many discoveries of new substances, and their novel applications in the arts, but also from the fact, that we are on the threshold, or rather, I should say, we have but lately passed the threshold of a change in the symbolic representation of chemical substances, and the groups they form, which must lead to much change in our chemical treatises, our chemical teaching, and our chemical examinations.

The new views have an important bearing on the advance of the science, and they already occupy a conspicuous part in the representation of the constitution of compound substances. I will take occasion, at the close of this discourse, to refer shortly to the more important doctrines recently being worked out, but which are apparently still merely the germ of what will follow, so that, as the change proceeds, and the new views germinate and are developed, we may be prepared in some measure for them.

During the last few years, two new modes of analysis or methods for the investigation of substances have been discovered, and have been applied to the examination of materials. One of these processes is the separation of chemical substances from each other, by allowing solutions containing such to pass through a diaphragm or septum, and has been styled Dialysis; whilst the other method of research depends upon the production of certain lines or bands by the various elementary bodies, when these or their compounds are burned in a flame, and the light is then decomposed by a prism, so that the spectrum is obtained. The latter mode of research is termed Spectrum Analysis.

The various researches in Diffusion Analysis have been worked out by Professor Graham, Master of the Mint; and the well-known investigations undertaken by that distinguished chemist in regard to the diffusion of gases, supplied us years ago with the laws regulating the passage of the various gases through each other, either in a common atmosphere or through porous plates. The later researches on diffusion have reference to liquids and the substances dissolved therein. Certain liquids have no power of diffusing through each other, such as mercury, water, and oil, which, when mingled together, arrange themselves according to their densities; and thus

the mercury remains at the lower part of the vessel, the water in the middle, and the oil floats above. Agitation of the liquids may cause a momentary intermingling of the substances, but subsequent quiescence will enable the mercury, water, and oil to separate again from each other, and arrange themselves as before. When water and alcohol are taken, however, and cautiously introduced into a vessel, so that the water may remain at the lower part and the alcohol is above, it is found that the two layers of liquid gradually intermingle; and in the course of time the water will rise in part through the alcohol, whilst the latter descends in part into the water layer; and at length the two substances, which were at first separate, will have become so thoroughly intermingled that there will be as much alcohol below as above, and the liquid will form a uniform mixture.

The diffusion of liquids may be observed by various arrangements, but the simplest apparatus is a tall glass jar, which is nearly filled with water, and to the lower part of which a solution of the substance to be experimented upon is carefully introduced by a long pipette. The saline matter in the lower stratum begins to rise slowly through the upper water, until in some instances it can be recognised at the summit of the column of water; whilst in other cases the process of diffusion proceeds so slowly that the material scarcely finds its way half up the water column, in the time which has enabled other materials to reach the top layer of water. Professor Graham has made an elaborate series of observations on the whole subject; and arranging that the conditions of the experiments were similar, and the temperature as nearly as possible uniform during the continuance of the trials, he has determined the approximate time required for the same degree of diffusion of many substances through water. Hydrochloric acid was found to be one of the most rapid in its rate of diffusion, succeeded by saline substances; and the slowest in diffusing themselves throughout the water were such bodies as albumen and caramel. Thus, whilst hydrochloric acid diffuses itself throughout the upper column of water in a time represented by 100, chloride of sodium will take 2:33, sugar 7.00, sulphate of magnesium

7.00, albumen 49.00, and caramel 98-00, or nearly 100 times the period required for the diffusion to the same rate of hydrochloric acid. The difference in the rapidity of diffusion has suggested the possibility of separating, to some extent, substances from each other, as in a mixture of chloride of sodium and sulphate of magnesium, where the former would tend to diffuse through upper layers of water at a rate three times quicker than the latter. Professor Graham has suggested that the substances which have a quick rate of diffusion should be called volatile or crystalloids, the latter term indicating that crystalline bodies have a ready diffusibility; whilst the substances which are tardy in their rate of diffusion should be styled fixed or colloids, the latter term being used owing to gelatine (collin) being representative of the class. A modification of the experiments in jar diffusion may be carried out, by placing the substance to be experimented upon in a phial, and introducing such into the lower part of a tall jar containing water, when a similar series of results will be obtained to those already referred to.

The special department of diffusion which is properly termed dialysis, has reference to the separation of substances dissolved in liquids by means of allowing them to pass through a diaphragm or septum. Various membranes will serve for the diaphragm, such as a bladder or part thereof, but the best material for the purpose is parchment paper. This material was first prepared by M. Gaine, and afterwards by De la Rue, and it is most easily formed by taking a mixture of one part by volume of water, and adding thereto exactly two volumes of oil of vitriol. When this acid mixture is allowed to cool, and ordinary bibulous or unsized paper is soaked therein, the paper undergoes a molecular change, becoming transparent to a certain degree, and when washed in dilute ammonia to neutralise the acid, and thereafter in a stream of pure water, and subsequently dried, the bibulous paper has been manufactured into parchment paper. This peculiar form of paper is tough even when wet, and very much resembles in general appearance a piece of ordinary parchment. The proportions of acid and water employed in its preparation require to be very exact, as a larger

quantity of either acid or water will destroy the paper and convert it into a pulpy substance. Instead of using oil of vitriol and water in the preparation of the parchment paper, Mr T. Taylor has suggested the use of a strong solution of chloride of zinc, which acts in the same way. In employing the parchment paper as a dialyser, the paper is placed between two hoops of gutta-percha or other material not readily acted upon, in the same manner that an ordinary sieve is constructed, or the parchment paper is firmly tied round the open mouth of a small bell jar shaped like an inverted cup, and having an opening at the upper part through which the liquid may be introduced into the bell jar or bulb.

The substance to be dialysed is placed in the upper part of the diaphragm of parchment paper, and the apparatus called the dialyser is then floated on the surface of water contained in a vessel placed below. Certain substances, especially those of a crystalline nature, such as metallic salts, arsenious acid, sugar, strychnine, morphia, and quinine, pass through the diaphragm of parchment paper into the water underneath with considerable rapidity, and the bodies of this class are styled crystalloids. Other materials, however, such as gelatine, albumen, pectin, animal mucus, vegetable gelose, and caramel, pass through the dialysing membrane very tardily, and are named colloids. As a class, the crystalloids are more or less sapid to taste, whilst the colloids are more or less insipid. When a mixture of a crystalloid, such as arsenious acid, and a colloid, such as gelatinous matter, is taken and placed on the dialyser, the crystalloid or arsenious acid passes comparatively rapidly through the diaphragm, whilst the colloid or gelatine is left in greater part on the upper side of the septum, and thus the process can be successfully followed in the separation of substances from each other. Thus, arsenious acid may be readily dialysed from the albuminous and other organic ingredients present in the stomach of an animal which has been killed with that poison; and in employing this mode of analysis in the separation of poisons from organic mixtures, it is interesting to observe that all the organic and inorganic poisons are crystalloids, and hence tend to pass

readily through the diaphragm. Professor Graham has taken a mixture of a solution of arsenious acid and albumen, and having coagulated the albumen by heat, he has dialysed the arsenious acid out of the mixture to the extent of fourfifths of the entire amount of the poison. In a mixture of arsenious acid and porter, which was thrown on the dialyser, the poison passed through the membrane, accompanied by a faint yellow colour, which stained the water, and at this stage one-half of the arsenious acid had been dialysed from the porter mixture. This common poison has likewise been separated by this process from blood, the contents of the intestines, &c. My late assistant, Dr Alexander T. Machattie, a Fellow of this Society, has suggested that, in cases of poisoning, the stomach or the intestines, or both, might be simply secured at the openings, and the organs, with their contents, be placed in a vessel of water, when the crystalline poison would dialyse through the membrane of the coats of the stomach or intestines, and pass into the water, which might then be examined. This operation might be carried out as a preliminary operation in cases of poisoning; and it would have this manifold advantage, that an indication of the poisonous ingredient might be obtained without actually disturbing the contents of the stomach or intestines, and without removing them from these organs. Mr Whitelaw of Glasgow has suggested that the large quantities of brine from salted meat, which are at present thrown away, might be utilised by being subjected to the process of dialysis. He proposes to place the brine from the salted meat in bladders provided with stopcocks, and to suspend these bladders in a tub or tank of water. The crystalloid chloride of sodium will quickly pass through the membrane of the bladder, and in a few days the contents of the bladder will be found comparatively free from salt, which will have passed into the water in the tub or tank, whilst the extract of meat, which possesses colloid properties, will have remained behind in the bladder, and may thereafter be utilised as extract of meat or beef-tea.

The importance of the process of dialysis in the physiological changes which occur in the animal economy can hardly