

It may appear to some that the chemical character which has been assigned to osmose takes away from the physiological interest of the subject, in so far as the decomposition of the membrane may appear to be incompatible with vital conditions, and osmotic movement confined therefore to dead matter. But such apprehensions are, it is believed, groundless, or at all events premature. All parts of living structures are allowed to be in a state of incessant change,-of decomposition and renewal. The decomposition occurring in a living membrane, while effecting osmotic propulsion, may possibly therefore be of a reparable kind. In other respects chemical osmose appears to be an agency particularly well adapted to take part in the animal œconomy. It is seen that osmose is peculiarly excited by dilute saline solutions, such as the animal juices really are, and that the alkaline or acid property which these fluids always possess is another most favourable condition for their action on membrane. The natural excitation of osmose in the substance of the membranes or cell-walls dividing such fluids seems therefore almost inevitable.

In osmose there is, further, a remarkably direct substitution of one of the great forces of nature by its equivalent in another force-the conversion, as it may be said, of chemical affinity into mechanical power. Now, what is more wanted in the theory of animal functions than a mechanism for obtaining motive power from chemical decomposition as it occurs in the tissues? In minute microscopic cells the osmotic movements should attain the highest velocity, being entirely dependent upon extent of surface. May it not be hoped, therefore, to find in the osmotic injection of fluids the deficient link which intervenes between chemical decomposition and muscular contraction? The intervention of the osmotic force is also to be looked for in the ascent of the sap of plants. The osmometer of albuminated calico appears to typify the vegetable cell; the ligneous matter of the latter being the support of a film or septum of albuminous matter, in which the active properties of the cell reside. With a vegetable salt, like oxalate of potash above, and pure water below such a septum, an upward movement of the lower fluid would necessarily ensue.

VII.-Chemical Composition of the Waters of the Metropolis during the Autumn and Winter of 1854.

By ROBERT DUNDAS THOMSON, M.D., F.R.S. L. & E.

THE important bearing of the composition of the waters supplied to the inhabitants of the metropolis on the public health, may serve as a sufficient apology for the addition of another to the many previous investigations which have been instituted on this subject. The valuable report communicated to this Society in 1851, details researches which were made under particular circumstances during the winter season, and at a time when the diluting influence of recent rain had tended to modify the constitution of the metropolitan waters. In the present paper it is intended to comprise a summary of an extensive series of analyses which were made from the 1st of September, 1854, to the beginning of the present year. From these results it is apparent that the waters supplied to London possess in many respects a fluctuating character, which nray tend in some measure to explain the antagonistic views that have at different times been entertained with regard to their adaptation to domestic purposes.

In order to determine the composition of the waters as supplied to the inhabitants, specimens of water were taken from houses generally in which the epidemic had proved fatal, and these were compared,

VOL. VIII.-NO. XXX.

H