Colloid

“ Almost all the substances of which animal and vegetable bodies consist are colloids, and this is, at all events, partly the reason why animals and plants have such varied forms, which have no resemblance to the crystalline forms of the majority of mineral substances. The colloid solid substances in organisms—that is, in animals and plants—almost always contain water, and take most peculiar forms, of networks, of granules, of hairs, of mucous, shapeless masses, &c., which are quite different from the forms taken by crystalline substances. ”

“ It is not until solid matter rises from the crystalline to the higher condition of the colloid, that it becomes capable of absorption, diffusion, and even of assimilation; that, in a word, it assumes relations to the external world which show that it possesses an individuality higher than the cyrstal, and is, in fact, endowed with many of the activities belonging to those masses of colloidal matter which biologists have agreed to call living. ”

“ When an electric current is passed through a colloid solution, the course of the molecules of the colloid is sometimes towards the cathode and sometimes towards the anode, according to the nature of the colloid and of the solvent. This displacement would appear to indicate a difference of electric potential between the molecules of the colloid and those of the solvent. Hardy has shown that in an alkaline solution the molecules of albumin travel towards the anode, while in an acid solution they travel towards the cathode. ”

“ The chemistry of colloids has now assumed such importance that it may be considered as a separate branch of the science. It has its own technical journal and deals largely with the chemistry of organic products. All living matter is built up of colloids, and hæmoglobin, starch, proteins, rubber and milk are examples of colloidal substances or solutions. Among inorganic substances, many sulphides, silicic acid, and the amorphous hydroxides, like ferric hydroxide, frequently act as colloids. ”

“ The belief that the agents of the chemical transformation in the cell are catalysators of a colloid nature is in complete accord with other facts that have been directly ascertained. What else are the chemists' ferments but colloid catalysators? The idea that the ferments are the essential chemical agency in the cell is calculated to meet the difficulty which arises from the smallness of the cell in appreciating its chemical processes. However large we suppose the colloid ferment molecules to be, there is room for millions of them in the smallest cell. ”

“ But the living cell grows in a totally different way, very much as a piece of glue swells up in water, by “imbibition,” or by interpenetration into and throughout its entire substance. The semifluid colloid mass takes up water, partly to combine chemically with its individual molecules267, partly by physical diffusion into the interstices between these molecules, and partly, as it would seem, in other ways; so that the entire phenomenon is a very complex and even an obscure one. ”

“ It is distinctive, but the use of the expression "colloidal solution" is to be strongly condemned, since it is so evident that substances in the colloidal condition are not dissolved, in the strict sense of the term. Colloidal particles are in a condition midway between solution and mechanical suspension, and they are held in this peculiar state of dispersion by virtue of their surface energy, electrical charge, their kinetic energy as manifested by the Brownian movement, and the adsorbed ions of electrolytes which are essential to the stability of all colloids. ”

“ After a consideration of all the terms, it would seem that "lyophilic" and "lyophobic," or more special derivatives, "hydrophobic" are the least objectionable terms, since most colloids answer to one of these designations. When our knowledge of colloids becomes exact enough to sharply differentiate between these two classes then we may find names for those systems which now seem to have properties which place them in neither of the above. ”

“ Jarvis, “is invisible under the most powerful microscope. The molecule is larger, but defies the microscope. But, having gone thus far, science had to go further. The next larger mass after the molecule, is the colloid. A colloid is a formless substance classified as a slime. It never takes a definite form like the crystalline substances. Solutions of gold can be made in the two forms—there is a colloidal gold and a crystalline gold.”
A look of stupefaction was on the faces of the inspector, the coroner and all that vast throng in the courtroom. ”

“ The matter of which a living organism is constituted consists essentially of certain solutions of crystalloids and colloids. To this we may add an osmotic membrane to contain the liquids, and a solid skeleton to support and protect them. Finally, it would seem that a colloid of one of the albuminoid groups is a necessary constituent of every living being.
We may say, then, that a living being is a transformer of energy and of matter, containing certain albuminoid substances, with an evolutionary form, the constitution of which is essentially liquid. ”

“ But long before the critical point of a colloidal solution is reached the variation in the charge of the colloidal particles alters their physical properties. An increase in their charge has the same effect as if the viscosity of the liquid were increased.
The bulk of our protoplasm consists of colloidal material, and the physical manifestations of life, such as muscular contractions and protoplasmic motions, and the innervations, are due to changes of the condition of these colloidal solutions. ”

“ We all know how the trajectory of a beam of sunlight through a darkened room is rendered visible by the particles of dust. In the same way if a colloidal solution is illuminated by a transverse ray of light, the light is diffused by the molecules of the colloid in semi-solution, and the liquid appears faintly illuminated on a dark background. ”

“ The chemical equilibrium of solids in contact with liquids, including the delicate mathematical conditions for the formation of crystals in mother liquor, is treated dynamically as a matter of stresses and strains, and this together with the theory of interfacial formations and liquid films will embrace the possible physics of colloid substances. ”

“ In the first place no one of those colloids which make up the mass of a living body, appears capable of carrying on vital changes by itself: it is always associated with other colloids. A portion of animal-tissue, however minute, almost always contains more than one form of protein-substance: different chemical modifications of albumen and gelatine are present together, as well as, probably, a soluble and insoluble modification of each; and there is usually more or less of fatty matter. ”

“ For it is becoming every day more apparent that the chemistry and physics of the living organism are essentially the chemistry and physics of nitrogenous colloids. Living substance or protoplasm always, in fact, takes the form of a colloidal solution. ”

“ Graham named the solids formed by the setting or coagulation of these liquids hydrogels. Solutions of colloids in solvents such as water and alcohol seem to be divisible into two classes. Both mix with warm water in all proportions, and will solidify in certain conditions. One class, represented by gelatin, will redissolve on warming or diluting, while the other class, containing such substances as silica, albumen, and metallic, hydrosulphides, will solidify on heating or on the addition of electrolytes to form a solid “gel” which cannot be redissolved. ”

“ We cannot argue from one substance to another; adsorptive attraction shews its effect on one and not on another; and we have not the least reason to be surprised if we find that the little granules of protoplasmic material, which as they lie bathed in the more fluid protoplasm have (presumably, and as their shape indicates) a strong surface-tension of their own, behave towards the adjacent vesicles in a very different fashion to the incipient aggregations of calcareous or siliceous matter in a colloid medium. ”

“ Lastly, the great difference in diffusibility between colloids and crystalloids, makes possible in the tissues of organisms a specially rapid re-distribution of matter and motion; both because colloids, being easily permeable by crystalloids, can be chemically acted on throughout their whole masses, instead of only on their surfaces; and because the products of decomposition, being also crystalloids, can escape as fast as they are produced: leaving room for further transformations. ”

“ In both plants and animals, as will be pointed out in the following chapter, practically all the vital activities of the cell protoplasm are definite manifestations of colloidal phenomena. Enzymes perform their catalytic functions by reason of their colloidal form. Proteins exist in colloidal form and are the seat of all vital functions. The regulation of the passage of materials into and out of the cell is governed by minute changes in the electrolyte concentration, etc., which produce enormous changes in the colloidal character of the protoplasm. ”

“ Through solution, all soluble elements, which would otherwise remain locked in the inactivity of the solid state, are enabled to enter upon a new phase of mobility, of chemical life, as we may say. Only in water could colloid carbon compounds first be built up, and only from such substances could life originate. ”

“ It is a mass of water made sufficiently solid to be reduced into shape and form, by rather less than twenty per cent, of solid matter, consisting of albuminous substance, saline substance, fatty substance. The mechanism for the supply of blood is most delicate, membranous; the mechanism for dialysis or separation of crystalloidal from colloidal substance is perfect, and the conversion of the compound substance of brain from one condition of matter to another is, if we may judge from some changes of water charged with colloidal or fatty substances, extremely simple. ”

“ The components of the system are, of course, water, salts, and the various organic compounds (fats, proteins, carbohydrates, and enzymes in all cells; and other groups, such as essential oils, tannins, pigments, etc., in cells which have certain special functions to perform) which constitute the solid phase of the colloidal mixture. ”

“ Importance of Gel-formation.—From the physiological point of view, gel-formation is undoubtedly the most important aspect of colloidal phenomena. In the first place, the ability to absorb and hold as much as 80 to 90 per cent of water in a semi-solid structure is of immense physiological importance. In no other condition can so large a proportion of water, with its consequent effect upon chemical reactivity, be held in a structural, or semi-solid, mass. ”

“ When the larger part of the mother liquor is washed away by the use of pure water, many precipitates show a tendency to "run through a filter," forming colloidal suspensions in the pure water in the filter and being again precipitated as the colloid mixes with the electrolytes in the filtrate. ”

“ Why apply any special term at all?
⁠When a hydrous ferric oxide sol is allowed to evaporate spontaneously, it will go through a jelly-like stage and finally become a hard scaly residue when all or nearly all of the water has left it. The jelly-like form will redisperse when warmed with an excess of water, while the amorphous form will not. Yet both forms are called "gel"! Zsigmondy distinguishes between these two different forms by calling the first one "hydrogel" and the second, "gel." A review of the texts on colloidal chemistry shows the following conceptions of this term. ”

“ It is extremely hard and extremely stable. Ascending to compounds of two, three, or more elements, we get substances which are more complex and less stable; and at last we arrive at combinations which involve many elements and are extremely complex. Among these latter substances are some, called colloids, which [78] are neither solid, like crystals, nor fluid, like liquids, but in an intermediate state, like jelly or the white of an egg, in which the molecules have great mobility and are at a considerable distance apart, so that water can penetrate their mass. ”

“ The terms "emulsoid" and "suspensoid" are very popular. "Lyophilic" and "lyophobic" are very expressive. The "stable-unstable" and the "reversible-irreversible" terms should also be abolished, since they describe the conduct and changes in state of colloids when subjected to external conditions and are not always sharply defined. For example, hydrous ferric oxide sol is fairly stable in the presence of neutral electrolyte (much more so than colloidal gold or platinum) , while it is irreversible when evaporated to dryness. ”

“ The power of the organic colloids to imbibe water, and to bring along with it into their substance the materials which work transformations, would not be continuously operative if the water imbibed were to remain. It is because it escapes, and is replaced by more water containing more materials, that the succession of changes is maintained. Among the higher animals and higher plants its escape is facilitated by evaporation. ”

“ And the first result of the researches upon colloids which should be emphasized is that there are in fact at least two kinds of dissolved or suspended substances retained by animal membranes, which differ so radically in their other properties that their inclusion in the same class is sure to lead to serious confusion, unless special pains be taken to discriminate between them. As types of these two classes of colloidal mixtures may be taken an aqueous solution of gelatine and one of colloidal arsenious sulphide. ”