water taking place, notwithstanding the higher sp. gr. of the substance which opposes this passage, is called the ‘diffusion of liquids.’ From the investigation of the phenomena of this diffusion, the late Prof. Graham derived the remarkable results upon which the method under notice is based. Different substances, when in solution of the same concentration, and under other similar circumstances, diffuse with very unequal velocity. “The range in the degree of diffusive mobility,” says Prof. Graham, “exhibited by different substances, appears to be as wide as the scale of vapour-tensions. Thus, hydrate of potassa may be said to possess double the velocity of diffusion of sulphate of potassa, and sulphate of potassa again double the velocity of sugar, alcohol, and sulphate of magnesia. But the substances named belong, as regards diffusion, to the more volatile class. The comparatively fixed class, as regards diffusion, is represented by a different order of chemical substances (marked out by the absence of the power to crystallise), which are slow in the extreme. Among the latter are hydrated silicic acid, hydrated alumina, and other metallic peroxides of the aluminous class, when they exist in the soluble form; with starch, dextrine, and the gums, caramel, tannin, albumen, gelatin, vegetable and animal extractive matters. Low diffusibility is not the only property which the bodies last enumerated possess in common. They are distinguished by the gelatinous character of their hydrates. Although often largely soluble in water, they are held in solution by a most feeble force. They appear singularly inert in the capacity of acids and bases, and in all the ordinary chemical relations. But, on the other hand, their peculiar physical aggregation, with the chemical indifference referred to, appears to be required in substances that can intervene in the organic processes of life. The plastic elements of the body are found in this class. As gelatin appears to be its type, it is proposed to designate substances of this class as ‘COL′LOIDS,’ and to speak of their peculiar form as the ‘colloidal condition of matter.’ Opposed to the colloidal is the ‘crystalline condition.’ Substances affecting the latter form will be classed as ‘CRYSTAL′LOIDS,’ The distinction is, no doubt, one of intimate molecular constitution.”[257] A certain property of colloidal substances comes into play most opportunely in assisting diffusive preparations. The jelly of starch, that of animal mucus, of pectin, of vegetable gelose, and other solid colloidal hydrates, all of which, strictly speaking, are insoluble in cold water, are themselves permeable when in mass, as water is, by the more highly diffusive class of substances. But such jellies greatly resist the passage of the less diffusible substances, and cut off entirely other colloid substances like themselves that may be in solution. A mere film of the jelly has the separating effect. Now, parchment-paper, when wetted, acts just like a layer of animal mucus or other hydrated colloid, by permitting the passage of crystalloids, but not of colloids; consequently this substance may be used for dialytic septa (see Dialyser, above). The following experiments recorded by Graham will give some idea of the results which may be obtained by dialysis:—
[257] ‘Philosoph. Trans.’ for 1861.
1. Half a litre of urine was placed in a hoop dialyser, which was then floated on a considerable quantity of pure water. Dialysed for 24 hours, the urine gave its crystalloidal constituents to the external water. The latter, evaporated by a water bath, yielded a white saline mass. From this mass urea was extracted by alcohol in so pure a condition as to appear in crystalline tufts upon the evaporation of the alcohol.
2. By pouring silicate of soda into diluted hydrochloric acid (the acid being maintained in large excess), a solution of silica is obtained. But in addition to hydrochloric acid, such a solution contains chloride of sodium, a salt which causes the silica to gelatinise when the solution is heated, and otherwise modifies its properties. Now, such a solution placed for 24 hours in a dialyser of parchment paper was found to lose 5% of its silicic acid (silica) and 86% of its hydrochloric acid. After 4 days on the dialyser, the liquid ceased to be disturbed by nitrate of silver. All the chlorides were gone, with no further loss of silica. What remained was a pure solution of silicic acid, which could be boiled in a flask, and considerably concentrated, without change.
3. Half a litre of dark-coloured porter, with ·05 gramme of arsenious acid added (1⁄10000th part of arsenious acid), was placed on a hoop dialyser, 8 inches in diameter, and the whole floated in an earthenware basin containing 2 or 3 litres of water. After 24 hours the latter fluid had acquired a slight tinge of yellow. It yielded, when concentrated and precipitated by sulphuretted hydrogen, upwards of one half of the original arsenious acid in a fit state for examination.
DIAMANTKITT—Diamond Cement. 50 parts graphite, 15 parts litharge, 10 parts milk of lime, 5 parts slaked lime, intimately mixed with enough linseed oil to make a firm mass. (Hager.)
DIAMANTTROPFEN—Diamond Drops (Dr Allinhead). A combination of the juices of mysterious herbs of tropical climes, which has the power to make all men transparent.
DI′AMOND. The diamond is pure carbon, and differs from the carbon of charcoal and lampblack simply in being limpid, colourless, and highly refractive of light, properties which are generally referred to its crystalline form. The weight, and, consequently, the value of diamonds, is estimated in carats, one of which is equal to 4 grains; and the price of one diamond, compared to that of another of equal colour, transparency, purity, form, &c., is as the squares of the respective weights. The
average price of ROUGH DIAMONDS that are worth working is about £2 for the first carat; that of a CUT DIAMOND is equal to that of a rough diamond of double weight, exclusive of the price of workmanship. “To estimate the value of a wrought diamond, ascertain its weight in carats, double that weight, and multiply the square of this product by £2.” (Ure.) Thus, a cut diamond of—
| 1 | carat is | worth | £8 |
| 2 | carats | ” | £32 |
| 3 | ” | ” | £72 |
| 4 | ” | ” | £128 |
| &c., &c. See Carbon, Gems. | |||