become the plastic elements. Although the two classes are widely separated in their properties, there is a certain degree of parallelism between them. The colloids, though chemically inert in the ordinary sense, possess a special activity of their own dependent on their physical properties. The rigidity of the crystalloids shuts out external impressions; the colloid, from its gelatinous softness, becomes the medium for liquid diffusion like water itself, and appears to possess the same penetrability for cementation in those colloids which exist at a high temperature—hence its wide applicability to practical purposes. The separation of the crystalloid from the colloid forms is readily effected by any septum composed of an insoluble colloidal material, but the best is the parchment paper manufactured by the Messrs De la Rue. This process consists in dipping the paper in sulphuric acid, by which it acquires the appearance of an animal membrane, and for experiments on osmotic force seems to act better than any other tissue. For experiments on osmotic force, or, as Professor Graham styles it, dialysis, all that is necessary is a hoop of gutta percha of 8 or 10 inches diameter, covered on one side like a sieve by De la Rue's parchment paper. If now a mixed solution of gum and sugar is placed in this simple apparatus, and the whole floated on a large basin of distilled water, in twentyfour hours three-fourths of the sugar is diffused through the water. Dr Graham explains the action on the true colloidal nature of the septum, and not on capillarity, the crystalloid sugar taking the water from the colloidal septum, and thus obtains a medium for diffusion, which the gum from its feeble power to separate the contained water therefore cannot escape with the same facility as the sugar. By these researches Professor Graham has proved what to me is most interesting, as it is another proof of the truth of my views on the aqueous origin of granite, and also most practical in its bearings on the useful arts, that silica can be held in solution in pure water alone to the extent of fifteen per cent. I hope that during this session we may add much to our knowledge of mortars and preservative processes by means of this new power which Professor Graham has placed in our hands, and to which he has given the name of dialysis. There is one subject to which I beg to call your attention, VOL. VI. Y of great importance and increasing interest,-namely, tramways. On my return from the British Association at Manchester, I made it my business to inquire into the merits of the two lines laid down, the one at Birkenhead, and the other from Water Street to the fort on the north side of the Mersey, at Liverpool. In these days, when a charge is made of 10s. 6d. between the two capitals of Scotland, a distance of only forty-four miles, it becomes us as a Society of Arts to inquire if no cheaper means of conveyance can be provided for our population, and it appears to me that the system of tramways will shortly be adopted on such roads as between Edinburgh and Glasgow. According to the system patented by Mr Haworth, neither cross-sleepers, chairs, nor tye-rods are required, nor, in fact, any other of the present railway appliances. The prominent feature of Mr Haworth's plan is that of the introduction of a middle rail, which, being grooved for the reception of a revolving disc attached to the carriage, forms a guide, and thus dispenses with the flanging of wheels and the raising of rails. By this method we avoid all danger on our streets or roads by the rails being level with the roadway; and further, the vehicles to which this guide-wheel is attached can at once leave the tramway by lifting the unicorn or guidingwheel. I hope that the directors of our railways will beware of the excessive charges put on the transit of goods, and if not, 1 warn them that ere long they will be called on to compete with tramways, which, formed at little expense, and worked economically, will shortly realise a profit seldom recorded in railway balance-sheets. I cannot conclude these few remarks without referring for a moment to that sad internecine war now waging in America, and which to us, dependent as we have been for so long on that country for the supply of cotton, is most momentous. But I am glad to say that Lancashire was never better supplied with its staple raw material than it is now; and let us trust that the men of Manchester-and they are, indeed, men of might-will no longer trust in a single country for the supply of an article the value of which equals the revenue of the three kingdoms,-namely, L.70,000,000 per annum. And, further, let us hope that that Providence which overruleth all things may make the wrath of man to praise God. 285 On the Water Supply of Edinburgh. By Mr ALEXANDER RAMSAY, Manager, Edinburgh Water Company.* THE history of the Water Company, and its sources of supply, have been made so familiar to the public of Edinburgh by the public press and otherwise, that any detailed notice of the one or the other to the members of this Society may seem somewhat a work of supererogation. In order, however, that what I am going to say may be more intelligible, I shall briefly, with the aid of the map now before you, point out the various sources from which the water supplied to Edinburgh and its vicinity is derived, and, at the same time bring under your notice any circumstances connected with its introduction and use, that may seem to possess sufficient interest to justify me in occupying your attention. Up to the year 1681, when the water of Comiston was introduced, the inhabitants were wholly dependent for the supply of water on public and private pump-wells. Such of these wells as were destined to public use, were few in number, and all of them subject to the inherent objections that the water was hard, and in dry seasons liable altogether to fail. I refer to a period of time before the introduction of water from the country by the Town Council; and although that period is far beyond any living memory, the distress which must then have been suffered was not inadequately represented by what the community endured in more recent times, and up to the year 1822, when the Crawley springs were brought in. The springs rising in the estate of Comiston, had been brought in by the Town Council in 1681, and in 1762 certain springs originating on the north slope of the Pentlands, on VOL. VI. * Read before the Society 23d November 1863. 2 P |