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DIALYSIS CRYSTALLOIDS, COLLOIDS.

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all the colloids, an action favoured by the thick coating of mucus which generally lines the stomach. This suggestion probably requires some limitation; otherwise starch, gelatin, and other colloids, unless previously converted into crystalloids, would be wholly unabsorbed after they have been swallowed. The process of dialysis, though most commonly exhibited in animal and vegetable textures, is not confined to them. For example, the cementation of iron, or the process of its slow carburation during its conversion into steel, is supposed to be due to colloid diffusion, the pasty condition to which iron is reducible at a certain elevation of temperature being referred by Graham to its assumption of the colloidal form.

Colloid bodies do not necessarily belong to the organic kingdom, though they are most frequently met with amongst its constituents; and owing to their tendency to undergo slow but perpetual molecular change, together with their peculiar relations to water, they seem to be especially suited to form the plastic materials required for building up the tissues of the living organism.

Indeed, the crystalloid appears to be the static, whilst the colloid is the dynamic condition of a body; and the usual tendency of the colloid is gradually to approach the crystalloid form.

The chemistry of a body in the colloid condition is very different from that of the same body in its crystalloid form. Hydrated or gelatinous silicic acid, soluble alumina, a particular soluble form of hydrated ferric oxide, and of chromic oxide, are instanced by Graham as belonging to the class of inorganic calloids. Each of them, in this state, possesses properties quite different from those which it exhibits in its ordinary or crystalline form. Some colloids are soluble in water, as gelatin and gum arabic; some are insoluble, like gum tragacanth: some form solid compounds with water, as for example, gelatin and tragacanth; whilst others, like tannic acid, do not. In colloids water of gelatinization appears to represent in some measure the water of crystallization in crystalloids. Colloids, though often largely soluble, are held in solution by a very feeble force. Fluid colloids appear always to have a gelatinous or pectous condition, and they easily pass from the liquid to the gelatinous state.

The combining proportion of colloids is generally high, although the ratio between the elements of the substance may be simple, and it seems not to be improbable that the grouping together of a number of crystalloid molecules may be one of the essential requisites for the development of the colloid condition.

(63) Flow of Liquids through Capillary Tubes.-An interesting and close connexion exists between the subjects which have just been considered and liquid transpiration, or the flow of liquids through capillary tubes. The most extensive and complete set of experiments hitherto made upon this branch of research is due to Poiseuille. (Ann. Chim. Phys. 1847 [3], xxi. 76.)

FIG. 37.

Fig. 37 will explain the method of conducting these experiments: A is a hollow conical metallic vessel which can be attached by a screw joint to a capacious receiver of condensed air, the exact pressure of which can be regulated by means of a gauge attached to it; B is a glass globe, of about half a cubic inch (8c..) in capacity, which contains the liquid under experiment; it is connected with the metallic vessel, A, by a glass tube of narrow bore. A similar tube proceeds from the lower part of the globe, and to this is attached the capillary tube, c, the diameter and length of which are carefully determined. The object of the little bulb, d, is merely to enable the observer accurately to define the termination of the capillary tube. G is a vessel which is filled with water, provided with an accurate thermometer, for observing and regulating the temperature. When an experiment is to be made, the end of the capillary, c, is introduced into the liquid, and the globe, B, is filled by attaching it to an exhausting syringe. When the liquid has risen a little above the line, e, the syringe is detached, and the apparatus connected with the vessel of condensed air. The pressure of this confined air continues without appreciable change during the experiment. By opening a stop-cock, the condensed air exerts its pressure upon the liquid, which is expelled through the capillary tube, c, and the column descends in the tube e f. By means of a stop-watch, the time at which it reaches the line, e, is exactly noted, and the time is again observed when the globe has become emptied, and the liquid has reached the lower line f. The object of the conical metallic vessel, A, is to act as a trap or lodging place for any particles of dust that might be suspended in the compressed air, and which, by obstructing the capillary tube, would mar the result.

From the inquiries of Poiseuille, it appears that when a tube exceeds a certain length (which is greater as the diameter increases), the following laws regulate the rate of efflux of the liquid-1. That the flow increases directly as the pressure; so that, with a double pressure, double the amount of liquid is discharged in equal times. 2. That with tubes of equal diameter, and under equal pressure, the quantities discharged in equal times are inversely as the length of the tube if from a tube I decimetre in length, 10 grammes escape in five minutes, from a similar tube, 2 decimetres long, only 5 grammes would flow out in the same time. 3. That in tubes of equal lengths, but of different diameters, the flow is as the fourth powers of the

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POISEUILLE'S EXPERIMENTS.

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diameters; for example, if tubes, one of o'4 millimetre, another of 0.2 millimetre in diameter, be compared together, the efflux from the larger tube would be 16 times as great as from the smaller, being in the proportion of 2*: 4*, or as 1: 16, although the diameter of the tube is only twice as great.

To the chemist, however, the most interesting part of these experiments is that which displays the effect produced by varying the kind of body which is allowed to flow through the capillary tube. The material of which the tube itself is made does not appear to influence the result; but the nature of the solution employed exercises the most marked effect. The liquids used were, in most cases, solutions in water of various bodies, especially of salts. In the majority of instances the flow of the solution was slower than that of distilled water. All the alkalies occasioned this retardation. In a few cases no sensible effect was produced. Thus, neither argentic nitrate, corrosive sublimate, sodic iodide, ferrous iodide, nitric, hydriodic, bromic, nor hydrobromic acid seemed to have any influence; whilst the hydrosulphuric and hydrocyanic acids, and a few of the salts of potassium and ammonia-viz., the nitrates and chlorides of potassium and ammonium, the iodide, bromide, and cyanide of potassium-increased the rapidity of the flow. but it is remarkable that concentrated solutions of potassic iodide above a temperature of 60° C., and of potassic nitrate above 40° C., actually flow more slowly than distilled water does. Strict attention to the temperature at which these comparisons are made is absolutely necessary, for both with water and with dilute solutions generally, a slight elevation of temperature produces a great increase in the rapidity of efflux. Water, for instance, at 45° C., escaped through the same tube with a rapidity of 2 times as great as it

did at 5° C.

Hitherto no connexion has been traced between the rate of efflux of the liquid and its density, capillarity, or fluidity. The capillarity of alcohol, as well as its density, increases as it is diluted with water, whilst its fluidity diminishes; but experiment has proved that a mixture of equal parts of spirit of wine and water flows out with considerably less than half the rapidity of pure alcohol, and with less than one-third of that of distilled water. The dilution of alcohol, therefore, to a certain point, retards its efflux, and beyond that point increases it: the minimum ate of efflux corresponds with that particular mixture of alcohol and water, which is attended with the maximum of contraction after admixture of the two liquids. The degree of solubility of a

body in water appears to exercise but a secondary influence on the phenomenon. Poiseuille shows it to be highly probable that the various solutions, when introduced into the blood of a living animal, provided that they do not cause the serum to coagulate, produce effects of acceleration or retardation on the capillary circulation, corresponding with those which are observed with the same liquids in capillary tubes of glass. He has proved this to be the case by direct experiment with potassic iodide when injected into the veins of the horse; and has shown that when various salts are mingled with serum, and the liquids are allowed to flow out through small tubes, retardation or acceleration occurs, as in the corresponding cases with their aqueous solutions.

Efflux of Liquids through Fine Tubes.

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The preceding table contains several of Poiseuille's results, numerically expressed. The solutions employed contained 1 per cent. of the various substances mentioned, except in the case of the last four liquids. They were exposed to a pressure equal to that of a column of water I metre (39:37 inches) in height, at the temperature of 11°2 C., unless otherwise noted; and escaped through a tube 64 millimetres in length, and o'24946mm. in diameter. The numbers in the table indicate the time occupied in seconds, for the efflux of equal bulks of the liquids used-viz., 6.6 cubic centimetres.

Arsenic acid

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Sulphuric acid

...

Pure serum, Ox

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586" 3 589.6 1048'5

Madeira wine

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Sparkling Sillery

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1134'I
1462'8

Jamaica rum...

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18319

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EFFLUX OF LIQUIDS THROUGH TUBES.

103 The observation of Poiseuille, that diluted alcohol has a point of maximum retardation coincident with the degree of dilution at which the greatest condensation of the mixed liquids occurs, or at a point in which I molecule of alcohol and 3 of water (CH ̧O, 3 H2O) are present in mixture, served as a starting-point to Graham for a new inquiry. (Phil. Trans. 1861, 373.) The rate of transpiration he has proved to be, in certain cases, connected with chemical composition. The 3-molecule hydrate of methylic alcohol, although not distinguished by any particular degree of condensation in volume, exhibits a peculiarity in its transpiration-rate similar to that of dilute vinic alcohol. The acids, also, in many cases, exhibit a characteristic retardation of transpiration at a particular degree of hydration.

As a result of his inquiries, Graham also concluded that as far as his observations upon different alcohols, ethers, and acids, extended, the order of succession of individual substances in any homologous series would be indicated by the degree of transpirability of these substances as clearly as it is by their comparative volatility.

The following table contains a résumé of some of the most interesting results obtained by Graham upon this subject. The transpiration time of water at the particular temperature employed is, in all cases, taken as the unit of comparison :

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In hydrated substances, the extent to which transpiration is affected by the annexation of water, is by no means in proportion to the intensity of combination. In sulphuric acid, for instance, the maximum transpiration time occurs with the hydrate (H,SO,H,O), in acetic acid with the compound (HC,H,O,,H2O),

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