Popular Science Monthly/Volume 67/July 1905/The Preparation and Properties of Colloidal Mixtures

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By Professor ARTHUR A. NOYES,


IT was by the well-known investigations of the English physicist, Graham, published in the seventh decade of the last century, that the general attention of scientists was first drawn to the existence of a class of homogeneous mixtures, differing materially in their properties from ordinary solutions, such as those of salt and sugar. Impressed by the fact that the dissolved substance as a rule separates from the one class of solutions in the amorphous and often gelatinous state, and from the other in the form of crystals, he designated the former substances colloids and the latter crystalloids, and their solutions have since been commonly known, respectively, as colloidal and as crystalloidal or ordinary solutions. During the period immediately following Graham's classical researches, the subject of colloidal solutions received comparatively little attention. Within the last fifteen years, however, this field has become a favorite hunting ground of both physical chemists and physiologists in their searches after new truths, and greatly has the store of our knowledge in regard to this important state of aggregation been thereby increased. Yet the difficulty in reaching general conclusions as to the properties of these solutions has proved to be a very great one, owing to the complexity of the phenomena and to the apparent contradictions between many of the results obtained with different colloids and by different investigators. Moreover, the original literature of the subject has become so extensive and so detailed as to be almost overwhelming to one who, with limited time to devote to it, desires to obtain a general survey of this field of work. A brief review of some of the more important principles thus far established may, therefore, be of general interest.

It seems appropriate to begin the consideration of the subject with a definition of the class of substances to which our attention is to be devoted. In accordance with the general use of the term, colloidal mixtures are most simply defined as liquid (or solid) mixtures of two (or more) substances which are not separated from one another by the action of gravity, however long continued, nor by filtration through paper, but which are so separated when the liquid is forced through animal membranes, the substance then remaining behind being designated the colloid. This distinguishes them, on the one hand, from suspensions of fine visible particles, and, on the other, from ordinary solutions, and it implies that the colloidal particles are intermediate in size between the particles of such suspensions and the molecules which are present in ordinary solutions.

It is obvious, however, that this definition is not based upon a really fundamental distinction either in the properties exhibited by the various mixtures or in the character of their particles. It would, therefore, not be surprising to discover that the so-defined group of colloids include substances having very different properties in other respects than that just considered. 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. The former possesses a much greater viscosity than that of water; the latter does not appreciably differ from it in this respect. The former gelatinizes upon cooling or upon evaporation, and passes again into solution upon heating or addition of the solvent; the latter does not gelatinize upon cooling, and if gelatinized by other means it does not redissolve upon heating. The former is not coagulated by the addition of salts (unless in excessive amount); the latter immediately gives an abundant precipitate. This difference may be readily shown by adding to a tube containing a one per cent, gelatine solution and to one containing a colloidal suspension of arsenious sulphide a little strong magnesium chloride solution, when no effect will be observed in the first tube; while a voluminous yellow precipitate will result in the second. We have, therefore, to distinguish the viscous, gelatinizing, colloidal mixtures, not coagulated by salts, from the non-viscous, non-gelatinizing, but readily coagulable, mixtures. The former class may be designated colloidal solutions, the latter, colloidal suspensions. This nomenclature is based upon the belief that a more fundamental distinction between the two classes of mixtures is the possession by the former of the characteristic properties of true solutions—osmotic pressure, diffusibility, and usually a limited solubility of the colloid at some temperature, and the absence of these properties in the members of the latter class and the manifestation by them of many similarities to macroscopic and microscopic suspensions. Even though this may not be a sharp line of division, it is highly probable that typical members of two classes exhibit these properties of true solutions in such a different degree as to make the differentiation an important one. Unfortunately, however, colloidal mixtures have not yet been satisfactorily enough investigated with respect to these properties to enable a classification to be based exclusively upon them.


Colloidal Solutions.

Let us now consider the characteristics of the two classes as manifested by typical representatives, beginning with the colloidal solutions. These substances are, for the most part, obtained directly from animal or vegetable sources and are purified by dialysis. Among the most important are gelatine, agar-agar, unheated albumen, caramel, starch, dextrine, and many natural gums.

A number of the important properties of these colloidal solutions have already been alluded to, but some of them deserve further consideration. The contrasts and similarities between them and ordinary solutions may first be mentioned. Such colloids possess a much slower rate of diffusion, a much smaller osmotic pressure, and a much slighter influence on the vapor-pressure, freezing-point, and boiling-point of the solvent than do corresponding weights of crystalline substances. So small are these effects that whether they exist at all is a question to which much investigation and discussion have been devoted. The now existing experimental data seem to show, however, that the gelatinizing non-coagulable colloids do possess these properties and influences in an appreciable degree. The results of the osmotic pressure determinations in the cases where it has been measured against a parchment or animal membrane, which would not retain the mineral impurities, are especially significant. Thus by this method it has been found that a 6 per cent, glue solution exerts a pressure of about one third of an atmosphere, and that a 10 per cent, solution of the colloids of blood-serum produces one of 40 mm. of mercury. Further investigations in this direction would be of great value. The results of Graham, too, seem to leave no doubt as to the existence of diffusion; he found, for example, that albumen diffused one seventh as fast, and caramel one fourteenth as fast as cane-sugar. Thus, these colloids exhibit the same properties as ordinary dissolved substances, but in a lesser degree—a fact which is explained in accordance with the modern theory of solutions by the simple assumption that they are true solutions, but that their molecules consist of aggregates of the ultimate chemical molecules and are, therefore, of much greater weight and complexity than those of non-colloidal substances.

This assumption seems, however, of itself alone, scarcely sufficient to account for the abnormal viscosity of these colloids, their power of gelatinization, and their property of permitting the free passage through them of non-colloidal substances, but preventing entirely that of other colloids of either of the two classes. This last property is the same one that is involved in the permeability of animal membranes for crystalloids and their impermeability for colloids, since such membranes are themselves nothing more than gelatinized colloids. Yet it deserves, on account of its great importance, a somewhat fuller consideration. This difference in behavior towards crystalloids and colloids may be readily illustrated by immersing sticks of gelatine or agar jelly in one experiment in a colored salt solution and in another in a colored colloidal suspension, and allowing them to remain for a day or more. Such comparative experiments may be made in a striking way with a solution of copper sulphate and ammonia and with a colloidal suspension of ferric ferrocyanide or Prussian blue made by mixing equal volumes of dilute solutions of ferric chloride and potassium ferrocyanide. Upon removing the sticks after some hours and cutting them in two, it will be noticed that the ammoniated copper sulphate has permeated the stick uniformly to its center, while the Prussian blue has not entered it at all.

Not only are gelatinized colloids permeable to salts, but, remarkably enough, they offer only a very slight, often scarcely appreciable, hindrance to the passage of these substances through them. Thus, accurate experiments have shown that the rate of diffusion of salts and mineral acids is the same, at any rate within one per cent., in a solid jelly containing 3 to 5 per cent, of agar-agar as it is in one containing only 1 per cent, of agar-agar, and it is, therefore, presumably the same as in water itself, though the accuracy with which this latter conclusion has yet been directly tested is much less. It has also been shown that the electrical conductivity of salts in a gelatine jelly is only a few per cent, different from that in pure water, and that there is no sudden change in its value when the jelly sets. This property is, however, dependent on the rate of motion of the ionized molecules of the salt through the medium between the electrodes, and the slight variation in it caused by the presence of colloids, even in quantity sufficient to produce gelatinization, proves that the flow of such molecules is but little impeded by the colloid.

Returning now for a moment to the other side of the phenomenon—the impermeability of one colloid by another—attention may be called to an apparently related fact of much importance, namely, to the fact that the presence of a gelatinizing colloid in a liquid in fairly small quantity prevents the coagulation of colloidal suspensions by salts, and, therefore, usually prevents the formation of a coagulated precipitate when the solutions of two chemical substances are mixed which, under ordinary conditions, give rise to such a precipitate. Thus when aqueous solutions of silver nitrate and sodium chloride are mixed, an abundant curdy precipitate is produced, but, if a little gelatine solution be first added to one of the salt solutions, only an opalescence results, and the silver chloride formed by the metathesis remains indefinitely in the state of a colloidal suspension. Glycerine, sugar and even ether in some cases have a similar influence. This result may arise from the fact that in the presence of the gelatine the particles of silver chloride after attaining a certain size are not capable of diffusing, and hence of coming into contact with one another. It is probable, however, that, at any rate in many cases, the gelatine prevents the coagulation by forming an envelope around the solid particle. Whatever may be the explanation of the phenomenon, it is one of great technical importance, especially in relation to photography; for upon it is based the preparation of the so-called emulsions of silver salts in gelatine, collodion or albumen with which dry plates, films and printing-out paper are coated.

Recent investigations have proved that the gelatinization of these colloidal solutions arises from the separation of a portion of the colloid in the solid state in more or less continuous masses. The resulting jelly, or gel, as it is technically called, has been shown to have an irregular sponge-like structure, the web consisting of a solid mixture of the two substances and the interstices being filled with a liquid solution of them. This has been proved in some cases by direct microscopic observation, and in others by separating the liquid from the solid portion by pressure and by analyzing these portions, which were thus shown to have a very different composition with respect to the proportions of the two constituents. Thus one of the investigators of this subject, Hardy, states that when a solution of 13.5 grams of gelatine in a mixture of 50 c.c. of alcohol and 50 c.c. of water is gradually cooled, it remains homogeneous until a temperature of 17° centigrade is reached. Then it separates into two liquid phases, and is seen to consist of small microscopic droplets suspended in a fluid matrix. As the temperature falls, these droplets cohere to one another and at 12° they have become solid, forming a framework built of little spherical masses. The mixture as a whole has then become a jelly. At 14° the droplets were separated and found to contain 18 per cent, of gelatine while the matrix contained only 5.5 per cent. The important statement is also made that the first appearance of the droplets is attended by a great increase in viscosity, while the subsequent increase is a continuous one. The abnormal viscosity of such colloidal mixtures is, therefore, probably always due to a physical heterogeneity of this kind. The investigations made with other gelatinizing colloids, such as agar, albumen, starch and even silicic acid, have led to a similar conclusion in regard to the structure of the jelly.


Colloidal Suspensions.

Methods of Preparation.—The mixtures of this class have been, for the most part, prepared artificially. The principles of some of the methods which have been employed for this purpose may, therefore, be first described.

Of these principles the most important one is, that when an insoluble substance is produced in the absence of electrolytes by a reaction between two chemical compounds, it almost invariably separates in the state of a colloidal suspension. By the term electrolyte is here meant any dissolved substance which is a good conductor of electricity, one, therefore, whose molecules are, according to the ionic theory, largely dissociated into electrically charged atoms or atom groups called ions. Most salts and strong acids or bases are such electrolytes; but water, neutral organic substances like alcohol or sugar, and very weak acids or bases, are not. Electrolytes must not be present in considerable quantity, for the reason that ions coagulate these suspensions. Thus, when a saturated solution of hydrogen sulphide, a slightly ionized substance, is added to one of arsenious oxide, also slightly ionized, no coagulated precipitate of arsenious sulphide results, but only a turbid yellow liquid, which, when poured through filter-paper, leaves nothing behind. It will be noted that in this case the other product of the reaction is water, an un-ionized compound. If this reaction be carried out with a solution of arsenious chloride, instead of with one of the oxide, the ordinary precipitate of arsenious sulphide is obtained; for, in this case, the hydrochloric acid produced by the reaction, being largely dissociated into hydrogen and chlorine ions, coagulates the colloidal suspension. So, also, upon adding hydrochloric acid to the colloidal mixture resulting from the former experiment, a large precipitate is immediately produced. As a second illustration of this method, hydrogen sulphide water may be added to a solution of mercuric cyanide. In this case also a black opaque colloidal suspension of the sulphide results; for the three substances involved in the reaction, hydrogen sulphide, mercuric cyanide, and hydrocyanic acid, are non-electrolytes; but, upon the addition of hydrochloric acid, or, still better, of magnesium chloride, to this solution, the precipitate immediately coagulates. It is not necessary, of course, that electrolytes be entirely excluded, but only that they be not present at any point at such a concentration as will produce coagulation. The method is, therefore, of fairly general applicability. Thus, a colloidal suspension of Prussian blue can be prepared by mixing dilute solutions of nearly equivalent quantities of ferric chloride and potassium ferrocyanide; for the other product of the reaction, potassium chloride, has a coagulating effect only at higher concentrations.

A second method which has until recently been even more commonly employed than that just described, consists in the dialysis of a salt solution in which a colloidal base or acid is present, either owing to natural hydrolysis or to the previous addition of an alkali or acid. Thus colloidal silicic acid may be prepared by dialyzing either a solution of sodium silicate alone, or one to which hydrochloric acid has been previously added. A dark red but perfectly clear colloidal suspension of ferric hydroxide is obtained by the dialysis of a ferric chloride solution which has been treated with ammonium carbonate until a permanent precipitate begins to form. This process of dialysis is commonly resorted to also for freeing colloidal solutions or suspensions prepared in other ways from mineral impurities. It is most conveniently carried out in parchment tubes, which are now an article of commerce. As the surface exposed by these is large, the process is a comparatively rapid one. The solution to be dialyzed is placed within such tubes, and these are immersed first in running tap water and afterwards in distilled water which is frequently renewed.

There is one other method of sufficient importance to deserve mention, and this is the process recently described of preparing colloidal suspensions of metals by producing an electric arc under water between electrodes of the metal in question. This is most readily carried out with the non-oxidizable metals, such as gold or platinum. When gold is used, red clouds of colloidal gold are formed near the arc, and in half a minute the whole liquid assumes a red color. The method depends on the fact that the metal is volatilized into the arc or spattered into it in an extremely finely divided form, and is then condensed or absorbed by the water, which, owing to the absence of electrolytes, has little tendency to cause aggregation of the particles.

Besides these colloidal suspensions artificially prepared from mineral substances, others can be obtained by dialysis and other treatments from animal and vegetable sources. Among the most fully investigated of these are heated albumen and gum mastic.

Properties indicating Heterogeneity.—Turning now to the properties of such colloidal suspensions, it seems appropriate first to refer to those which indicate that these mixtures really are suspensions of minute particles and not true solutions. The fact that the components of the mixture are separated by filtration through animal membranes or close-grained porcelain filters is not of itself an evidence of physical heterogeneity; for by copper ferrocyanide membranes, prepared by depositing a precipitate of this substance in an unglazed porcelain cylinder, sugar and even salts can be separated from true solutions. In some cases, the presence of particles in suspension in so-called colloidal mixtures has been proved directly by microscopic observation; thus this is the case with the colloidal mercuric sulphide and with colloidal arsenious sulphide when prepared under certain conditions, but not under others; the same is true of blue colloidal gold, which can be produced in a variety of ways, for example, by, the reduction of gold chloride solution by hydrazine. In most cases, however, the colloidal particles can not be seen even under the best conditions; they are, therefore, smaller than one seventh of a micron (1/7000 mm.), which is about the limit of microscopic visibility. It will be of interest to determine whether they can not be detected in many other cases with the help of the new Zeiss microscope, which, by employing quartz lenses and ultraviolet light (having a much shorter wave-length than ordinary light) and obtaining the image photographically, extends the limit of visible diameters to about one half of its present value. With such an 'ultramicroscope' a German investigator, Raehlmann, has already observed the suspended particles in an albumen solution. By the optical method of Sidentopf and Zsigmondy, in which the colloidal mixture is intensely illuminated by a thin beam of light, and the diffused light reflected from the suspended particles at right angles to the beam is viewed with a powerful microscope, the presence of still smaller particles having a diameter of 1/100 micron has been detected in red-gold suspensions and in other colloidal mixtures.

A strong indication of the heterogeneity of colloidal suspensions is furnished also by the familiar optical phenomenon, which is often called the Tyndall effect, and is observed when a beam of light is passed through any medium containing particles in suspension. The beam becomes visible, as does a sunbeam in dusty air, owing to a diffuse reflection of light from the particles. This can readily be shown to occur with colloidal suspensions of gold and of arsenious sulphide. Moreover, in every case where reflection takes place from non-metallic surfaces the reflected light is polarized, and this is found, in fact, to be true of the rays diffusely reflected from a colloidal suspension by examining them with a rotated Nicol prism. It has been shown, to be sure, that not only colloidal solutions (colloidal mixtures of the first class), but also ordinary solutions of some substances with complex molecules like sugar, exhibit this phenomenon, so that it is not a decisive criterion of a suspension. They do so, however, in an incomparably less degree than do typical colloidal suspensions, so that it at least furnishes evidence that the particles in the latter mixtures are of much larger size than are those in the former.

Whether the well-defined colloidal suspensions possess in appreciable degree what may well be regarded as the best single criterion of a true solution—a measurable osmotic pressure—does not, in spite of its importance, seem to have been the subject of investigation by the direct osmotic method. Nor is there conclusive evidence that they show the closely related phenomenon of diffusion. If the existence of these properties to an extent corresponding at all to the size and number of the particles should be demonstrated, it would, of course, prove that the distinction between colloidal solutions and suspensions is not one of quality, but only one of degree.

Properties related to the Electrification of the Particles.—A quite distinct class of properties may be next considered, which depend not on the size of the colloidal particles, but apparently upon the presence of electric charges upon them.

The most direct evidence of this electrification is furnished by the migration of the colloidal particles through the liquid under the influence of an applied electromotive force. This effect may be well illustrated with colloidal suspensions of arsenious sulphide and ferric hydroxide contained in two U-tubes. The tops of the U-tubes are covered with goldbeaters' skin and are surrounded by wider tubes containing pure water in which platinum electrodes are placed, so that the products of electrolysis collecting around them may not influence the colloid. These tubes are then connected in parallel with the terminals of a 110-volt circuit in such a way that the current will flow through each of them in the direction from left to right. It is some minutes before any result is observed. Then it is seen that the ferric hydroxide has moved down with a sharp surface of demarkation on the side where the current enters, leaving a clear layer of water above, and that the arsenious sulphide has done the same, but on the opposite side of the tube. In other words, the ferric hydroxide particles are moving with the positive current towards the cathode, the arsenious sulphide with the negative current towards the anode. The former are, therefore, positively, and the latter negatively, charged. These results are typical ones: such movement, or migration, as it is commonly called, is exhibited by all colloidal suspensions, and, it may be added, also by fine microscopic suspensions, like those of quartz, kaolin and lampblack. Other basic hydroxides, like those of aluminum, chromium and thorium, and certain dyestuffs, migrate to the cathode just as does the hydroxide of iron. The suspended particles of almost all other substances, whether colloidal or microscopic, migrate to the anode. This is true, for example, of silicic acid, stannic acid, metallic sulphides, salts like silver iodide and Prussian blue, and metals like gold and platinum. Of special interest with reference to the explanation of the phenomenon is the recently discovered fact that an egg-albumen suspension migrates towards the cathode in an acid liquid and towards the anode in an alkaline one.

In regard to the cause and character of the electrification two assumptions deserve consideration: one is that it is simply an example of contact electricity, the colloid particle assuming a charge of one sign and the surrounding water that of the other. This correlates this phenomenon of migration with that of electrical endosmose; for the motion of suspended kaolin, for example, through water against the positive current is obviously the converse of the flow of water through a porous clay diaphragm with the current. It does not, however, give an obvious explanation of the facts that the basic colloidal particles become positively charged and the acidic and neutral ones negatively charged, or of the peculiar behavior of albumen. The other assump- tion accounts for these facts. According to it the phenomenon is a simple case of ionization, the character of which may be best illustrated by specific examples. Thus, each aggregate of ferric hydroxide mole- cules may dissociate into one or more ordinary hydroxyl ions and a residual, positively charged colloidal particle, and each aggregate of silicic or stannic acid molecules into one or more hydrogen ions and a residual negatively charged colloidal particle. Albumen, which is known to be capable of forming salts with both acids and bases, would, acting as a salt, dissociate into an ordinary positive ion and a colloidal negative one in alkaline solution, and into an ordinary negative ion and a colloidal positive one in acid solution. To explain the behavior of neutral substances like gold or quartz by this hypothesis, it is neces- sary to supplement it by the assumption that in these cases it is the water or other electrolyte combined with or adsorbed by the colloidal particles which undergoes ionization. It seems not improbable that there may be truth in each of these hypotheses, contact electrification occurring in the case of the coarser suspensions, and ionization in the case of those which approximate more nearly to colloidal solutions. It should be noticed that these hypotheses do not differ as to the charge on the colloidal particle itself, the existence of which is in fact experi- mentally demonstrated, but only as to the location of the accompany- ing charge of opposite sign, namely, as to whether it is on the water itself or on ordinary ions dissolved in it. This matter is not essential to our further considerations.

Let us turn now to the extremely important phenomenon of the coagulation of colloidal suspensions. It will be seen that this phe- nomenon is certainly closely related to the electric charges on the col- loidal particles. Indeed, it seems highly probable that they remain in suspension because of their electrification. Thus it has been found that egg-albumen, whose particles are shown by their migration to be positively charged in acid solution and negatively in alkaline solution, immediately coagulates when the solution is made neutral. Attention may also be called to another interesting fact having, apparently, the same significance; namely, to the fact that when two colloidal suspen- sions, whose particles have an electric charge of the same sign, are mixed, they have no influence upon each other, but when two suspen- sions, with particles oppositely electrified, are brought together, the two colloids combine with each other, and with proper proportions a complete coagulation results. Thus, upon mixing suspensions of colloidal gold and of arsenious sulphide, no coagulation occurs, but when the suspensions of ferric hydroxide and arsenious sulphide, which we have seen from their behavior on migration have opposite electric charges, are poured together, there is immediate coagulation, and in a few minutes the precipitate settles, leaving the liquid clear above.

Crystalloid substances are also to be divided into two classes with respect to their effect in coagulating colloids. Non-electrolytes, whether organic or inorganic, have no tendency to produce coagulation; indeed, we have seen that organic substances, like ether, glycerine or sugar, often increase the stability of the suspension. On the other hand, strong electrolytes, that is, substances which are themselves largely dissociated into electrically charged particles or ions, cause coagulation, when their concentration in the solution becomes sufficiently great. Although complete coagulation does not occur suddenly as the quantity of electrolyte is increased, yet the interval between the concentration at which, in a given time, the turbidity becomes visibly greater and that at which the particles have become large enough to settle out or to be retained by a filter is usually so small that a fairly definite concentration can be specified at which each electrolyte causes a certain, experimentally determinable, degree of coagulation in a definite time. Now recent investigations have demonstrated the remarkable fact that this coagulation-concentration is nearly the same for different ions having the same electric charge (or valence), but that it diminishes enormously with increase of the electric charge of the ion of unlike sign to that of the colloid, while it is not affected by a change in the electric charge on the ion of like sign.

These principles are well illustrated by the results given in the table, which were obtained by Freundlich, on the one hand with the negative colloid, arsenious sulphide, and on the other with the positive colloid, ferric hydroxide, by determining the concentration of various salts which in two hours caused such coagulation as would prevent any of the colloid from passing through a hardened filter.

Coagulation-concentration in milli-equivalents per liter
of AS2S3, a negative colloid, by of FeO3H3, a positive colloid, by
NaCl 71 MgCl2 2.0 AlCl3 0.39 NaCl 9.3 K2SO4 0.41
KNO3 70 Ba(NO3)2 1.9 Al(NO3)3 0.41 KNO3 11.9 K2Cr2O7 0.39
K2SO4 91 MgSO4 2.3 Ce2(SO4)3 0.40 BaCl2 9.6 MgSO4 0.43

It will be seen by comparing the first two values in each column that so long as the electric charge or valence of the ions of the salt remains the same, their chemical nature has no influence. It will be further seen from the third value in each column that a variation of the electric charge of the negative ion has no great influence upon the coagulation of the negative colloid, and that this is true of that of the positive ion in the case of the positive colloid. On the other hand, by comparing the numbers in the same row, it will be seen that the coagulation of the negative colloid takes place at a much lower concentration when the salt produces positive ions with a higher electric charge, and that an increase of the electric charge upon the negative ion has a similar effect upon the coagulation of the positive colloid.

It is evident from these facts that it is the ion with a charge opposite to that of the colloid particles that is mainly responsible for their coagulation, but what the mechanism of this coagulation is, is not yet understood, though it has been the subject of much discussion. Interesting though they are, it is not worth while to describe the explanations that have been suggested; for, in the opinion of the writer, mere speculative hypotheses, that is, hypotheses which have not been shown to facilitate to an important extent a knowledge of the actual phenomena, are of little value except to the investigators of them, and to them only because of the possibility of their future development into really useful conceptions. The recent literature of colloids furnishes a striking example of the unfortunate tendency even of our modern investigators and text-book writers to attach greater importance to hypothetical interpretations of imperfectly known phenomena than to a determination and presentation of the laws in regard to them.

Even an elementary consideration of the properties of colloids should include a discussion of the absorption or coprecipitation of other substances with them when they are gelatinized or coagulated—a phenomenon which is of great importance in analytical chemistry, as well as in other directions. But, for lack of space, this side of the subject will have to be entirely omitted. Moreover, only a mere reference can be made to the importance of a knowledge of the properties of colloids, not only in the industrial applications of chemistry, but also in many other sciences and arts. It must suffice to mention that the industries of dyeing, of tanning, of glass-making and coloring, and of the manufacture of photographic materials and of modern explosives have to deal primarily with substances in this peculiar state of aggregation; that the clarification of syrups and other liquors by charcoal, and that of water and sewage by precipitation, are based on the phenomena of absorption by colloidal substances; that it is with these substances as constituents of living bodies that physiology is mainly concerned; that they constitute the culture-media of the bacteriologist, to the employment of which the development of his science is largely due; and that to the geologist the phenomenon of the sedimentation of mud and slimes, which is closely related to that of the coagulation of colloidal suspensions, is one of much interest.

  1. This article is based upon a presidential address delivered by the author at the Philadelphia Meeting of the American Chemical Society, December 29, 1904.